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What is a Hull?

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  • Written By: Jonathan Stevens
  • Edited By: Bronwyn Harris
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  • Last Modified Date: 27 November 2016
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The hull is the basic structure of a boat, not including the masts (if any), rigging, above board constructions, or attachments of any kind. Typically, it is curved along the bottom, angular toward the bow (front) and somewhat squared at the stern (rear). There are flat-bottom boats and different bow and stern shapes, but most hulls are especially designed with hydrodynamic considerations as described above.

There are three basic hull designs, and no matter which is used, all watercraft create a bow wave as they push through the water. At slow speeds, this is a relative non-factor but at greater speeds, the wave becomes larger and higher and the boats will try climbing it. Some boats are not built to ride a bow wave and will remain deep in the trough of their own waves. Others are purposely designed to ride up the bow wave and move atop it, while still other hulls will only partially rise up.

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The first kind is called a displacement-hull. Any boat of this design pushes through the water, displacing it at a volume equal to the boat’s weight. Once the higher speed is attained and the bow wave is of sufficient size, the displacement-hull cannot rise up to ride the wave and is therefore limited in speed. Longer hulls of this type can reach higher speeds due to their greater weight which, in turn, displaces a larger volume of water. To calculate a theoretical top-end speed of such a shape, it's first necessary to find the square root of the length of the vessel. This is multiplied by 1.34. The resulting number is the estimated top speed the vessel can reach, measured in nautical miles per hour (knots).

The second type of design is called a planing-hull. This type is built to ride up and atop the larger bow wave created by faster speeds. Once the boat rises to plane, the energy previously required to push the water aside is repurposed into forward movement on top of the water. When operating at such speed, the planing-hull design displaces less water than its own weight. For the most part, the basic design feature that allows planing is found in the shape of the bottom surface of the hull. It is usually less rounded than the displacement-hull designs.

A third type of design is a hybrid of the above two types and is commonly known as a semidisplacement hull. This type can lift the boat to a median plane; however, it cannot attain a true planing position.

Knowing which kind of hull is best requires understanding the advantages and disadvantages of each. Displacement designs are typically safer, more fuel-efficient, and easier to handle in rough weather and choppy seas. Freighters are a good example of a displacement hull in practice. Planing-hull designs are fast, less efficient with fuel, and work best on calm waters. Speedboats and racers are good examples of this type.

Semidisplacement hulls are not as fast as a planing-hull and not as fuel efficient as a displacement hull, but they can go faster than the latter and handle rougher seas than the former. Recreational cruisers are good examples of this type.

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