Articles

Sorensen Maritime publishes technical, professional, and consumer articles focused on real-world vessel functionality, operational safety, seaworthiness, seakindliness, maneuverability, crew workload, and operational decision-making.

These articles are written for owners, operators, designers, and acquisition professionals who have an interest in how hull-form design and configuration choices actually play out at sea—beyond marketing claims, paper specifications, or theoretical compliance.

The emphasis throughout is on operational reality: how vessels are run, how crews experience them over time, and how early design decisions shape long-term performance, safety, and usability.

Selected Articles

Sorensen’s Guide to Powerboats (DRAFT- 2026 edition revision)

Chapter 4: Planing Hulls

In this country it's a good thing to kill an admiral now and then to encourage the others.
—Voltaire

Table of Contents

I. Introduction

II. Fundamental Principles

• How Planing Works: From Buoyancy to Dynamic Lift

• Planing Speed: S/L Ratios and the Volume Froude Number

• The Physics of Lift and Resistance

• The Dynamic Lifting Surface

III. Centers: The Geometry of Balance

• Longitudinal Center of Gravity (LCG)

• Longitudinal Center of Buoyancy (LCB) and Center of Flotation (LCF)

• Center of Dynamic Lift (CDL)

• The Interplay: How They Work Together

• LCG in Practice: Fuel Tanks, Repowers, and Load Management

IV. Hull Geometry Essentials

• Deadrise: Angle, Impact, and Performance

• Buttocks: Profile, Pressure, and Performance

• Chines: Definition and Function

• Other Key Geometric Elements

V. Hull Types and Variations

• Hard-Chine Planing Hulls

• Round-Bilge and Hybrid Hulls

• Large Planing Hulls: Scaling Challenges

VI. Performance and Dynamics

• Understanding Resistance

• Weight, Bottom Loading, and Efficiency

• Performance Realities: Speed, Weight, and Power

• Trim: Angle, Attitude, and Control

VII. Seaworthiness in Practice

• Ride Quality and Human Factors

• Course-Keeping and Handling

• Dynamic Stability and the True Turn

VIII. Design Refinements

• The Refined Bottom

• Strakes and Spray Rails

• Trim Tabs

• Appendages and Fairing

IX. Dynamic Instabilities

• Understanding Dynamic Instability

• Oscillating Instabilities

• Constant Instabilities

• Ventilated Appendages

• Solutions

X. The Hunt Legacy and Modern Excellence

• Ray Hunt's Revolution

• Hunt Design Principles

• Lessons for Today

XI. Practical Hull Assessment

• Reading a Hull: Dockside Evaluation

• Underway Evaluation

• Red Flags

XII. Conclusion

I. Introduction

You'll probably get more from this chapter if you read chapters 2 and 3 first. Many of the concepts discussed here are explained in those earlier chapters, and reading them will help you gain a more complete understanding of what makes a planing hull work.

Good planing hulls are amazing creations. They have to run reasonably well and handle predictably at displacement speeds, and also be capable of climbing up on plane and skimming along the water's surface. Besides the static pressure of buoyancy acting on any partially submerged body, planing hulls at speed are subject to - and supported by - the dynamic forces of rapidly moving water. The ski boat, convertible sportfisherman, express yacht, and outboard-powered center console are all examples of planing boats.

Whereas displacement hulls can move huge amounts of cargo slowly while using very little fuel per ton of cargo, planing hulls can move small cargo loads, including people, very fast and very inefficiently, using a lot of fuel in the process. Though semidisplacement (or semiplaning if you prefer) hulls can rise partially on plane, supported by buoyancy and dynamic lift, a planing hull can essentially fly on the surface of the water and reach tremendous speeds.

Why Planing Hull Design Is Complex

Planing hull dynamics can be remarkably complex, and the study of these forces keeps some of our best naval architects busy. Why the complexity? For one thing, unlike a displacement hull, a hull's behavior on plane is nonlinear and therefore hard to predict. This means that steady changes in dynamic influences, like trim, have an inconsistent influence on the boat's behavior. Increasing trim by two degrees instead of one does not necessarily mean that lift is doubled, nor does dropping the bow a degree have the opposite effect of raising it a degree. A one-degree increase in trim may suddenly cause a dynamic instability like porpoising, while a three-knot increase in speed, say from 25 to 28 knots, may suddenly result in chine walking. So while the static buoyant forces acting on a displacement hull can be accurately predicted, why a planing hull acts the way it does under all conditions at high speed is another issue.

It would seem obvious, then, that designing a planing hull is a job for an expert with a thorough understanding of hydrostatics and hydrodynamics. But in fact many boats are designed by people who apparently rely on their boating experience and their intuition rather than on a comprehensive study of the principles of planing hull dynamics. The problem with this approach is that the behavior of planing hulls can be counterintuitive or evade intuitive powers altogether. So, while many planing boats are marvels of engineering, more than a few are inappropriately designed for their intended purpose. Others are designed to satisfy market demand for the roomiest possible boats for a given length, with wide, relatively flat bottoms and full, blunt entries. These floating condos should never venture outside calm, sheltered waters.

In theory, a hull can be made to plane easily by keeping it flat below the waterline, light, and wide. In real offshore sea conditions, however, that same boat will punish its passengers. At the other extreme, a hull that slices cleanly into waves may struggle to get on plane with the installed power if bottom loading is excessive or if excessive deadrise is carried too far aft.

The art lies in balancing the forward sections for wave impact absorption with the aft sections for dynamic lift, while keeping bottom loading - displacement relative to bottom area - within a workable range. This is also where deadrise distribution becomes critical (as discussed below). The faster the boat, the more deadrise it needs- and the further aft it needs it since wave impact shifts aft as the hull rises bodily out of the water with increasing speed.

In practice, many recreational boats perform poorly offshore because builders pursue interior volume by flattening hull sections and increasing beam. The result is a compromise in both hydrodynamic efficiency and ride quality. As boats become wider and heavier, they require more power to maintain a given speed—driving up cost, fuel consumption, and structural loads.

These tradeoffs, and the ways skilled designers manage them, are explored throughout this chapter.

II. Fundamental Principles

How Planing Works: From Buoyancy to Dynamic Lift

Let's consider first in broad-brush strokes how a planing hull works. Since boats have mass (weight), they have to be supported by something. An object that floats, including a planing hull, is supported by buoyancy when at rest, or at low speeds. When a planing (or semidisplacement) hull is moving above displacement speed, dynamic lift created by the pressure of moving water along the hull supports an increasing portion of the vessel's weight. Once a hull is moving fast enough, and this speed depends mostly on the hull's length and weight, dynamic water pressure predominates over buoyancy, doing most of the work of supporting the vessel. At this point we say a boat is "on plane."

A planing hull not only floats—it flies on the surface of the water. The pressure created by high-velocity water flow has a longitudinal center of effort called the center of dynamic lift (CDL). It's called dynamic lift simply because it depends on the constant motion of water flow to be present. Dynamic lift tends to dominate the force of buoyancy at speeds over 25 knots in larger planing hulls.

We'll also see that a planing hull's overall dimensions mean little when it comes to its performance; what really matters is the wetted hull length and chine beam. And this wetted area, the part of the hull that's in regular contact with the water, is surprisingly small in relation to the boat's overall size.

Planing Speed: S/L Ratios and the Volume Froude Number

In chapter 3 we saw that a vessel is considered to be in the semidisplacement mode at speed-to-length (S/L) ratios between 1.34 and 2.5 (8.5 to 15.8 knots for a boat with a 40-foot waterline length, for instance). A boat is generally considered to be fully planing when traveling at speeds in excess of S/L 2.5 to 3 (for a boat 40 feet long at the waterline, that means over 15.8 to 19 knots). Concrete evidence of a boat on plane, though, is a noticeable rising of the hull relative to the water—that is, the boat has "emerged" from its hole in the water. A hull planes because it has a suitable shape and sufficient power; when going fast enough, it's supported primarily by dynamic rather than buoyant pressures.

Speed-to-length ratios notwithstanding, a heavily loaded boat may still be in semidisplacement mode at S/L 2.5, while the same hull lightly loaded may be on plane at a S/L ratio of 2.3. The speed at which a hull actually rises bodily out of the water depends to a large degree on its bottom loading.

If, upon achieving a certain speed, a 42-foot, 20,000-pound lobster boat rises vertically at the stern as well as the bow and leaves a clean wake astern, is it on plane? The answer is yes, since there has been a rise in CG, and a clean wake indicates that significant dynamic pressures (lift) are being developed. But what about the same hull, now 18,000 pounds heavier with a load of fish on board and running at the same speed? Probably not, since the same dynamic forces acting on the bottom of the hull aren't likely to be sufficient to lift the hull vertically and make it plane. The only way to add dynamic lift is to add speed, and that requires more power. If it's available, the boat will plane.

So, although S/L ratios are useful for calculating wavemaking resistance, they aren't the whole story when it comes to predicting planing speeds; this is where the volume Froude number comes in. Naval architects use the volume Froude number, rather than the S/L ratio, or another Froude number based on length or beam, when evaluating planing craft. That's because the ability to plane is partly a function of the vessel's displacement, especially in the transition stages from semiplaning to planing.

The Physics of Lift and Resistance

When a planing hull impacts a wave, a deep-V absorbs the energy incrementally, and decelerates slightly more slowly than a flatter bottom, reducing the vertical accelerations felt by the boat and its human occupants. The hull meets a wave incrementally—first the keel, then the garboard, then the midsection, and finally the chines. A flat-bottom hull, on the other hand, meets the same wave all at once—wham! Both boats follow the surface of the waves, but the deep-V adjusts to the contour of the water's surface with a lot more ease and finesse; the difference in terms of comfort between the physical effect of a deceleration that's spread out over a few milliseconds, and one that's virtually instantaneous, is significant.

As a planing hull generates dynamic lift, it also creates resistance. The drag caused by a hull's interaction with the water is called resistance, which we break down into three types. Frictional resistance results in a boundary layer of water that the hull drags along with it due to the friction of the hull's surface disturbing water molecules in the immediate vicinity. This boundary layer gets thicker as a hull gets longer and deeper in the water, and may reach 4 to 6 inches on a sportfisherman and 18 inches on a navy destroyer. Wavemaking resistance is caused by the hull's displacement of water as it moves along. Hull shape and trim have the biggest influences on wavemaking resistance. Appendage drag is the resistance caused by struts, shafts, rudders, and propellers sticking out into the waterflow. Then there's aerodynamic drag, caused by the above-water hull and superstructure moving through the air; a modestly proportioned displacement hull will hardly notice a stiff head wind, but a broad-beamed convertible with a flybridge and tuna tower might easily be slowed 4 or 5 knots by air drag when running upwind.

The Dynamic Lifting Surface

The dynamic lifting surface is the wetted surface of the hull that supports the vessel when on plane. It is bounded by an area termed the stagnation zone forward, by the spray rail or chines to the sides, and by the transom aft. On high-speed planing hulls, this surface is roughly triangular in shape and may be only 20 to 30 percent of the boat's length. The dynamic pressure on this small area equals the displacement of the boat. And ultrafast surface drive-powered boats may also be supported partially by the surface-piercing propeller's considerable vertical lift.

