Hull form and Geometry
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General
A ship's hull is very complicated three-dimensional shape. With a few exceptions an equation cannot be written that fully describes the shape of a ship. For this purpose, graphical description of the hull form is used. Traditionally, it is represented in a lines plan (lines drawing).
Hull form characteristics
Depth (D)
The depth of the hull is the distance from the keel to the deck. If the deck is cambered, or curved, the depth may be defined as the distance from the keel to the deck at the intersection of the deck and side. The symbol used for the depth is D. The depth of the hull is significant when studying the stress distribution throughout the hull structure.
Draught (T)
The draught of the ship is the distance from the keel to the surface of the water. The mean draught is the average of the bow and stern draughts at the perpendiculars. The mean draught is the draught at amidships.
Freeboard
Freeboard is the difference between Depth (D) and Draught (T).
Categorization of Hulls
Displacement Hull
- the hull is supported exclusively or predominantly by buoyancy. They travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always.
Semi-displacement or Semi-planing Hull
- the hull form is capable of developing a moderate amount of dynamic lift, however, most of the vessel's weight is still supported through buoyancy
Planing Hull
- the planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag. They are sometimes flat-bottomed, sometimes V-bottomed and sometimes round-bilged. The most common form is to have at least one chine, which makes for more efficient planing and can throw spray down. Planing hulls are more efficient at higher speeds, although they still require more energy to achieve these speeds. (see: Planing (sailing), Hull speed).
