Hull shape and stability

Boats can be split into two categories: the ones that dig holes in the water and the ones that travel on top. Race boats, runabouts, fast cruisers, sport fisherman and military patrol craft are part of the planing fraternity. A few racing sailboats and wind surfers fall into the same cult, but when all is said and done, it takes considerable energy to coax a hull onto a plane and it’s not easy to get that from a sail plan. Water’s hold on the hull comes from the double team effort of skin drag and wave making, phenomena experienced by all craft as they accelerate. Hull speed is defined as the point where the very crest of the bow wave is at the stem of the vessel and the crest of the stern wave is at the transom – in essence, the vessel is neatly tucked into the trough of a wave of its own creation. To get on a plane requires enough energy to break free of the stern wave and climb onto the crest of the bow wave shackling the effect of hydrodynamic lift (hull speed = 1.34 x square root of the design waterline length (DWL). Planing is a speed length ratio of 2 or greater).

Displacement cruisers (power and sail), merchant ships and small, low-powered recreation vessels, along with kayaks and rowing shells, make up the second category, one that favors hull shapes that avoid the desire to plane and emphasize an ability to slip through the water with as little resistance as possible. The issue of resistance to forward motion is comprised of several factors and, at low speed, skin drag reigns supreme. In short, water next to the hull adheres to the skin and a thin layer gets dragged along for a ride. Narrow, pointy-ended, round bilge craft with as little surface area submerged as possible afford the least drag in a displacement mode. The smoother and fairer the underbody of the craft, the less water gets towed along, and for decades coatings manufacturers have been looking for the magic touchstone in paint technology – a formula that lessens skin drag and actually makes a boat more slippery in the water.

Once a vessel starts to approach its hull speed, resistance from wave making becomes the biggest obstacle to progress. With enough power, any shaped hull can be thrust over the “hump” and launched onto a plane. But the real art in high-speed yacht design is balancing a vessel’s proclivity to plane with its behavior once on top of the water. Wide, flat-bottomed boats leap onto the surface but then tend to porpoise and pound, even in calm conditions. Juggling the influence of gravity, buoyancy, dynamic lift, drag and thrust is the design challenge, and small changes in things like trim angle can make a big change in performance. As a rule of thumb, the trim angle that seems to give the best performance is around 2° to 4°.

With fuel prices soaring, designers and builders are taking a second look at the hull shape, boat speed and the cruising efficiency of the vessels being built and those already in play. It’s no surprise that motor boats are least efficient when they are trying to climb over their own bow wave to get on a plane. It maximizes wave making, negatively impacts fuel consumption and decreases range. The half plane hull form/power package is a vestige from an era of inexpensive fuel and is hard to justify at current energy prices. The new trend is toward boats that plane at fairly modest speeds and can be throttled back and still remain on top of the water. There’s a resurgence of interest in fuel efficient, low-powered displacement hull shapes.

Catamarans came to the U.S. with masts stepped and sailors as the target market. Meanwhile, the Australians were running gas turbine-powered, high-speed multihull ferries across Jackson Bay and designers quickly recognized the fuel efficiency and decent load carrying ability of smaller renditions of these power cats. Their high beam-to-length ratio hulls didn’t suffer as much from wave making drag, and builders were cognizant of the ill effect of excess weight. The result was a variety of fuel-efficient workboats and recreational craft that are popular on the market today.


Staying afloat is a major desirable yacht characteristic, and stability has a big role to play in the process.

The Greek scientist and inventor Archimedes was ahead of his time when he acknowledged that buoyancy and displacement were correlated factors associated with the shape and behavior of objects floating in water, and that the weight of the water displaced by a floating object was equal to its weight. Since then, everyone involved with boat design has been keenly aware of buoyancy and weight, and how they relate to each other. The first part of our stability discussion involves the shape of a vessel and how it contributes to staying up right. A wide, short, hard chine skiff has an abundance of athwartship form stability – you can stand near the rail and not fear capsizing. This is a plus, but if you tried to row the boat there was so much skin drag from the wide flat bottom it was like rowing the dock. A longer, leaner, round bilge skiff would be reluctant to plane with a small outboard and would send you for a swim if you stood near the rail but would row like a dream. From these simple design comparisons you get a hint of the dilemma faced by every naval architect dealing with the art of design compromise.

Let’s hypothetically deck over the small 9-foot skiff, make it watertight and discover another factor associated with form stability. We have recognized that, all things being equal, the wider a boat the more it resists heel. However, if it heels to a point where the center of gravity and the center of buoyancy line up vertically, we have the proverbial nickel standing on its edge – the vanishing point of positive stability. The stability curve of a wide boat initially looks good but the large area under the negative portion of the curve is disconcerting. The form stability associated with buoyancy derived from a wide beam now works to keep the vessel in the inverted position. To mitigate the effect of stability in the negative range, or in simple terms a boat’s desire to stay upside down, designers introduce shapes and structures that create a secondary righting moment.

This extra boost to stability is best seen in sailboats in the form of ballast keels and their heavy pendulum-like influence that resists the forces of wind and sea. Like any teeter-totter relationship, the effect of ballast has to do with both the weight involved as well as its distance from the fulcrum – in this case, the center of buoyancy. Designers calculate the vessel’s vertical center of gravity (CG), a point within the vessel around which its weight is evenly distributed. The lower the CG, the more stable the vessel becomes and the higher in degrees its limit of positive stability.

Adding a large cabin structure can also change the heeled center of buoyancy and, at extreme angles, may contribute to the vessel’s self-righting ability. However, the structure also raises the CG and often adds the issue of down flooding created by placing large vulnerable windows in contact with violent seas. Tradeoffs have to be carefully considered when designing oceangoing powerboats. The concept of scantings, or the overall strength of a vessel, comes into play at this point. For example, an expeditionary craft designed and built to range into high latitudes is likely to endure more sea-induced stress and strain than the average weekend cruiser. Therefore, its range of stability, structural integrity and ability to avoid down flooding should be greater. In lay terms, it makes sense that the former be stronger and more resistant to capsize than the latter.

Too much of a good thing?

Seakindliness is another important design consideration and, ironically, when it comes to stability there is a down side to too much of a good thing. A case in point is how excess beam creates a snap roll effect that can be more disconcerting than a slower pendulum-like swing through a greater degree range. The flat, wide bottoms of catamarans and some monohulls offer significant initial stability. Their powerful righting arm associated with the large distance between the at-rest CG and CB creates more acceleration when returning a vessel to its upright position than is found with narrower beam vessels. This jerky motion, especially when navigating beam seas, can be disconcerting.

Every vessel is an amalgam of ideas, some working in complement with each other, while others have a parasitic effect. Windage is one of the latter and as cabin accommodations grow, so does the stack-of-boxes look of the boat. This is not a subjective concern over aesthetics, it’s a safety issue associated with excess surface area and the influence of wind pressure on light displacement vessels with very little hull surface submerged. If the vessel is primarily a live-aboard accommodation, and the need for performance is little more than ambling from protected estuary to protected estuary in benign weather, the windage issue may be of less consequence.

Moderation has proven to be the naval architect’s friend and avoiding excess, whether it’s beam, cabin size, horse power or window size, makes sense. Good design is about harmony, and the naval architect’s role was best explained by poet John Masefield when he described a schooner as: “Built out of so much chaos brought to law.” n

By Ocean Navigator