The stagnation zone is where the high-speed waterflow is tangent to the hull surface, with waterflow breaking off both ahead and astern. Since the waterflow impinges at a right angle to the hull, dynamic pressure is greatest here. This is called the stagnation zone, or line, since the water hitting the hull along this line, for the briefest moment, is not moving in relation to the hull. Dynamic pressure on the hull drops off both forward and aft of this region—rapidly forward, as water is deflected and turns to spray, and more gradually aft.

III. Centers: The Geometry of Balance

Longitudinal Center of Gravity (LCG)

If one element is crucial to planing hull performance, it is longitudinal center of gravity. As we saw in chapter 2, LCG is defined as the precise location between bow and stern from which a boat would balance if suspended in the air. When a boat is designed, the weight of the hull, superstructure, decks, machinery, auxiliary equipment, fuel, water, furnishings, appliances, and everything else that will be placed in the finished boat must be precisely accounted for to ensure a well-balanced LCG. That's because the location of LCG is key to a planing hull's performance.

If LCG is too far forward or aft in relation to CDL, the boat will run off trim, handling poorly and running inefficiently. And if the boat is riding bow-down, low pressures (below the pressure of buoyancy) can develop forward in the hull, causing dynamic instabilities. If bow-high, for example, porpoising can result. So the interplay between CDL and LCG is all important; the hull has to have the right shape and trim angle for proper lift, LCG influences trim, trim affects CDL, overall displacement affects wetted surface and lift, and around and around we go. And the faster the boat, the farther aft LCG should be to maintain control and prevent instabilities at high speed. That's why some repower jobs get into trouble; high-horsepower diesels are installed, LCG doesn't shift aft, and the boat becomes a performance and handling nightmare.

Longitudinal Center of Buoyancy (LCB) and Center of Flotation (LCF)

The longitudinal center of buoyancy (LCB) is the center of the buoyant forces acting on the hull; or to put it another way, it's the LCG of the water displaced by the hull. An immersed body is buoyed up by a force equal to the weight of the displaced fluid, and the fore-and-aft center of buoyant force (LCB), determined by the underwater shape of the hull, lines up precisely with the fore-and-aft center of the boat's weight (LCG). If you add 500 pounds of weight, the LCG will move aft and so will the LCB, and the boat will trim down by the stern accordingly. The actual LCG must be located directly at the hull's design LCB, or center of the hull's submerged volume, for the boat to float properly at its design waterline.

The longitudinal center of flotation (LCF) is the longitudinal center of the hull's waterplane area (or footprint on the water's surface) about which the hull trims. Weight added at LCF will cause the hull to settle without any change in trim.

Center of Dynamic Lift (CDL)

The dynamic pressure acting on the planing hull varies from point to point along the bottom; it's high forward where the bottom first makes contact with the water, while back aft, near the propellers, it's quite a bit lower, in terms of pressure-per-square-foot of hull surface. CDL is determined by the shape of the bottom and the vessel's trim. Trim, in turn, is determined by CDL and LCG, so all these elements are interdependent.

CDL also changes with trim; it moves forward when weight is added at the bow, and aft when weight is shifted toward the stern. The faster a boat, the farther aft LCG should be positioned to keep CDL properly balanced and maintain control at high speed.

The Interplay: How They Work Together

A planing hull's longitudinal center of buoyancy (LCB) is generally about 35 to 40 percent of the way forward of the transom at the waterline—farther aft than on a typical displacement hull. Why the difference? The planing hull's buttock lines aft must be nearly horizontal, or parallel to the waterline, so the hull can plane efficiently. On a displacement hull, the buttock lines normally sweep up aft to reduce drag and equalize fore-and-aft buoyancy at displacement speeds. Such a hull will lack the dynamic lift necessary to plane. The hull at the waterline must also be wide enough back aft to develop sufficient dynamic lift so the vessel can get up on plane.

The planing hull also has its longitudinal center of flotation (LCF) farther aft, which reduces the impact of fuel level on trim when the fuel tanks are near the stern. The fullness of the planing hull's waterplane is reflected in its prismatic coefficient (see pages 36-37). Compared to its displacement and semidisplacement cousins, with their finer ends, the typical planing hull has a large Cp of about 0.70 or greater, which it needs to achieve dynamic lift.

LCG in Practice: Fuel Tanks, Repowers, and Load Management

From a performance perspective, the optimum location for fuel tanks is about 5 percent of the waterline length ahead of the full-load LCG. Bow-up trim tends to increase as fuel is burned off and the LCG shifts aft, and tends to decrease as speed picks up with the loss of weight. The result of these opposing forces is a net trim change of zero. The problem is, it's just about impossible to get this balance just right on a marketable boat, since fuel tank location isn't as high on the average owner's priority list as, say, closet size and squeezing in that stackable washer-dryer unit.

Saddle tanks installed outboard of the engines is one way to get the tanks close to the LCG, but this also cramps the engine room and reduces maintenance accessibility. Saddle tanks usually force the engines closer to the hull centerline, and this of course reduces the boat's maneuverability at low speed, with the propellers and rudders being closer together. This tank location also places the tanks higher in the boat, raising the CG and decreasing ultimate stability.

The convertible's largest fuel tank must usually be placed under the cockpit, which is well aft of the LCB. As a result, most convertibles will change trim markedly as fuel is consumed, and fuel has to be pumped between forward and after tanks to compensate. In nonsportfishing layouts, V-drive configurations with the engines mounted well aft have a lot to commend them. This allows the large fixed weights (the engines) to be in the stern and moves the variable weight of fuel forward, nearer the LCG. But an aft engine room arrangement, which also isolates machinery noises and vibrations more effectively from living spaces, is not practical if the design calls for a cockpit or aft cabin.

Some larger boats are designed so the fuel tanks are centered over the LCF, which means that trim won't change noticeably as fuel is consumed. Such a boat needs trim tabs, then, only to adjust trim to suit the sea state and wind conditions.

IV. Hull Geometry Essentials

Deadrise: Angle, Impact, and Performance

What Deadrise Is

Deadrise is the angle of the hull bottom (keel to chine) upward from the horizontal in station or cross-sectional view. The first place to look is the transom; a flat-bottom boat has no deadrise (0-degree deadrise) while a deep-V racing boat typically has 24 degrees of deadrise at the transom. The more deadrise, the smoother the ride. Larger angles of deadrise result in a smoother ride, allowing a hull to slice through the waves. However, this improvement in ride quality comes at the expense of dynamic lift, propulsion efficiency, and form stability. The flatter the bottom, the more easily a boat is pushed along the surface of the water. But calm-water speed is gained at the expense of rough-water speed, because the low-deadrise boat will have to slow down sooner as the waves build to avoid pounding.

Why Deadrise Distribution Matters

Transom deadrise, while an important at-a-glance indicator of how a boat will ride at speed, is by no means the whole enchilada. In fact, it's possible for a boat with 16 degrees of transom deadrise to ride better than the boat in the next slip with 22 degrees of transom deadrise, if the deadrise farther forward is greater. Deadrise in the bow and midship sections is important to ride quality because this is where the hull first meets the waves (except in ultra-high-performance racing boats, which can often become airborne and land stern-first). Planing hulls can have constant-deadrise or warped-V hulls, as we'll see.

How Speed Affects Deadrise Requirements

As increasingly powerful lightweight diesels boost a boat's speed capability, it's essential that the deadrise in modern designs increase proportionately to keep slamming loads down. A sharp entry for the first 20 percent of the waterline length isn't enough; the increased deadrise must continue aft to the hull's midbody, because that's where most of the wave impact is taking place at higher speeds. For most hulls, there must be at least 15 degrees of deadrise at the transom, and 25 to 35 degrees in the hull's midsection. There's no way around the need for deadrise here.

Over the years, many production boatbuilders like Tiara, Grady-White, and Viking have steadily increased deadrise forward as their boats have evolved from cruising at 16 to 20 knots in the 1970s to 32 knots (or faster) today. But these boats and yachts aren't lightweights, so there's a limit to how much deadrise is practical with a 30-knot cruising speed in demand. Bertram and Viking both opt for transom deadrise in the 15- to 18-degree range, delivering good lift at speed, but with enough deadrise for a smooth ride and improved course-keeping. The Bertram 46 (a stretched 43), as much as any other convertible of its size, is a smooth-as-silk marvel at speed in a chop.

Deadrise and the Keel Effect

At 30 knots, a sudden evasive maneuver or collision-avoidance turn demands a hull that remains composed in hard turns. A deep-V with a radiused keel and moderate warp or twist in the aft half of the hull excels here because it manages lateral forces progressively. As the boat heels into the turn, the water flow around the keel forward remains attached rather than separating abruptly which causes hooking and coursekeeping instabilities when running downsea. The result is a controlled turn. A hull with a keel that comes to a hard edge forward is much more susceptible to hooking and bow steering downsea since the bow turns into a rudder with more authority than the real rudders back aft. 

Buttocks: Profile, Pressure, and Performance

What Buttocks Reveal

Buttock lines are run longitudinally from bow to stern, like slicing a loaf of bread lengthwise from the top. You can see the curvature of the hull bottom in the buttocks, and the degree or radius of curvature spells the difference between a hull that runs well downsea and one that does not. The shape of the buttocks in profile can also create or prevent dynamic instability, and in fact is a primary design factor distinguishing displacement, semidisplacement, and planing hulls. Waterflow at high speed very nearly follows the buttock lines, so their shape has a great deal to say about how a hull will perform.

Buttock Curvature and Hull Type

As we saw in Chapter 3, a displacement hull's buttock lines sweep up aft to reduce the hull's wavemaking at slow speeds. The planing hull's buttock lines aft, on the other hand, must run parallel, or nearly so, to the waterline, in order to generate lift and allow waterflow to detach from the transom, to develop dynamic lift with appropriate running trim (bow rise), which is to say preventing the stern from squatting at speed. If the hull has too much bow rise, it either has to be corrected with trim tabs or interceptors, which add drag as well as lift, slowing the boat and burning more fuel. The best approach is to design the hull so it runs correctly in the first place, but not all builders have figured this out. Also note that excessive bow rise not only adds form drag (which shows up visually in the wake size as well as in fuel consumption) with a more deeply immersed transom, it also increases slamming in linear fashion.

The 30-Knot Hard-Over Turn Test

A true test of dynamic balance is the 30-knot hard-over turn. A well-designed boat tightens its turn, and slows due to the extra resistance, but the stern never slides out abruptly, even with the rudder hard over. It also heels commensurately with the turn rate so that centrifugal force is directed straight down through your feet, just like you see in an airplane or on a bicycle. A poorly designed boat "hooks"—the stern slides out, the bow trips, and the boat yaws violently. The difference lies mostly in hull geometry. With full bows with keels that come to a point, the center of frictional and lateral resistance shifts forward, increasing the couple between the rudders and the bow. On the other hand, a finer bow with a keel that’s radiused in cross section will have less hull surface in contact with the water, reducing frictional resistance forward, and the rounded keel is free to slide sideways without gripping and digging in. This reduces both bow steering hen running downsea and any hooking propensity in a hard turn. When you drive a balanced hull, corrections at the helm are minimal- the boat coms close to steering itself, and indeed will wander a few degrees wither side of your desired heading, constantly correcting itself with very littl3e work on the part of the helmsman.

