Hull Design 101; Part 3...

SPECIFIC VARIATIONS AND TRAITS...

“DEEP “V” HULLS: Smaller, lighter performance hulls typically use a 22 degree deadrise hull with “step” shaped strakes that are conducive to good lift and hull aeration, while providing predictable tracking and grip when cornering. These same hulls sometimes feature a light hook at the rear of the hull to reduce porpoising, as the length of the hull, center of gravity, etc., are not always in harmony in choppy conditions.

A deeper deadrise hull, say 24 degrees or greater, as is typical with most performance oriented larger boats, will provide a smoother ride in rough conditions, but will sacrifice some lateral stability, whether at rest or at speed. Typically, a deeper deadrise hull will corner/handle better with less temptation to spin-out, as its keel creates a hydrodynamic slot through the water to hold a desired path.

“FLAT BOTTOM” HULLS: In contrast a flat bottom hull with shallow “V”, say less than 12 degrees affords good stability at rest or idle speeds, but lacks the deadrise depth for aggressive handling and smooth ride in rough water. Because the prop placement is generally closer to the same plane as the rest of the hull, ventilation may occur more easily in less than ideal water conditions. This configuration is not conducive to higher speeds in rough conditions as the prop unloads more easily. In cornering with this hull, air is easily inducted from the sides of the hull, especially given the amount of sliding associated with a shallow deadrise design.

MODERN YACHT HULLS: This is basically a planning hull that features a sharper “V” toward the bow and transitions to a shallower deadrise toward the rear for increased stability and lift (speed). The progressive “V” shape provides a smoother ride through rough water, while the planning area affords increased speed and stability. The displacement (weight) of larger hulls generally add to stability and ride comfort.

“CATHEDRAL”: The cathedral hull features “channels” that force air and water to travel backwards under the hull, as opposed to displacing outwards. This helps generate lift and aeration. The channels are a by-product of longitudinal appendages sometimes referred to as hard chines, but really fall into a category of sponsons. The actual chines on this hull are reversed, thus channeling water under the hull, as opposed to letting it slip outward. This configuration creates parabolic areas at each side of the “V” center. In turning, this hull should remain relatively flat, but theoretically should create excellent tracking when cornering, although in reality will feel a little “quirky” at times. The largest drawback of the cathedral design is it does not offer the smoothest ride in choppy water. However, it offers outstanding stability and a very shallow draft.

TUNNEL “V”: This design has proven itself in Formula One Grand Prix Boats. Tunnel “V’s” develop extreme lift (surfacing hulls). For recreational boating, this is not a versatile design. Tunnel “V’s” have amazing cornering capability due to the outside sponson building a hydrodynamic berm when cornering and the inside sponson working like a rail to hold the combined sponsons in a designated track.

STEPPED HULLS: Step ventilation technology induces air under the hull to help reduce drag. The design concept has been adapted by many of the leading performance boat builders around the world. It is also referred to as Step-Ventilation. Steps are generally placed where the designer determines that a need for aeration exists on the bottom of the hull, to aid in reducing drag. Some designers use multiple steps to achieve the greatest aeration under the hull, however too much aeration can lead to less than favorable handling, especially when cornering.

Other designers, such as Harry Schoell, take a different approach. Harry’s design is really two hulls in one; a forward Delta Conic hull section having a conical entry with a delta shaped constant planning area. Aft of the main hull section, a constant planning area displacing another delta pattern is created when on plane. The forward hull creates a bow wave that surges under the hull (rooster tail), causing the aft section of the hull to ride on this wave of higher pressure. As speed increases, this wave moves further aft, lifting the stern and creating greater leverage against bow rise (anti-porpoising). This system gives quicker planning, much like a cavitation plate mounted lifting wing (Doel-Fin). It essentially gives a longer hull without the penalty of increased wetted surface.

TRANSOM NOTCHES / RECESSED STEPS: Usually incorporated into the original mold, this design feature allows the propeller to be raised higher out of the water flow coming off the transom, thus reducing drag and usually increasing rpm’s, resulting in greater speed.

TRANSOM BRACKETS: Essentially create the same effect as a recessed “step” just before the transom, allowing the prop to be raised higher (“X” dimension) and reducing drag. In addition, an extended bracket adds overall length to the hull, which aids in wave-spanning capability or technically speaking; improves longitudinal stability.

“AIR INDUCTION”: An air induction system can be incorporated to aerate the hull, similar to steps, but drawn in via air inlets channeled from inside the hull. This is accomplished via an inlet from the bilge that draws air under the hull, based on the vacuum created by a raised strake located just forward of the exposed air inlet hole on the bottom of the hull. This is an innovative form of hull aeration, but only serves to aerate a small portion of the hull, which might be just right, because too much aeration sacrifices handling.

“REVERSED STRAKES”: One of Ocke Mannerfelt’s (remember the “batboat”) neat little product ideas called “Speed Rails”, which were designed as a bolt-on appendages to traditional hull strakes. These clever little rails help capture water flowing outward, toward the side of the hull and force it to flow toward the rear of the hull, naturally increasing lift. Because the rails protrude straight down from the lip of the strake, it also aids in cornering, by creating grip. Hence the name; Speed Rails.

Coupling these technologies and design exercises appropriately and you have significant factors that can extract speed and fuel efficiency from a hulls’ design.

CONCLUSION...

There are many elements that make up a hull’s bottom. The shape of the bottom’s sections, how rapidly the sections sharpen as they move forward, and they blend toward the transom. A good hull has the right blend of design elements and it is the designers job to create a hull that incorporates the best features and performance for a given application. Hence, the rapid development of specific hull designs for runabouts, cruisers, muscleboats and raceboats.

Despite having to meet general performance goals, every boat has its own personality and quirks. Personality can be cosmetics, ergonomics, special features and lay-out. Quirks will most often manifest themselves in the form of handling. This is why hull design is crucial to the overall package. The perfect hull does not exist, because each person will place emphasis on different areas.