Hull Design 101; Part 2...

Continued from Part 1...

POWERBOAT HULL DESIGN:

Most high performance craft utilize some version of Deep “V” hull, as it offers a smoother ride in rough water. This is most pronounced in boats that are fast & light. The Deep “V” provides driveline and propeller placement that is conducive to keeping the prop in the water when riding high atop the waves, as it is located at the deepest part of the hull as possible.

The deep “V” is a variation of planning hulls. A displacement hull moves through the water, thus displacing water as it moves forward. In contrast, the planning hull develops lift as it moves forward through the water. Lift is primarily a function of the sectional shape and the length of the leading edge of the waterline plane while under way. Hydrodynamic lift replaces static buoyant forces as a hull moves forward. A hull continues its planning mode as speed increases until very little hull area is left underwater. Once a hull is fully on plane, the greatest part of total resistance is drag or friction because of contact with water and air. Water friction is a function of speed, area, and to a varying degree, wetted surface length. With conventional propeller driven boats, a hull’s speed is largely limited by the horsepower available.

With jet-driven boats, the hull’s speed is primarily limited by the inefficiency of jet-pumps at higher speeds, or the inability for water to turn upwards (at high speed) via the vacuum created at an intake housing. Pumps are forced to ventilate at high speeds. In other words they do not run completely saturated or loaded with water and thus lack the thrust to continue into higher speeds, even with increased horsepower.

What separates the Deep “V” from other planning forms is not complicated, and is easy to see. In short, a deep “V” uses a wedge to drive into and split a rough water surface. As speed increases, so does hydrodynamic lift. The faster you travel, the further backwards lift (water) moves, thus reducing the amount of wetted surface and the associated drag.

Some high performance deep deadrise hulls use a “pad”, located at keel level to develop lift and increase speed, but this can result in a rougher ride at higher speeds in choppy water. If the “pad” area is too large (wide), it will create excessive lift, causing the hull to ride on much of the pad area, thus leaving the outboard areas of the hull with little or no hydrodynamic support. This will ultimately result in the greatest speed, but will sacrifice lateral stability (outboard wetted surface) and “chinewalking” can commence. “Chinewalking” is a side-to-side oscillation caused by too much lift at the pad, which in-turn causes the chines to run dry and thus loosing the lateral stability afforded by the chines. A “V” bottom with little or no “pad” area will not develop as much lift, therefore creating a larger wetted running surface, which will aid in stability, but may not travel as fast. When chinewalking occurs, the hull will lean to one side in search of stability. When it leans over enough, it will develop excessive lift which will push the hull over to the other side, where the scenario will repeat itself.

Chinewalking should not to be confused with “Broaching” which is the result of too much lift at the transom, thus causing the nose of the craft to plow. At this point, lift at the transom creates a leverage effect that is transferred to the front of the hull, which generally will have a steeper deadrise angle (as the bottom of the hull turns upward at the bow) and will be forced to roll to one side or the other, again in search of lateral stability. Broaching is best described as a hull that will not stay centered (upright) at higher speeds.

Follow this link to part 3....

http://www.yachtforums.com/forums/showthread.php?t=202