If I were evaluating a boat and found it prone to hooking in such a test, or to bow steering downsea, I wouldn't buy it—no matter what else it offered. That single trait reveals a fundamental flaw in the hull's geometry, one that can't be corrected with power or tabs. Keep in mind that the downsea part is key to this discussion, since bow steering means undesired and unexpected yaw, and since rolling in something boats do a lot of downsea, a combined yaw and roll can easily and quickly lead to a broach and even capsize

Chines: Definition and Function

What Chines Do

The chine is the corner formed by the intersection of the hull's bottom and its sides. Planing hulls typically have "hard" chines, with true corners, although there are also round-bilge planing hulls. The gentle radius seen on displacement hulls and sailing boats is properly referred to as a round bilge, not a soft chine.

The hydrodynamic purpose of the chine is to provide waterflow separation from the bottom of the hull, which reduces frictional drag. Compared to a round bilge hull, a hard chine bottom has more effective lifting surface, which decreases dynamic bottom loading for improved efficiency. And, since the chine forward projects farther outboard than the corresponding section of a round-bilge hull, the hard chine flattens the buttock lines forward, which helps prevent dynamic instabilities. In a round-bilge hull, the intersection of hull side and bottom, seen in section, is a radiused curve. With no chine to separate the waterflow, when the round-bilge hull is running at high speed, water tends to flow outward around the turn of the bilge and up onto the hull sides, and negative pressures can result. If the hull is light enough and properly trimmed, this problem can be overcome and the hull will run perfectly well. Spray rails can be added forward to not only reduce spray but also improve waterflow separation along the hull sides. Water climbing up the hull side also adds frictional drag, slowing the boat and decreasing range.

Chine Flats and Modern Refinements

Chine flats, in which the deadrise flattens out or even reverses downward a few degrees in the outboard six inches or so of the hull bottom, are de rigueur in modern planing hulls. They are wider back aft where the added lift helps but doesn’t increase slamming like it would forward if too wide. They add buoyancy outboard, which increases form stability (measured as GM, or Metacentric Height), and they deflect water and spray at planing speeds, increasing dynamic lift and contributing to a drier ride. The original deep-Vs, like the Bertram 31, which did not have chine flats, could run like the wind when slicing through a choppy sea, but were notoriously tender at rest since the chines were clear of the water at rest. Chine flats also help control heel in a turn, since they create more lift – much like airplane flaps- as water flows laterally in a hard turn. When Volvo’s IPS pod drives came out in the 2010s, builders tried installing them in their existing hulls which were designed for inboard shafts. The problem was that the pod drives had a much lower center of lateral lift compared to rudders, and this created more couple or force x distance. This in turn caused the boats to heel excessively and Volvo had to cut back the rudder angle in high-speed turns to keep the boat from heeling dangerously. This caused the boats to turn like battleships since rudder angle is what determines turn rate (along with the hull shape). Adding down-angle, or reverse deadrise, in the chine flats helped once the builders figured this out, since this had the happy effect of increasing lift, like adding flap angle in an airplane wing. Also Volvo has added automatic ride control systems (RCS) which help control heel by adding lift on the inboard side of the turn. But too often appendages like tabs and interceptors are bandaid for a hull that was poorly designed in the first place.

While we’re talking about heel angle on a turn, it’s not just an academic talking point. A true turn, with the well design planing boat heeling the same degree as a plane or bicycle in a turn, is not only safer for its passengers, it’s also more seaworthy than a boat that turns too flat or that heels excessively since much of the latent energy in the righting arm curve is retained so the boat has greater reserve stability. A boat with a keel, on the other hand, is the worst possible boat to be in in a high-speed hard turn, since it will heel outboard in a turn, which is dangerous for passengers, especially when they are not warned in advance, and it also eats up a lot of the vessel’s righting energy, making it more susceptible to capsize.   

Double Chines and Other Illusions

Some boats have double chines, which are intended to increase wetted beam at rest (with the upper chine submerged) and, once up on plane, reduce wetted surface and drag (with the upper chine clear of the water). However this design is in reality usually ineffective since the hull will have the same overall wetted surface unless the upper chine is completely clear of the water, which just doesn’t happen unless the boat is very light and very fast. It’s also true that bottom loading actually goes up, not down, if the upper chine is clear of the water. 

Where the Chine Enters the Water

Walking down the dock at a marine, you can tell a lot about how well a boat will run offshore by the chine elevation, or height above the waterline. If the chine go underwater- the chine immersion point- close to the bow, say at station 2 or 3, the boat will almost certainly pound and be wet- the two go together- at relatively slow speeds. On the other hand, if they enter the water farther aft, say at station 5, the boat has a lot more deadrise where it’s needed- in the center of the hull. That’s because the higher the chine, the more deadrise there is at the that station.

Other Key Geometric Elements

Forefoot and Entry Design

A hull's entry, or forefoot, plays a crucial role in determining both ride quality and course-keeping ability, which is the vessel's natural tendency to stay on course at all speeds and in all directions to the sea. Regulator, Buddy Davis, and Blackfin sportfishing boats have deep, fairly fine entries that essentially eliminate pounding in a head sea. It is possible for a forefoot to be too deep and fine, resulting in a boat that bow-steers in a following sea. A destroyer like (long and narrow) bow with low displacement forward may develop too little buoyancy initially when immersed below its static waterline, such as when the stern is raised by a quartering sea. Especially when the stern is wide and fairly flat, this imbalance results in a boat that is difficult to keep on course when running downsea; the bow digs in, and the broad, buoyant stern gets thrown around.

A fat and flat bow entry will be far more susceptible to pounding in a head sea since it meets each wave almost instantaneously rather than more gradually as with a deeper-deadrise hull, and this impact can be measured in milliseconds. A balanced hull has a moderately fine and deep entry to deliver a smoother ride in a head sea, and enough deadrise aft to act as a keel for directional stability while also creating adequate lift for efficient low speed planing. As in all human endeavors, moderation is key, including in hull form- including deadrise distribution, spray strake size and placement,

Boats powered by waterjets require special care in the shape of the entry: with no underwater gear aft to keep them going in a straight line, these hulls are inherently susceptible to unreliable tracking downsea. A waterjet-propelled hull must have fairly shallow sections forward, with the hull radiused at the keel to help keep the bow from digging in downsea.

Since a hull is a three-dimensional object, the angle of entry – the shape of the bow when viewed from above - is also evidence of how well a boat will run offshore. A fine, sharp entry means the hull meets each wave more incrementally rather than all at once, creating a smoother ride. A sharper entry also sends spray out and aft which make it a drier ride since the boat runs past the spray. A fatter entry not only pounds, but it make for a wet boat since the spray is sent forward and up, giving the wind plenty of opportunity to throw the spray up over the boat.

Flare

Flare is the concave curvature in a hull in section view: a hull with flare curves inward between the chine and sheer. Flare makes a boat look better, of course, but more importantly it adds buoyancy as the bow submerges; no amount of flare, though, can make seaworthy a hull that is too fine forward at the waterline to start with, or that has its LCG too far forward. And relying on flare alone to provide a dry ride is also an exercise in futility; the geometry of the bottom and the placement and shape of the bottom strakes and chines have a greater influence on whether a hull will deliver a wet or dry ride. Exaggerated flare sometimes hints at a boat's heritage, such as that of a North Carolina-built sportfisherman, but this too can be taken to extremes and can actually inhibit seakeeping, especially when running downsea.

Section Shape

The shape of the hull bottom in cross section is also important to ride quality and dryness. Underwater hull sections can be convex in cross section, making for a ride that is smooth yet wet. Concave hull sections produce a dry ride but are hard-riding in a seaway, since water gets trapped in the hollow sections, especially when the hull is heeled over. Straight sections are often a good compromise, offering a reasonable combination of smoothness and dryness. Some sophisticated hulls are bell shaped in cross section, with convex curvature near the keel (to soften wave impact) and concavity near the chines (to deflect spray), mixing the best of both worlds for a smooth and dry ride.

Hook and Rocker

Hook and rocker describe the curvature, in profile (viewed from abeam), in the aft 10 to 25 percent or so of the boat's buttock lines. Hook is a concave curvature; rocker is convex. Adding hook or rocker is a good way to control trim. Hook makes a boat run flatter by shifting the center of dynamic lift (CDL) aft, thus raising the stern slightly. Rocker makes the hull run lower at the stern, trimming up the bow and shifting the CDL forward. A designer might add a little hook to compensate for a large amount of weight in the stern, but you have to be careful here since the faster the boat goes, the more lift is created and the more the bow comes down. Both hook and rocker can help correct dynamic instabilities, but rocker can also be problematic since the bow comes up more as speed increases, adding resistance and slamming.

One way to get trim where you want it in varying sea conditions – you generally want bow up downsea, bow down upsea- is to shape the aft sections so the boat runs a little bow high, then use tabs or interceptors to raise the stern when wanted. You can get a similar result with a slightly aft LCG, so the boat sits and runs a bit bow high. In fact, hook in the buttocks aft acts like a big set of trim tabs by shifting the CDL aft. Hooked buttocks minimize drag at low speed, straight buttocks minimize it at intermediate speeds, and slightly convex buttocks (a little rocker) produce the least drag at high speeds since the bow comes out of the water more, reducing frictional resistance and bow steering. That is, if it’s down right1

Length-to-Beam Ratio (L/B)

This is the relationship between the boat's length and beam, usually at the waterline. Along with deadrise, displacement, and longitudinal center of gravity (LCG), the L/B ratio has a profound effect on ride quality in a seaway. A longer, narrower boat will have more comfortable motions and lower accelerations in rough water, and be more efficiently propelled, than a shorter, wider boat of the same size and displacement. The long and narrow vessel may not have a lot of room for its length, but it has ample room for its displacement. A short, wide hull may have more room for the LOA, but it will burn more fuel than its longer, narrower cousin and will have to slow down more and sooner when the going gets rough. The long-and-narrow advantage applies both to planing hulls and to displacement hulls. Unfortunately, market forces have driven the demand for shorter-wider boats, which is too bad since most people have no earthly idea how smoothly and efficiently a same-size, but longer-narrower, boat can run.

The longer a hull, and the greater the length-to-beam ratio, the flatter it tends to run. This makes it easier to see over the bow of some 60-foot express cruisers than many 35-footers which tend to have much more beam for the length.

Speed-to-Length Ratio (S/L)

The S/L ratio is the ratio of a hull's speed in knots to the square root of its waterline length. A boat that is 49 feet at the waterline makes 7 knots at a S/L of 1, or 21 knots at S/L 3. A hull’s maximum S/L is also used to define which category a hull belongs to. In general terms, with sufficient power, a displacement hull can reach S/L 1.34 and a semidisplacement hull S/L 2.5, while a planing hull theoretically has unlimited speed potential. The displacement hull’s maximum speed in turn is determined by gravity, which governs how fast an open-ocean wave of a given length travels. A 100 ft waterline-length hull can only travel as fast as a wave of the same length since it cannot climb over its bow wave.

V. Hull Types and Variations

Hard-Chine Planing Hulls

Flat-Bottom Hulls

Flat-bottom planing hulls are generally found on skiffs and other small craft that ply calm bays and lakes. Able to carry a lot of weight, the easiest to push through the water, and the most stable at small angles of heel, flat-bottom boats also produce a kidney-jarring ride in a chop.

Deep-V Hulls

The deep-V hull is the most common planing hull out there today. It is seen in its most basic form in the Bertram 31, the Ray Hunt hull that started it all. A deep-V typically has 20 degrees or more of deadrise at the transom, with 24 degrees a common standard on very high speed hulls. The deadrise angle on a deep-V is constant from the stern forward for over half of the boat's length, and then starts increasing from slightly forward of amidships, rising to the stem, where the chines start to rise out of the water and narrow up.

