Hull Forms—What’s the Difference?

By Christopher Swanhart

Walking the docks of any boat show, you’ll see a plethora of boats. Our industry is filled with use-driven designs—motor yachts, expedition yachts, high-speed center consoles, low speed trawlers, the list goes on, including one of my favorites, sportfishing yachts.

While it is easy to see differences in deck design, or in the engine room, it’s not so easy to see the differences below the waterline. The ideal hull form depends on many factors, such as intended use, expected conditions, desired speed and others. Let’s take a look at the characteristics of some different hull forms and how they work.

Low- and High-speed Hulls

While not as prevalent in sportfishing, low-speed, or displacement hulls, do have their place. If a hull will remain in the displacement regime, a round bilge hull form is typical. This section shape does not have a hard point at the chine. It’s not necessary, or even desirable, as the water does not separate from the hull like it does with its higher speed relatives. This hull form provides lower resistance at speeds below hull speed, when the hull is still supported almost solely by hydrostatic pressures (buoyancy). Benefits of a displacement hull include relatively low fuel burn (longer range) and good seakeeping. Fabrication materials can be just about anything as weight has little impact on performance.

While this type of design has seen a bit of a resurgence, it will not get us to the fishing grounds, or anywhere, quickly. Semi-displacement of semi-planing hulls are moving faster, for their size, than displacement vessels.

The term “high-speed displacement” has gained traction in describing this type of hull. These hulls, while underway, are seeing a mix of static and dynamic pressures. Lift is being generated, but the hull isn’t reaching hump speed—the maximum peak of resistance.

In this speed regime hull forms may still be round bilge, but we start seeing features that help generate lift and allow the water flow to separate from the hull at appropriate locations. Strakes become useful as do partial chines, which, at the very least, help knock down wave patterns and spray. Also seen here is a double chine hull, which provides benefits of both round bilge and hard chine shapes.

Performance Hulls

While plenty of yachts operate in the low- to mid-speed ranges, the sportfishing world, from small to large vessels, is dominated by high performance. At high speeds, hydrodynamic forces dominate. Lift is generated in extreme amounts, allowing the hull to fully overcome maximum resistance (hump). Once planing speeds are achieved, a hard chine hull is the obvious choice. The sharp edge at the chine, usually a chine flat, allows for a nicely formed stagnation line, where pressures are highest—the transition between solid water and spray.

And sharpness is critical. Edges where we want water to separate must be sharp to minimize drag. This includes the chine, strakes and the hydrodynamic transom. It is at these speeds that weight and center of gravity become critical. These must work in concert with the hull form to maintain dynamic stability. Hull design variables are too many to discuss in detail here, but typical hard chine features include relatively high deadrise forward, section geometry including convex and concave shapes, and straight buttocks aft. The designer will adjust hull variables to suit the application or to maximize a particular operational feature.

Want maximum top speed? Less deadrise and a wider planing surface helps. Want the best possible ride quality? Higher deadrise and narrower form is the way to go. The trick is balancing all these variables to achieve a good performing, predictable hull with a soft dry ride. And one that raises fish!

Stepped Hulls and Multi-hulls

One variation on the deep-v monohull is a stepped hull. These have grown in popularity and can improve top-end performance if done properly. Stepped hulls have become more common with the advent of high-power outboards and owners’ desire for higher and higher speeds. The ventilated transverse steps introduce air to the bottom, reducing wetted surface, and thus, drag. However, stepped hulls only help reduce resistance when the boat is going fast enough. If the boat does not achieve a high enough speed, steps can actually increase drag, negatively impacting performance. And if not designed appropriately, with proper vessel weight and weight distribution, a stepped hull can exhibit undesirable dynamic characteristics and operate quite unpredictably.

Multi-hulls are yet another family of hulls that vary in popularity and use. Catamarans offer great transverse stability and can provide high performance. However, they are not as popular for fishing because of their relatively high deck heights, which are necessary to ensure good performance.

Catamaran hulls have their own design challenges including demi-hull spacing, wet-deck height and tunnel shape. Lower speed cats call for symmetric hulls, while asymmetric hulls work well at very high speeds. Monohull, deep-v, multi-hull, steps, hard chine, round bilge and many other hull forms and features exist. While sportfishing vessels tend to have hard-chined monohulls, many of these hull forms can provide a superior fishing platform. It is the job of designers and naval architects to identify the right features and employ them appropriately. Objectively speaking, no one form is best; they each offer benefits and are each optimal for different situations.

Originally from Annapolis, Chris moved from aerospace to the marine industry early in his career. For more than twenty years, he has taken pride in being able to build lasting relationships with many world-class yacht builders. He has managed countless new designs and builds, including patrol and rescue craft, production boats and high-end custom sportfishing yachts, with a design focus on high-performance hydrodynamics, hull forms and structures.