Hull Design 101; Part 1...

DESIGN & DEVELOPMENT: HULLS
by Carl Camper

The design of a hull determines the effectiveness of its application. There are several types of hulls and many variations. Hull designs are generally a compromise of stability, efficiency, and function. The following list describes the three basic hull types...

DISPLACEMENT: A displacement hull is normally considered a relatively slow hull. It is supported by the weight of the water it displaces (static water pressure). This hull typically runs with a large amount of wetted surface.

PLANNING: A planning hull operates much like a displacement hull at slower speeds, but with sufficient power (speed), this hull develop hydrodynamic lift and will rise towards the water’s surface and operate in a “skimming” mode, or planning mode. With sufficient speed, this hull is supported primarily by water’s velocity pressure. Planning hulls are generally faster and more efficient than displacement hulls at higher speeds, whereas a displacement hull is generally more efficient at slower (off-plane) speeds.

SURFACING: A surfacing hull incorporates several hydrodynamic and aerodynamic effects. A surfacing hull will achieve the greatest speed due to lift generated by increasing water velocity and air pressure.

With a basic description of the three hull types, the following list outlines the most common variations and designs associated with modern hulls...

FLAT BOTTOM: A hull bottom without a “V” shape is flat from one side of the chine to the other, viewing the hull from the stern. A cross-sectional view would have little or no angular shape from side to side.

“V” BOTTOM: A hull bottom with a “V” shape is angled down towards the center of the hull, viewing the hull from the stern. A cross-sectional view would feature an angle, measured in degrees, from the center of the hull up to each chine. Variations of the “V” include; Deep-V and Modified Deep-V’s.

CATHEDRAL OR TRI-HULL: A hull bottom with more than one “V” shape. This hull typically is a combination of a “V” bottom hull and a tunnel hull, featuring “hard” chines, similar to sponsons.

TUNNEL OR CATAMARAN: A hull bottom with the “V” sectioned-off or separated at center, creating a passage-way between the sections to entrap air.

TUNNEL “V”: A hull bottom featuring a shallow “V” section at center with twin-tunnels on each side of center. Similar to a cathedral design, but more accentuated.

TERMINOLOGY...

To facilitate a better understanding of the phenomenon’s associated with hull design, the following list will help get you better acquainted with terminology associated...

CHINES: That portion of the hull where the bottom of the hull meets the side of the hull. This is usually a sharp angle and can be referred to as a Hard Chine or a Soft Chine. A Soft Chine may offer a smoother ride, but may not offer the spray deflection and stability of a Hard Chine. A Hard Chine will generally offer greater speed.

DEADRISE: The angle of the “V” shape from the center of the hull, measured in degrees to the chines, is referred to as the deadrise. The deeper the deadrise, the deeper the center portion of the hull will ride in the water and thus a smoother ride may result. A shallow deadrise hull may not ride as smooth in rough water, as it develops more lift , generally riding higher, but will offer greater lateral stability at rest or at slower speeds. Deadrise is the angle on each side of the keel, that the bottom of the hull would create if an imaginary horizontal line ran through the keel, usually measured at the transom. Deadrise angle is not always constant from stern to bow.

STRAKES: A usually protruding appendage running lengthwise along the bottom of the hull to enable greater lift, stability, speed, tracking, and aeration of the wetted running surfaces. Strakes are generally Step Shaped, V-Shaped, or Reversed. A strake can add grip when cornering or resistance against lateral slippage. Strakes are normally used, to a varying degree, in multiples and sometimes staggered placements. Among their many attributes, strategically placed strakes can be used to deflect spray and soften a hulls ride while adding stability, handling and speed.

HOOK: A usually intentional design where a concave area, running fore and aft, near the transom, increases lift thereby pushing the bow of the craft down. This enables faster planning with reduced bow-rise and generally aids in reduced porpoising at higher speeds. However, this increases the wetted running surface of a hull, which may result in a loss of speed or poor handling characteristics.

ROCKER: The curvature of hull’s bottom from the bow to the stern (best viewed from the side of the craft). The bottom of the hull will appear to have on outward convex shape. This shape can result in more bow-lift and reduced running surface, but can cause porpoising. Some hull designs feature a rocker for favorable characteristics and hook to cancel out unfavorable ones.

PAD: A flat portion of a deep “V” hull at the bottom dead center, running lengthwise from the transom, forward to the bow. The Pad will generally taper-off into a sharper “V” shape as it progresses up the bow. A Pad is a flat running surface that develops lift allowing the hull to plane on a smaller surface area, thus reducing drag and increasing speed. A Pad can be long or short, wide or narrow. Each has benefits and drawbacks.

LENGTH: A hulls length (and the added weight associated) can add longitudinal stability, especially in rough water conditions. A longer hull has the ability to “straddle” choppy water, with less porpoising and a resulting smoother ride. A shorter hull will generally be more maneuverable and lighter.

BEAM: This is the width of hull. A wider hull will add greater lateral stability at rest or when underway. A wider hull may be more susceptible to “wave-slap” and therefore may not offer as smooth a ride as a narrower design. A wider hull may be utilized for more load carrying capability.

FREEBOARD: This is the height of the hull from the waterline to rub-rail. A boat with greater freeboard will generally offer a drier ride, but may have a higher center of gravity, and thus less stability.

DRAFT: This is the amount of water (or depth) that a hull displaces at rest. Generally, a hull with greater draft will have greater stability and greater resistance to outside forces, i.e., wind or waves, as the total submerged area of the hull provides a greater foundation to cancel the effects of outside forces.

Follow this link to part 2...

http://www.yachtforums.com/forums/showthread.php?t=203