The great advantage of the deep-V is the smoothness of the ride in a rough chop at speed. A deep-V can keep on charging when a more flat-bottom boat has to slow to a crawl. The deep-V's high angle of deadrise, carried all the way back to the transom, allows a boat to slice smoothly through the waves. A deep-V can maintain speed without pounding in sea conditions that force boats with less deadrise to back off. On the flip side, excessive deadrise will add resistance unnecessarily, since a flatter bottom is a more efficient lifting surface. The best way to get some of that speed back is to reduce weight, and some builders have done a great job of doing so, without sacrificing strength, through the use of composite sandwich construction (see chapter 6). Deep-Vs can also gain speed through refinements that add lift, like reverse chines and spray strakes. But for two otherwise identical boats, the one with less deadrise will go faster in calm water since a flatter bottom is the more efficient lifting surface.

For high-performance craft that cruise fast enough to emerge most of the length of their bottoms on a regular basis, a deep-V bottom is the way to go. These very fast (50-knots-plus) hulls often become airborne in rough water, and when they land on the aft 30 to 40 percent of their bottoms, they need every bit of that 24-degree deadrise back there to smooth out the landings. The extra deadrise aft also helps a deep-V to track better downsea, since the hull shape lends directional stability since the hull shape creates more resistance to lateral motion aft. A hull shaped like a shoe box aft is just as happy going sideways as straight ahead, and that make it hard to keep on course. Well-known, superior deep-V hull designs include the original Bertrams, the Regulator- and Contender-type center consoles, and Formula's cruising and race boats.

Modified-V Hulls

Modified-V planing hulls typically have between 12 and 18 degrees of deadrise at the transom and 20 to 25 degrees amidships. This design works well with larger, heavier planing hulls that need all the dynamic lift they can get back aft and rarely come more than half their length out of the water at high speed. The flatter sections usually extend farther forward, as well, than in a deep-V hull. In a monohedron hull, deadrise is constant in the aft half of the hull, with the keel and chines running essentially parallel to the waterline. This can also create a tendency to run bow-high compared to a hull with warp, or twist, since the center of dynamic lift is further forward. Since buoyancy in a flatter hull is centered higher in the hull, the modified-V hull has greater initial stability than a deep-V, which adds form stability, helpful on boats carrying a lot of weight topside. Unless a boat rides with half its length out of the water at cruising speeds, cruising in the low to mid 20-knot range, a modified-V can be a good choice.

Warped-V Hulls

The warped-V is so named because deadrise changes along the length of the vessel, decreasing all the way to the transom. While the keel remains very nearly parallel to the waterline, the chine continues to run downhill as it approaches the stern, resulting in a warp, or twist, to the bottom. A planing hull with a warped-V bottom will often run at a flatter trim than a constant-deadrise deep-V, since the CDL is farther aft, with more lift developed by the stern's flatter sections, thanks to the bottom's slightly twisting sections. And because the warped-V hull's chines are deeper in the water at the transom, LCB shifts aft as well, making this hull form an excellent candidate for an aft engine room with V drives. The warped bottom also produces an excellent lower-unit-powered boat, since the added buoyancy in the stern can easily accommodate the weight of a stern drive or a pair of big outboards cantilevered off the stern on a transom bracket.

Warped-V hulls have gotten a bad rap because excessive warp, such as in some of the WWII PT boat designs, produces a poor hull design with low dynamic pressures aft in the hull on plane. Designers have since learned how to get it right. With their superb ride quality and excellent propulsion efficiency, Eastbay, Alden, Palmer Johnson, Grady-White, and some of the latest Chris Craft retro-yachts are examples of the warped-V hull form done right. Trim tabs, incidentally, are needed less with these hulls, since they climb on plane and run at a low trim naturally.

Stepped Hulls

Short, wide (high-aspect) surfaces are more efficient than long, narrow (low-aspect) ones in terms of frictional drag on water. Lift generation is just far more efficient with a large beam-to-length ratio surface. So, the idea behind a stepped bottom is to reduce wetted surface by allowing the hull to plane on two or three high-aspect planing surfaces rather than one large, low-aspect surface. And the popular notion that any added speed from a stepped bottom is due to a layer of bubbles blanketing the hull bottom is true to a degree but generally exaggerated. Entrained air bubbles undoubtedly reduce frictional drag to some extent, but the real saving is in minimizing the hull area in contact with the water, specifically by presenting two or three wide and short surfaces to the water instead of one long, narrow one.

Like any true design advance, though, the technology can be misapplied. While stepped bottoms work admirably on high-performance boats, they do little or nothing to improve performance on slower boats. In general, data indicate that if a boat can't cruise easily at close to 30 knots or more, it can't go fast enough to ride up on hull steps, so steps would only add drag. More specifically, this means that a gas-powered family cruiser with steps should be able to cruise fully loaded at 30 knots, not just reach this speed at full throttle. Otherwise, the extra cost of tooling and the added time and cost spent laying up a stepped hull is wasted, and the stepped bottom is just a marketing gimmick. Some runabout builders even carve out a little scoop at the chine amidships, which I suppose is meant to suggest that the bottom is stepped, when in fact the bottom is as straight as an arrow.

Assuming that it has enough power to cruise in the 30-knot range, what's the downside to a stepped hull? Well, since stepped bottoms create localized areas of high pressure (dynamic lift) at the steps and at the transom, these boats tend to make up their own minds about trim angles. Fore-and-aft stability is significantly increased compared to a nonstepped hull, and stepped bottoms tend to follow wave contours more closely. Even in an outboard- or stern-drive-powered stepped-hull boat, at least at higher speeds, you can forget about raising the bow by trimming the lower units up in a following sea.

The area behind each step has to be adequately ventilated, and the airflow must not be cut off by waves, turns, or rolling. If airflow is lost, resistance is immediately created that can sharply reduce speed or, if airflow is cut off to one side of the hull only, the vessel may turn suddenly and unexpectedly. Builders often provide large inlets to the areas behind the steps, and a few even provide air paths through ducts that lead to the trailing vertical edge of the steps.

Round-Bilge and Hybrid Hulls

Though they are the most efficient at high speeds, not all planing hulls have hard chines. Some have round bilges (radiused chines), which have their own advantages, such as producing an easier, more comfortable motion in a seaway, and reducing resistance at slower speeds. Or a hull may have hard chines aft (to increase lift) and round bilges forward (to improve ride quality). A hard-chine planing hull creates more wavemaking resistance at displacement speeds and requires more power for a given displacement speed than a round-bilge craft. The round-bilge hull will tend to require less power to achieve a given speed up to S/L 2, whereas a hard-chine vessel will require less horsepower, and be more stable dynamically, above S/L 2.5. If the priority is low-speed efficiency and range, the round-bilge hull is the best choice. The larger waves created at slow speeds make the hard-chine planing hull a poor choice for operating extensively in no-wake zones such as canals and other protected waters. This is especially true of deep-V hulls, which leave a large wake and plane at higher speeds than flat-bottom boats.

There's no significant difference between round-bilge and hard-chine hulls of similar dimensions and displacement in pitch and heave at semidisplacement speeds. However, round-bilge vessels do have more comfortable motions at displacement speeds, and planing hulls often have superior ride quality at planing speeds. The hard-chine vessel rises significantly out of the water due to dynamic pressures, increasing the hull's effective freeboard, improving visibility from the helm station, and helping to keep spray and solid water off the deck.

Compared to the round-bilge hull, the hard-chine vessel at planing speed will often have better directional stability or course-keeping, especially with seas abaft the beam, and will be less prone to broach when operating near wave speed. The hard-chine craft will also have a stiffer, shallower roll and will tend to ship less water on deck in rough water. A hard-chine vessel running downsea at displacement speed may yaw more than a displacement hull, due to its flat buttocks lines aft, wide transom, and buoyant stern. It will also tend to squat less when coming up on plane, and once planing. And because of its superior course-keeping, a hard-chine, deep-V hull is generally better suited to waterjet propulsion than a round-bilge hull, in part because jets do not do well at semidisplacement speeds.

A round-bilge hull would need a full keel for acceptable directional stability, which has the downside of making the hull heel outboard in a turn, and possibly bilge keels or active stabilizers to reduce roll, all of which add drag to the hull. Naturally, there's a relation among course-keeping, yaw, and roll, since a single degree of yaw in a round-bilge hull can develop up to five degrees of roll. In fact, reducing roll angle in any vessel does wonders to improve course-keeping. And with a round-bilge hull's greater buttock lines curvature forward, bow diving becomes more likely.

Hybrid Configurations

One type of hybrid hull starts with a round bilge forward, the radius of which decreases gradually until a hard chine appears near the stern. The hard chine adds buoyancy and dynamic lift aft and reduces squatting at low planing speeds, helping to turn a semidisplacement hull into a planing hull. The hard chine aft also contributes to directional stability, improving course-keeping at higher speeds. Another hybrid design puts it the other way around, with the round bilge aft and the hard chine forward, a combination thought to improve course-keeping as well as transverse dynamic stability at semidisplacement speeds.

Other hard-chine design advantages include greater internal volume than a round-bilge craft of the same dimensions, improving load-carrying capacity and habitability. And since the hard-chine hull has greater form stability, it can carry more weight up high with greater impunity.

Large Planing Hulls: Scaling Challenges

Large planing hulls, over about 75 feet, tend to run too flat in trim at high speeds, increasing frictional resistance and diminishing the vessel's handling. An easy way for the designer to drop the stern (and raise the bow relative to it) and improve matters is to put a little rocker in the buttocks aft. On many hulls, trim can be varied within a range of some five degrees in this manner.

VI. Performance and Dynamics

Understanding Resistance

We've already touched on the types of resistance affecting planing hulls, but it's worth examining them in more detail since they determine how much power is needed to achieve a given speed.

Where the boundary layer contacts the hull, it moves along as fast as the boat, but it slows gradually until at its outer edge the surrounding water remains undisturbed. Frictional resistance is largely a function of the immersed hull's surface area and roughness along with the size and shape of its appendages. The most efficient trim angle is, up to a point, the one that minimizes wetted surface, and therefore frictional drag.

Weight, Bottom Loading, and Efficiency

Why Weight Distribution Matters as Much as Weight

Weight per square foot of bottom area—bottom loading—is the single most important measure of planing efficiency. The heavier a boat for its bottom area, the higher its pressure loading and the greater the energy required to maintain speed.

For example, a 10,000-lb boat with 250 square feet of wetted bottom has a bottom loading of 40 lb/ft². If a similar boat has 300 square feet of bottom area, bottom loading drops to 33 lb/ft²—a major difference in how easily it planes and how efficiently it runs. Custom builders sometimes lengthen the waterline just to keep bottom loading under control as equipment accumulates—granite countertops, heavy joinery, or an icemaker every few feet. Nothing affects speed and efficiency more directly than weight. Add weight and the boat becomes slower, wetter, and more sluggish in response. High bottom loading is the hidden inefficiency behind many overbuilt modern cruisers. Builders pile on amenities—generators, towers, elaborate interiors—without realizing that every extra pound exacts a performance penalty. Some production boats now struggle to exceed 0.4 nautical miles per gallon at cruise, even with enormous horsepower.

By contrast, a lighter, narrower boat of equal length—say, 28,000 pounds with 320 square feet of wetted bottom—will run faster, flatter, and softer with less power and fuel. Its lower bottom loading reduces wave impact and wake size alike. The lighter boat can also remain on plane at lower speeds, which is invaluable when running home in rough weather. Instead of wallowing at 15 knots, it can stay comfortably on plane at 12-13—a small difference that, in heavy weather, feels enormous.

Weight, Displacement, and Reality Offshore

A well-balanced hull at cruise should look visually level—neither plowing nor straining. If you see a boat at 25 knots running with its bow high and wake boiling off the transom corners, it's telling you it's out of balance. The hull's geometry and weight distribution are working against each other.

The bow sections of a planing hull must be full and buoyant enough so that when the stern is lifted by a following sea, the bow is not excessively immersed. If the bow roots, or digs too far into a following sea, yawing (or in extreme conditions, broaching) is introduced, with the bow turning into what is effectively a fixed rudder forward. The stern can't be so wide and buoyant that the bow tends to stuff when running downsea, and the bow shouldn't be so full (wide at the waterline forward) that it makes the boat pound upsea in a stiff chop.

Performance Realities: Speed, Weight, and Power

Speed versus Deadrise

If two boats of similar dimensions weigh about the same and have similar drivetrains, and one is still significantly faster than the other, the reason is likely to be found in the deadrise. I'll take the deeper-deadrise, slower boat over the flatter, faster boat, though, since the first will deliver solid all-weather comfort and performance while the second will be a miserable ride in a chop and limit when and where you can go.

Speed versus Weight

Since adding weight and deadrise reduces speed, all else being equal, a boat's speed can be increased by keeping the weight down. But two boats of similar size can vary in weight for a variety of reasons; the heavier boat isn't necessarily stronger or better built, but it may be. If Brand A's 36- by 12-foot express cruiser weighs significantly more than Brand B's, ask the builder to spell out specifically why that is. Is it because Brand A is heavily built with fiberglass-encapsulated plywood stringers and bulkheads, a solid glass hull, and plywood-cored decks, whereas Brand B benefits from advanced composite engineering with foam-cored stringers and bulkheads and a resin-infused, post-cured, cored epoxy hull and deck laminate? Or are the two boats built pretty much the same, except for the thinner laminates and smaller scantlings in the hull grid system of the lighter one?

Update Those Old Hull Designs!

The problem with some boats on the market today is that their hull designs originated thirty or more years ago. In the late 1960s, a 42-foot, low-deadrise sportfisherman might be fitted with a pair of 300 hp, 2,800-pound GM 671 diesels and would run along nicely at 16 to 20 knots. Nowadays you can fit 610 hp MAN diesels in about the same space as the old 671s, and suddenly a decent 16- to 20-knot fishing boat is transformed into a hard-riding, flat-bottom 32-knot race boat.

Some of the venerable New Jersey-built sportfishermen, like the Post 42—a well-engineered and beautifully finished boat otherwise—and the Jersey 40, are prime examples. They're fast for their size and power precisely because of their relatively flat bottoms. But with their low-deadrise bottom sections forward, they pound noticeably in a stiff chop; there's no way to get around the laws of physics! With low or no deadrise aft, boats with similar hull shapes also tend to wander about their course with seas abaft the beam unless trimmed bow-up (which is possible only with plenty of fuel in the under-cockpit fuel tank and trim tabs raised). If you value speed in calm water above all else, these boats may be right for you. But don't expect to comfortably keep up with the rest of the fleet when the wind picks up.

The bottom line is that if a manufacturer claims that their boat is the fastest around with a given power plant, you should pull the string and find out why. If the boat isn't a whole lot lighter, it's probably because it has a flatter bottom than everyone else. You should care deeply about this if you plan on heading out past the jetties when the wind's blowing. Adding horsepower to an older hull can also introduce dynamic instabilities if LCG isn't carefully managed during the repower.

Propulsion and Efficiency

Diesel engines are more efficient than gasoline engines, both because of their higher thermodynamic efficiency and because diesel fuel contains more energy per gallon. As a rule of thumb, a gasoline engine produces about 11 horsepower per gallon per hour of fuel flow, while a diesel produces about 20. A 200-hp, 6000-rpm outboard will therefore burn around 18 gallons per hour at full throttle (roughly half that at 4500 rpm). This is a good reality check for marina "fuel economy" claims—anyone saying their 40-footer gets 10 miles per gallon at cruise is either mistaken or creative.

Trim: Angle, Attitude, and Control

Static and Dynamic Trim

Trim is the fore-and-aft inclination of the vessel, measured in degrees, and is often referred to as bow rise when on plane. A planing boat has a static trim (at the dock) and a dynamic trim (when on plane). When a boat runs with its bow raised 3 degrees higher than when floating (on its design waterline) at the dock, it is said to be running at 3 degrees of trim. For a given hull shape, static trim is determined by LCG. Dynamic trim is determined predominately by LCG, and also by CDL, with the latter in turn influenced by the direction and location of propeller thrust, and the shape of the hull, in particular its buttock lines. LCG is a moving target, since it changes as fuel is consumed (on most boats) and as people and gear move around.

Optimal Trim Ranges

Trim directly affects ride quality and propulsion efficiency. Planing hulls tend to run best in a trim range of 2 to 5 degrees (on warped-V and constant-deadrise hulls respectively), depending on bottom type. But most efficient doesn't mean most comfortable; a bow-up attitude is more efficient than bow-down, but the latter delivers a smoother ride. The trick is to find the sweet spot, which will depend on how rough it is, what direction you're running in relation to the seas, and how fast you're going, that delivers a reasonable combination of efficiency and ride quality without causing instabilities and poor handling.

Effects of Incorrect Trim

Excessive bow-high trim increases slamming loads (vertical accelerations), increases the tendency to porpoise, increases fuel consumption, and interferes with visibility from the lower helm station. Bow-down trim increases wetted surface and frictional drag, slowing the boat, although it produces a smoother ride in a chop. Bow-down trim also creates a tendency to bow-steer (a deeply immersed bow reduces the rudder's ability to control heading), so things can get out of control quickly in a following sea, and you'll be in for a wetter ride with spray developing farther forward. Running with a slightly bow-high attitude reduces drag by reducing wetted surface. Running with a degree or two less trim slows the boat a whisker, but reduces vertical accelerations in a chop.

VII. Seaworthiness in Practice

Ride Quality and Human Factors

Understanding Vertical Accelerations

Ride quality refers to the comfort of a boat's ride measured in vertical accelerations, or slamming loads. As a hull encounters waves at high speed, impacts of varying degrees occur repeatedly. These impacts are measured as accelerations using the force of gravity as a standard. A boat high and dry in a parking lot subjects its cradle (and its occupants) to a G-force of one. But if the same boat is slamming into a wave with sufficient impact to produce a G-force of 1.5, a 200-pound person standing in the bow of the boat will momentarily feel as if he weighs 300 pounds. Standing near the stern of the same boat, since the boat effectively moves vertically about a fulcrum just aft of amidships when pounding through small waves, the same man would experience less G-force and consider the ride more comfortable. Thus, where you happen to be standing in a high-speed boat is important to your morale. Remember if you are sitting farther aft than your passengers that they are being subjected to higher accelerations than you are, especially in a bowrider.

Lateral Accelerations and Passenger Safety

Beyond vertical accelerations from pounding, lateral accelerations—side-to-side forces during turns or quartering seas—pose a serious risk to passengers and crew. U.S. Navy and Coast Guard studies show that many onboard injuries stem not from vertical impact but from lateral neck and spine stress.

Hull design, then, isn't just about speed—it's about human survivability. The best hulls reduce both structural and physiological loads. When a boat moves through the sea smoothly, its crew fatigue less, make better decisions, and arrive safely.

The Comfort Equation

Comfort, in this sense, isn't luxury—it's endurance. The difference between arriving after three hours of relaxed running and arriving battered and exhausted is the measure of real-world performance. A hull that maintains speed in a seaway with minimal impact loads and low fatigue is, by definition, a safer and more seaworthy hull.

If a boat's ride is so jarring, its motions so violent, or its controllability so poor that speed is limited not by engine power but by human tolerance—or by unpredictable coursekeeping—it is not as seaworthy as it should be, no matter how strong its structure or high its freeboard. In this sense, crew comfort drives seaworthiness. A less-fatigued, more alert crew is inherently safer and more capable of making sound decisions after hours or even days at sea.

Speed Capability in a Building Seaway

Consider two boats returning from an offshore tournament, 80 miles out, as weather builds from 3- to 8-foot seas. The better-running hull, able to maintain 30 knots safely, arrives one to two hours earlier than a similar-length boat forced to throttle back to 15 knots. The first gets home while seas are still 6 to 8 feet; the second faces 10 to 12s. That difference can mean the line between a comfortable return to port and a dangerous one. In this way, a faster hull offshore is also more seaworthy.

Course-Keeping and Handling

High-Speed Directional Stability

Course-keeping and boat handling have to do with the ability to control a boat at both high and low speeds. High-speed course-keeping is largely a function of hull form, which determines whether or not a boat will naturally tend to track in a straight line with minimal helm input. Here again, the deep-V has a distinct advantage over a boat with flatter sections aft, since a flat bottom tends to wander more about the desired heading, especially with seas abaft the beam. A full keel on a flat-bottom boat helps, but the added wetted surface increases drag and slows you down, and dynamic instability can occur at high cruising speeds. That's why a deep-V is so much better suited to waterjet propulsion (which has less directional stability than other drives) than a flat-bottom boat.

The Role of LCG and VCG

The LCG must not be too far forward, nor the VCG too high, to permit good seakeeping. In fact, proper placement of the LCG is essential to good performance, and this includes handling at speed and course-keeping. Put it too far forward and the boat rides bow down, making it directionally unstable and wet to boot. Too far aft, and the boat takes excessive power to propel through the water, decreasing range and wasting fuel, to say nothing of the bad effect on visibility from the lower helm station. A VCG that is too high will make the boat yaw excessively in a following sea, too.

The best test for course-keeping is a quartering sea some 20 to 40 degrees off the stern. If a boat tracks well in such a sea, then hats off to the designer and builder for getting weight distribution right. Boats tend to handle better running at a greater trim angle (bow up) in a following or quartering sea, while lowering the bow by depressing trim tabs makes for a smoother if wetter ride in a head sea.

Throttle and Helm Response

Good handling characteristics presume rapid response to throttle and helm inputs and a hull that is not so directionally stable that it's hard to turn. For throttles to respond well, horsepower alone is only the beginning. Just as important, or more so, is how the power is delivered to the water. Larger reduction ratios with larger, slower turning propellers respond better to low-speed clutch and throttle commands than smaller gear ratios with smaller, faster turning props. The volume and velocity of water being moved by the propeller is the key to responsiveness. Just try docking a typical 40-foot express cruiser with its shallow (1.64:1) gear ratio and too-small (22-inch) props; the boat hardly moves when you put an engine in gear at idle. While docking such a boat in Florida a few years ago, I backed the starboard engine only to have the bow fall off to port until I applied a strong burst of power. The same boat and engines with deeper gears and larger props would undoubtedly handle much more responsively and would likely reach the same cruising speed, though top speed (a mostly irrelevant and academic figure) might fall off slightly.

Dynamic Stability and the True Turn

The "True Turn" Defined

In a well-designed planing hull, a turn at speed should feel as natural – and hard to detect- as a plane banking in a turn. When a boat heels inward just like a bicycle, centrifugal force presses you into the deck rather than sliding you across it, and the heel is proportional to the speed and turn rate. You feel secure, grounded, and in control. The heel angle in turn is determined by the VCG, the shape of the hull below the waterline, and the center of effort of the propellers and rudders. Keep in mind that a boat with a keel will turn much flatter and can even heel outboard in a turn which as we discuss elsewhere robs righting arm and stability and makes the boat unsafe for its crew. 

Hull Geometry Protects People and Structure

A good running hull is not only kinder to its passengers but also to its own hull and deck structure and to the electronic instruments and other mechanical and electrical equipment onboard

Since seaworthiness is in part dependent on how fast a hull can go in a seaway, we’ll also mention here that seaworthiness depends on many interconnected factors: hull design (bottom shape, freeboard, and bow geometry above the waterline); structural strength and integrity; propulsion system performance (including throttle response and the "traction" needed to climb the face of waves and escape breaking crests astern); and steering control at both low and high speeds. Finally, seaworthiness also depends on the vessel operator's experience and judgment. Together, these elements define how seaworthy a vessel truly is when conditions turn extreme.

VIII. Design Refinements

The Refined Bottom

Besides waterline L/B ratio, the secret to efficiency and ride quality lies in refinements to the hull shape, including deadrise distribution from bow to stern; hull shape in station view (whether concave, straight, convex, or bell-shaped); the precise shape and size of the chine flats; the shape, size, and location of bottom strakes or spray rails; and the hull trim resulting from LCG and bottom hook or rocker. A padded keel, also known as a ski or delta pad, is a flattened keel section that adds lift, reduces hooking (spinning out) in a high-speed turn, and shifts buoyancy from the keel area to the chines, and can marginally increase stability. Chine flats (which improve form stability and add dynamic lift) can also improve a planing hull design. Expect most modern planing hulls, whether modified-Vs, warped-Vs, or deep-Vs, to sport a combination of these refinements.

Hull shape and refinements influence ride quality, handling characteristics, and optimum speed, and so do underwater appendages such as shafts, struts, strikes, fins, keels, and rudders. These appendages should be faired to minimize drag and to provide a smooth flow of water to the propellers and rudders.

Strakes and Spray Rails

Bottom Strakes

Bottom or running strakes add some dynamic lift and deflect spray, but another important function is to define the boundary of the hull's wetted surface when running on plane. The lift generated by bottom strakes can be significant forward, where water and spray direction is at an outward angle from the hull's centerline. Farther aft, waterflow lines up with the hulls' keel, or centerline, so dynamic lift from strakes is minimal or nonexistent, though they may still contribute to a small degree to tracking and roll attenuation.

Bottom strakes, like those we typically see on a deep-V hull, look great and, if properly designed, can improve performance. It seems intuitively obvious that a soft radius in the inner corner of a bottom strake deflects spray most efficiently and effectively. Water tends to "crunch" up against strakes with hard inside corners, increasing slamming loads. It's also important that the edge away from the keel comes to a sharp point so waterflow breaks cleanly away, minimizing drag. Bottom strakes have to be carefully positioned, or they can create channels for air to flow to the propellers, struts, rudders, through-hull connections, waterjet inlets, and transducers farther aft.

Spray Rails

These are larger strakes used on round bilge hulls to act as hard chines. These spray rails are something like oversized bottom strakes, and a single pair is usually fitted starting well above the waterline at the stem and gradually sloping down as you move aft. They may terminate just forward of amidships or continue to the transom. Overly large spray rails may increase slamming loads, and when improperly shaped and located on the hull may generate spray as well as deflect it. But all in all, they're indispensable when it comes to controlling spray forward, they generate lift forward, and they can encourage waterflow separation at the bilge aft.

Trim Tabs and Interceptors

Function and Application

Trim tabs are small flaps, usually made of thin stainless-steel plate, mounted to port and starboard on the transom just above the bottom of the boat. They're controlled from the helm station via a hydraulic piston that projects through the bottom of the boat or from the transom. In the Up position, they are on the same plane and at the same level as the hull bottom. When lowered, they generate lift by increasing their angle of attack. The effect of this lift, then, is to raise the stern and lower the bow.

Interceptors are flat plates that drop down vertically at the transom, interrupting waterflow at the hull bottom, counterintuitively perhaps, this interruption crates lift, and in fact does so more efficiently than trim tabs do once running fast enough. In other words, they have a higher lift-to-drag ratio. While tabs create more lift at semidisplacement speeds, say 10 to 15 knots, interceptors come into their own over 20 knots and also can respond extremely quickly to input and for this reason are used in automatic ride control systems (RCSs).

Planing hulls can use trim tabs or interceptors to advantage in order to run at optimum trim through any likely combination of speed, loading, and sea state. They can be used to correct for heel (caused by dynamic forces including a beam wind, waves or a turn ) or list (caused by a weight shift creating weight distribution), to decrease time to plane, to help stay on plane at lower speeds, or to depress the bow in a head sea to move the wave impact forward and reduce slamming loads. Trim tabs will also increase a boat's speed range at low planing speeds, allowing to get and stay on plane a lower speeds. Note that at 10 to 18 or so knots, tabs work better than interceptors at creating lift, but interceptors come into their own, generally around 18 or 20 knots, and have a better lift-to-drag ratio.  

Depressing the tabs might allow a planing vessel to slow down a couple more knots without falling off plane, a useful attribute in rough water since a hull on top (planing) at 13 knots is much more efficient than one that's fallen off plane. Tabs, then, can be used to "finesse" a boat's trim, meaning the skipper can tweak the angle of the hull so it operates at its most fuel-efficient, best-handling, smoothest-riding attitude.

When Tabs Indicate Design Problems

Some boats need very large tabs to compensate for a hull that runs bow high due to a flawed hull design. They a lot of lift aft not only to get on plane at lower speeds, but to correct for inappropriate LCG (too stern-heavy) once on plane. With these boats, trim tabs are used to help compensate for poor design. Though there are exceptions, if a boat needs tabs just to run well when normally loaded in calm water, or to get on plane without aiming for the clouds, something was amiss in the design phase.

Using Trim Tabs and Interceptors

Tabs can be lowered and raised together or independently. When both are lowered, the stern comes up and the bow drops, shifting the CDL aft. Lowering the bow smooths the ride in a head sea by immersing the sharper bow sections and shifting wave impact forward, and by reducing the angle of wave impact incidence with the bottom. Wetted surface and drag are also increased when depressing the bow, slowing the boat and increasing the amount of spray generated forward. In a following sea, a depressed bow will often lead to bow steering and degraded directional control.

When only one tab is lowered, that side of the stern is raised and the opposite bow is depressed. Using a single tab is a good way to correct for a small list caused by uneven weight distribution (say, with a 1,000-pound tuna to port), single-engine propeller torque, or a strong beam wind. Interestingly, a boat running on plane often tends to heel into the wind rather than away from it. That's because a small amount of rudder is needed to counteract the wind and keep the boat going in a straight line, and this steering effort induces a lever-arm that tends to heel the boat. So, depressing the starboard trim tab in a strong starboard beam wind will often return the boat to an even keel, using one trim tab to counteract that right-rudder lever arm.

As discussed elsewhere, getting the LCG right during the design of a boat is crucial to proper performance and handling. If a boat runs excessively bow-high during all conditions of loading, the LCG is simply too far aft. Such a boat needs trim tabs at all times to achieve its best running trim, and having to rely on trim tabs can be a Band-Aid approach for ill-conceived boats. On the other hand, a few, usually well-designed planing boats don't even come with trim tabs, running naturally at their 2- to 5-degree trim angle, but you give up the ability to offset list or depress the bow in a stiff chop. Length helps to minimize bow rise when accelerating up on plane, so a 70-footer will aim for the sky less than the 30-footer, all else being equal.

And tabs can help a boat get up on plane in less time while burning less fuel and with better visibility from a lower helm station, thanks to less bow rise. Certain dynamic instabilities can sometimes be corrected with trim tabs. For instance, porpoising can usually be controlled by dropping the tabs a bit, shifting the CDL aft.

Proper Sizing and Angle Indicators

It can be aggravating to run a boat that needs trim tabs to operate acceptably, only to find that the tabs are too small or that there are no trim tab angle indicators. The tabs must be large enough to actually lift the stern at cruising speed. I tested a Chris Craft 300 (a nice-running and handling boat otherwise) fitted with tabs that managed only to slowly turn the boat when depressed individually. There was no noticeable difference in trim when depressing both tabs until the engines were running over 3,500 rpm, which is above their cruise rpm. So, if you don't see three to five degrees of change in trim from full-up to full-down tabs when running at a comfortable cruise rpm, they're probably too small for the weight and length of the boat.

Bennett Marine, the largest trim tab manufacturer in the world, recommends an inch of tab width per side for each foot of boat length, but also takes boat weight and speed into account. So there would be 30 inches (called span) of trim tab width per side for a typical 30-footer. Their tabs are usually 9 inches long (fore-and-aft length referred to as chord) but are available up to 12 inches long for heavier, slower boats, or if space for mounting the tabs on the transom is limited. Speed is an important issue when sizing tabs, since those that would be large enough to work well at slower speeds could cause the boat to get out of control at, say, 50 knots.

Trim tab angle indicators are usually not included by the builder, but I wouldn't want to leave home without them. That's because it can be pretty tormenting fooling around with the tab rocker switches at the helm without having a clue as to their actual angle. These handy little tab angle indicators use a series of LED lights to show the angle of each tab, which depress 20 degrees when fully lowered.

Trim Control with Outboards and Stern Drives

Outboard and stern-drive power offer a natural advantage with regard to trim: the ability to control the direction of propeller thrust vertically as well as horizontally. It's the ability to trim the lower unit up (raising the bow) that separates a stern drive or outboard from an inboard-, surface-drive-, or waterjet-powered boat. Raising the lower unit allows an operator to find the hull's sweet spot, where wetted hull area and drag are minimized but the boat is still not porpoising, or to select a bow-up attitude for better following-sea control. Even though you can trim the lower unit down to raise the stern, trim tabs get the job done more efficiently, allowing the propeller to run level and just push the boat.

Of course, trimming a drive down will depress the bow as trim tabs do. The inefficiencies of a stern drive or outboard's smaller propeller and complex, energy-robbing bull-gear arrangement are offset by this ability to adjust propeller thrust to a more horizontal inclination, and by the lessened drag of the lower unit in comparison with an inboard's fixed gear (shafts, struts, and rudders). It's also worth noting that in a small boat, three or four people moving from stern to bow or filling a fish box and livewell will alter trim significantly, and a lower unit's ability to compensate for a bow-heavy condition due to these variable loads is a welcome feature.

Though its ability to "dial in" optimum trim gives a lower-unit-powered boat with poor weight distribution a speed advantage, an inboard with optimum LCG can perform nearly on a par with the stern drive or outboard, giving up at most 10 percent speed. The result is a boat that will run at its sweet spot all day long without trim tabs, with the tabs still available to depress the bow in a chop, raise the stern when the fish box or livewell are filled, or correct for heel or list. Provided you don't need an outboard's light weight and you get the weight distribution right, the cheaper, more corrosion-resistant inboard just might be the ticket to your boating Nirvana.

Appendages and Fairing

Underwater appendages—shafts, struts, rudders, and propellers—all contribute to drag and must be carefully designed and faired to minimize resistance and provide smooth waterflow. Poorly faired appendages can also cause ventilation problems that lead to dynamic instabilities.

Rudders can ventilate (and cause the stern to lift) when they're located too close to the transom. The low pressures created by the rudders can suck air in and cause the rudders to stall as well as generate lift. The best fixes are to move the rudders farther forward away from this ready air supply, to install horizontal cavitation plates above the rudders that project aft of the transom, or to notch out the rudders' upper trailing edges. Speaking of rudders, adding larger rudders or skegs to solve course-keeping problems will probably prove ineffective if the root cause is low-pressure regions forward in the hull.

IX. Dynamic Instabilities

Understanding Dynamic Instability

Predictability is greatly favored by astronauts and boaters alike. No one appreciates the unexpected happening when traveling along at cruising speed. When things get out of control, it's invariably because of outside forces acting in an unanticipated way.

How Low Pressure Develops

As we mentioned earlier, a planing bottom can develop low-pressure areas that destabilize handling and controllability. To understand how underwater hull shape produces positive and negative pressures, we return to our airfoil analogy. Note that the buttock lines on a planing hull describe a similar curve forward (seen in profile) to the top of an airplane wing (seen in section). The wing, which is more highly curved at the top than the bottom, develops lift not so much because of the high pressure on the bottom of the wing, but from the low pressure on the top. Low pressure develops on the wing's top surface because the air has to travel farther and therefore faster along its curved upper surface, creating a low-pressure region of aerodynamic lift.

Now here's where the bottom of a planing hull acts like the top of an airplane wing. Normally just the aft half of a planing vessel is in contact with the water, with the bow high and dry on plane in calm water. But when the forward hull bottom is immersed—when the stern is lifted by a following sea, or the hull is heavily loaded, or trim is too low, or the boat is slowing down—low hydrodynamic pressures are created forward even if this area of the hull is at a positive angle of attack relative to the surrounding waterflow. Under these circumstances, the forward bottom sections may actually develop pressures that are less than atmospheric, pulling the bow down.

These negative, less-than-atmospheric pressures may be exacerbated when bow-down in a roll, since the roll presents the most negative pressure-inducing hull profile to the water. As a result of the loss of lift forward, the CDL shifts aft, the bow dives farther, and the boat becomes dynamically unstable. High pressures also develop at the bow, but the net change is a bow-down trim. You'll see the same effect if you dangle a spoon under a running faucet; it takes some effort to separate the spoon from the waterflow, even when the spoon surface seems to be at a positive angle of attack.

One dynamic instability called bow diving (the bow pitches down deeply into the water in the absence of a clear cause, such as waves acting on the hull) is compounded in a heavily loaded hull because the forward sections are already more deeply immersed at speed. The more highly curved the buttock lines forward are, the less lift is developed, so the bow of a poorly designed and overloaded planing hull can be literally drawn down into the water by low pressure that's insufficient to support the hull's weight. In fact, one of the reasons a hard-chine hull makes a better planing boat than a round-bilge hull is that there's generally less curvature in the buttock lines forward, because the hard chines provide a natural place for the buttock lines to terminate. Low pressure can also develop locally due to depressions in a metal hull's plating or a fiberglass hull's imperfect tooling. The result can be chine walking, bow steering, and bow diving.

What Causes Dynamic Instability

In simple terms, a dynamically unstable vessel is one that, when up on plane and supported by the pressure of fast-moving waterflow, does not run like it's supposed to. This usually means the boat porpoises, chine-walks, runs bow-down, heels over to one side, yaws unpredictably, or is subject to some combination of these motions. The reason it runs at the wrong attitude (in trim, yaw, and heel) probably has to do mostly with hull shape and weight distribution and the resulting distribution of dynamic pressures (or lack of pressures) acting on the hull, hence the term dynamic instabilities. (There may be other reasons, too, such as appendage problems, loose steering, and improper handling.)

As we've seen, the area of the hull bottom where the water pressure is greatest is called the stagnation zone. This is the forward area of the hull at the on-plane waterline where waterflow impacts with and is perpendicular to the hull bottom. Dynamic pressure drops off quickly aft of this point, but there should be some degree of positive pressure all the way aft to the transom. Certain underwater areas of the hull are under normal pressure while others, due to inappropriate hull curvature or hull-skin irregularities, can actually be under less-than-atmospheric pressure (slight vacuum). When you sum these pressures, their distribution and intensity don't combine to support the hull in its proper attitude. A normal, dynamically stable vessel, on the other hand, is suitably supported by the sum of hull pressures and will return to a state of equilibrium (usually an even keel) by restoring pressure or forces after being subjected to wave action.

Factors That Contribute to Instability

So what factors are believed to cause a boat to be unstable dynamically? Speed is at the top of the list—the faster a boat can go, the more apt it is to become unstable. That's one reason why driving a semidisplacement hull to planing speeds can be a bad idea. Depending on hull length, dynamic lift generally starts to predominate over the static lift of buoyancy at about 25 knots. Instabilities resulting from dynamic forces alone are relatively uncommon below this speed. Hull loading, or the psi (static and dynamic) acting on the bottom of the hull, is another factor; the heavier the loading, the greater the chances of an instability developing. LCG is also critical: putting it too far forward will invariably result in a dynamic instability. It's worth noting that adding a cockpit extension effectively shifts LCG forward (as a percentage of waterline length), which can easily be a recipe for introducing dynamic instabilities, so this common hull-form alteration must be carefully engineered.

Too much buttock line curvature forward, excessive hook or rocker aft, and underwater hull appendages can also contribute to dynamic instabilities, and rudder and trim tab movement can upset the stability apple cart. Dynamic instabilities may also result when VCG, LCG, or displacement are wrong for the hull's shape and dimensions. So, a high-powered, heavy boat with a forward LCG, highly curved buttock lines forward, and ventilated appendages is likely to disappoint in terms of stability on plane. Sometimes the solution is simply to slow down a couple of knots.

Some dynamic instabilities result from putting too much power in an older design. Production builders are taking older hull designs, which behaved well with moderately sized engines and repowering with 75 to 100 percent more horsepower. Adding power usually necessitates shifting LCG aft (usually by moving the engines aft), but this requirement is all too often ignored. Now these boats are really operating in an altogether different performance regime, and their builders and owners seem to wonder why they're no longer well-behaved at speed.

Oscillating Instabilities

Instabilities can be either oscillatory (varying, moving back and forth) or constant. Oscillatory instabilities include chine walking: this is a roll oscillation in which the boat heels over on its chine, rights itself to an even keel, and then repeats the cycle. The cure for chine walking in outboard-powered boats may be as simple as tightening up the steering or shifting the LCF aft. Repowering with larger engines without moving the LCG aft can also result in chine walking.

Porpoising is an oscillation in pitch and heave, with the bow alternately rising and falling. The center of dynamic lift (CDL) changes as speed increases and trim and the area of the immersed hull change; porpoising results when the CDL constantly shifts forward and aft relative to a stationary LCG. Both chine walking and porpoising are associated with hard-chine hulls (chine walking is also sometimes found in certain boats with long length-to-beam ratios), and the intensity of these oscillations is often a function of hull speed. They can occur in calm or choppy water without any helm or throttle input from the operator, and they are often predictable.

Porpoising, in fact, like other oscillatory instabilities, can be anticipated by the operator once it's happened the first time, since it's based on a known combination of speed, trim (and trim-tab or drive position), and weight distribution. The operator may learn that her boat will never porpoise below a certain speed or engine trim setting. An owner may relocate her outboard from the transom to an aft bracket; this weight shift would tend to increase the chances of the boat porpoising. Porpoising can be stable at a constant speed and trim, or it can be unstable, increasing while these conditions remain constant. A naval architect's ability to accurately predict the conditions in which a new boat design will porpoise, before it ever hits the water, is limited, but it's also one of the best understood planing instability phenomena.

Constant or Steady-State Instabilities

Constant dynamic instabilities usually occur on large, relatively fast-moving, heavily loaded craft with LCGs that are too far forward. These instabilities are potentially the worst kind, since they can occur rapidly and unexpectedly under pushing-the-envelope conditions of weight and speed, and the results, including broaching and erratic helm response, can be dangerous. Instabilities can be created entirely by internal influences such as a vessel's weight, weight distribution, and shape, or they can be triggered by external forces like wave action. A broach while traveling down the face of a wave is a dynamic instability in yaw and roll, for example. A broach can be initiated by a static instability if a hull is moving at the same speed as the wave supporting it, but the hull's inertia makes things worse. Sometimes, a one-in-a-thousand combination of wave impact, rudder angle, trim-tab setting, and LCG will cause a dynamic instability in an otherwise satisfactory boat.

Low-Speed Instabilities

Instabilities at semiplaning (semidisplacement) speeds are often caused by static as well as dynamic pressures, and can diminish both directional and transverse stability. Loss of transverse stability, for example, can result when gravity, acting on the bow wave, creates a deep trough alongside the midbody of the hull, diminishing form stability by exposing the turn of the bilge to the sea breeze (and low atmospheric pressure). In fact, positive and negative pressures distributed along the underwater portion of the hull affect planing and semidisplacement, and round-bilge and hard-chine hulls.

When round-bilge hulls are pushed to semidisplacement or planing speeds, the boat's GM (its metacentric height, the measure of initial stability derived primarily from hull shape), is decreased because of the positive and negative pressures and changing waterlines that develop. In fact, some dynamic inclining experiments show a 20 to nearly 40 percent loss of GM at a speed-to-length (S/L) ratio of 1.7 and 3, respectively, resulting in a high degree of roll sensitivity. A reserve of static stability must be designed in to compensate. Round-bilge boats that roll excessively at speed usually benefit from increasing bow-up trim, either by shifting the LCG aft, reshaping the hull by adding rocker aft, or adding wedges forward, just aft of the stagnation zone. Much as lowered trim tabs create lift at the transom, wedges forward provide lift at the bow, especially on the side more deeply immersed by heel and roll, thus countering excessive roll.

Ventilated Appendages

Hull spray strakes, which can as easily channel air as water, must be carefully positioned so they don't direct air to propellers, rudders, and underwater appendages. This may be counterintuitive, but if struts and rudders ventilate, they can cause dynamic instabilities, including roll moments and bow-down trim angles, by generating lift at the stern. An off-center ventilated strut or rudder will also cause the vessel to roll, potentially initiating a yaw or broach. The designer also has to make sure that cooling-water inlet through-hulls, depth sounder transducers, and waterjet inlets get a clean supply of solid water. Ventilation problems are best addressed by closing off air paths and reducing local disturbances with improved appendage fairing (streamlining).

Solutions

Dynamic instability fixes that may work include shifting weight aft, building rocker into the hull to depress the stern, widening the chines or adding running strakes forward, fairing hull appendages, and adding wedges at the bow to introduce air to ventilate the wetted surface, eliminating the low pressure areas. Naval architects can conduct tests to determine the causes of dynamic instabilities, including a trim-speed test to look for low dynamic pressures forward that produce bow-down trim at speed, and dynamic inclining experiments to measure changes in transverse stability from dead in the water to full-speed. Older, overpowered hull forms need to be replaced with newer designs that can accommodate today's powerful diesels. The simplest fix, though, may be just to throttle back and enjoy the scenery.

While this discussion may give the impression that naval architects have a good handle on dynamic instabilities, this is not always the case. The interface between wave and hull, and the other elements that influence boat behavior in a seaway at speed, are complex. It may be easy to recognize a dynamic instability when it happens, but most of these phenomena are poorly understood even by those who make a living studying them.

X. The Hunt Legacy and Modern Excellence

Ray Hunt's Revolution

The deep-V was born not from computer modeling but from observation and intuition. Ray Hunt—first and foremost a sailor—was intimately familiar with wave, current, and tide. Watching how boats behaved in the rough waters off New England, he noticed how some hulls slammed and skittered while others seemed to breathe with the sea. He studied why, and drew lines that allowed the latter behavior to be repeatable. The proof came in the early 1960s with the legendary Bertram 31 Moppie, which dominated offshore racing and established the Hunt deep-V as a revolutionary form. From that foundation grew an entire generation of Hunt-inspired boats—Bertrams, Eastbays, several Regal, Boston Whaler, and Wellcraft models, Grady-Whites (which uses Hunt hulls exclusively), the standard 11-meter US Navy RIB, and the majority of the pilot boats in the US.

Hunt Design and other top designers have over the years carried that lineage forward with the same disciplined understanding of why the geometry worked. They refined the rate of warp, keel radius, buttock curvature, and the alignment of strakes and chine flats, scaling the design seamlessly from small outboard dayboats to 80-foot commercial pilot vessels. These designs are iterative, not static. Each refinement comes from at-sea feedback and mission requirements—whether a 40-knot sportfisherman or a 25-knot cruiser.

Their refinements over decades—adjusting chine width, twist, and strake placement for different propulsion types—represent the kind of deep empirical understanding only long experience can yield. A good planing hull doesn't rely on gimmicks like fake bottom steps, hull dimples and steps on a sub-30 knot hull- it relies on geometry. It's the difference between a design evolved by the sea and one conjured up in a classroom.

Hunt Design Principles

The Geometry That Endures

For a 30-kt hull, the hull form formula has developed into an art form. Twist or warp in the last half of the hull, a radiused keel, the alignment of strakes, and the very gradual change in buttocks curvature forward create a good running and utterly predictable hull offshore. Such a hull can also plane at very low speeds—as low as 12 knots if you define planing as being over the hump with the bow down and a clean wake astern. Let's look at each of those attributes and see why they matter.

Twist from Station 5 to 10

The twist, or warp in the aft half of the hull (vs. the monohedron's constant deadrise) shifts the center of dynamic lift (CDL) aft so the boat typically runs at just 2 to 4 degrees of trim with less stern immersion, which in turn creates less form drag. It also results in lower vertical accelerations due to a lower angle of incidence between hull and waves, and because the sharper sections further forward in the bow absorb more wave impact. Lower trim improves visibility ahead from the helm, even when seated. And all this without having to resort to bandaids—trim tabs or interceptors—just to raise the stern to achieve the same moderate trim level.

Radiused Keel

A hull that comes to a sharp point forward at the keel becomes a rudder when it's immersed, which is not what you want when running in a following or quartering sea. That's why boats bow steer—when the rudder forward overcomes the rudders back aft. A sharp, pointed keel forward can also make a boat hook, or spin out in the turn. That's because when a boat turns hard, especially with outboard or sterndrive power, the stern lifts up and the bow comes down, immersing the keel and increasing the wetted surface forward, which does two things: the distance from the props to the center of resistance increases, increasing the couple (force × distance) between the two and making hooking more likely. And since the sharp keel naturally grips the water laterally, the two forces make hooking inevitable. Many of the best-known center consoles and other saltwater boats hook, incidentally—I've run most of them in my competitive analysis work.

Further, a boat with fatter sections forward, when immersed in a turn, naturally immerses more hull area, increasing frictional resistance and increasing the couple mentioned earlier. On the other hand, a hull with a radiused keel is not susceptible to tripping since the keel is free to slide sideways, so there's nothing to trip on. It also has a higher elevation since the bottom of the keel is effectively "sanded" off relative to a pointed keel, so there is less immersion to start with at a given trim. The best bow has a radiused keel and a sharper entry, which further diminishes the amount of hull area in contact with the water in a turn. Finally, a radiused keel impacts the waves with lower accelerations or peak impacts than a sharp keel, and you feel that as a smoother ride.

Buttocks Curvature

Less buttocks curvature forward results in less negative pressure, particularly important when running downsea, through the trough and up the front of the next wave. This is when the bow is most deeply immersed for the longest period, and excessive buttocks curvature forward develops negative pressures dynamically, like the top of an airplane wing. Hold the back of a spoon into a stream of running tap water and watch the water pull it into the stream. The same thing happens with your boat if it had a lot of buttocks curvature between stations 2 and 5. The reason the hull with twist aft can have less buttocks curvature is that deadrise increases from, say, 20 degrees at the transom to 25 or 26 degrees up at station 5, so the increase in deadrise from station 5 forward can be less abrupt moving forward. A monohedron hull, on the other hand, has the same deadrise at station 5 as at station 10, so it's a steep climb to create even marginally adequate deadrise up in the bow. If you are following and can appreciate this discussion from experience, you already know more than the great majority of boat designers in the recreational and commercial markets.

Appendages

By appendages I mean bottom strakes and chine flats, which are essential to get right for optimal performance. The strakes have to be the correct size and cross section, commensurate with the size and displacement of the hull, and they should be in the right position on the hull to do their job most effectively. Strakes in the bow have four distinct but interrelated functions:

1. They create flow separation to reduce the amount of hull surface in contact with the water, reducing frictional resistance, or drag.

2. They deflect spray for a dryer ride, which results in better visibility and therefore improved situational awareness at the helm, and increased safety of operation.

3. They create some lift, since for every action there is an equal and opposite reaction, getting the bow out of the water a little more for reduced hull immersion and drag.

4. The dynamic pressure of water flow at the outer strakes and especially at the chine flats increases lateral stability.

5. The hard corners at the outer edge of the chine flats create flow separation away from the hull side, further reducing frictional drag. The chine flats also have the effect of increasing bottom area, decreasing both static and dynamic bottom loading and making the hull more efficient on plane.

When hull form, powertrain, and weight distribution are properly aligned, a boat achieves that elusive state of grace—the one where you throttle up seamlessly from 12 to 30-plus knots and feel nothing change except the wind in your hair. That's the mark of good design.

Hull Form Defines Performance and Survivability

Hull form defines a planing hull more profoundly than it does a displacement or semidisplacement vessel because the dynamic forces at work when on plane are central to its performance—and its survivability. Seaworthiness isn't just built into a boat's design—it's shaped into the hull. And when done right, it endures for decades. The sea hasn't changed, and neither have the laws of physics. What has changed are our priorities. The best designers remember what matters: performance, efficiency, and, above all, trustworthiness offshore.

Lessons for Today

Moderation as a Design Philosophy

A lighter boat doesn't just run better—it's easier to maintain and less costly to operate. Yet modern builders often add complexity and amenities until the boat becomes an apartment that happens to float. Every system—from granite countertops to washer/dryers—adds weight, which compounds inefficiency.

Simplicity can be its own form of luxury. Fewer systems mean fewer failures; lighter structure means lower power requirements, and smaller engines mean less machinery weight and less fuel required for greater efficiency. Together, these yield greater range, less fatigue, and more enjoyment.

XI. Practical Hull Assessment

Reading a Hull: Dockside Evaluation

What to Look For at the Dock

You can learn a lot about a planing hull just by walking around it at the dock. Look at where the chine enters the water at rest. If it immerses well forward—say at Station 2 or 3—that's evidence that the boat has a full, blunt entry that will pound in a head sea. If the chine is dry forward and doesn't immerse until back at Station 5 or even 6, the bow is sharper and the hull has more deadrise not only in the bow but in the center of the hull. That's because chine elevation signals how much deadrise there is at that station. The reason that more deadrise farther aft matters is because the faster a hull goes, the more it rises out of the water, and the farther aft waves predominantly impact the hull. A hull with 20 degrees of deadrise at station 5 might be fine for a boat that cruises at 16 or 18 knots, but it will jar your kidneys at 25 or 30 knots in a seaway.

You can also see transom deadrise at the dock. Too flat—below around 15 degrees—and the boat will have little coursekeeping tendency, essential when running downsea. The deadrise acts like a keel, creating resistance to lateral movement, and without it, the stern is just as happy sliding sideways as keeping its course. 20 degrees at the transom is about perfect for a boat that cruises at 25 to 35 knots, since with a warped vee hull you can get to 25 or more degrees of deadrise at station 5 where wave impact occurs at these speeds. Above 35 to 40 knots in a seaway in a boat up to 50 feet or so, you'll want more than 20 degrees at the transom since the hull will be landing back here more than occasionally. So deadrise distribution from bow to stern should be appropriate to the speed and size of the hull. A larger, heavier 40-knot 65-ft convertible does not need as much deadrise aft as a smaller boat since the mass and immersion of the hull will give it directional stability.

Also at the dock, look at the angle of entry in the bow—the bow's footprint in the water. If it's fat, say over 32-34 degrees half-angle of entry, it will have a harder ride than a boat with a sharper bow. The great majority of cruising boats at the marina have fat angles of entry, low chine immersion and low deadrise in the forward half of the hull, and you will be very limited in when you can head out past the jetties, and how fast and far you can go once offshore.

Red Flags

Excessive Tab Dependence

If a boat needs trim tabs at all times just to run acceptably in calm water, or can't get on plane without pointing at the clouds, LCG is too far aft or the hull is flawed, or both. This is a Band-Aid for poor design.

Wake Turbulence

Aerated, turbulent wakes indicate and wasted energy. Clean, glassy water leaving the transom and on either side of the rooster tail are signs of efficient flow separation.

XII. Conclusions

Ultimately, seaworthiness is geometry made humane. It's not about mathematical perfection or aesthetic beauty; it's about motion—how a hull translates the chaos of the sea into manageable, predictable forces. When a boat rises cleanly over a wave, lands softly, tracks true downsea, and responds smoothly to the helm, that's geometry at work—the sum of thousands of small choices in line, proportion, and balance.

Every successful boat, from a 25-foot express to a 68-foot offshore cruiser, begins with the same design and performance principles. Getting the fundamentals right—LCG, deadrise distribution, buttock curvature, L/B ratio, bottom loading—is what separates a boat that performs predictably and safely from one that doesn't.

The best planing hulls, like the best displacement hulls, embody moderation and balance. They don't specialize in one condition at the expense of others. They run well upsea, downsea, beam-sea, and quartering-sea. They're fast enough for their mission but not so fast that they sacrifice comfort and safety. They're light enough to plane efficiently but strong enough to endure years of offshore use. They're complex enough to perform well but simple enough to maintain.

It’s both healthy and productive to challenge the conventional thinking, the marketing departments, the demands for ever-wider beams and more amenities—to encourage better design. Hull design in much of the cruising yacht sector has actually regressed in the last few decades with many major brands succumbing to marketing pressure with every-expanding, voluminous hulls. But the laws of physics haven't changed. The sea hasn't changed. What's changed is our willingness to accept compromised performance in exchange for interior volume.

Good planing hulls are still being built today, though, by builders who understand these principles and resist the temptation to pile on interior volume, weight and beam. When you find one—whether it's a classic Hunt design or another modern interpretation of timeless principles—you'll know it the first time you take it offshore.