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There are two essential types of powerboat hulls: a displacement hull and a planing hull. A boat with a displacement hull may ordinarily be distinguished by a deep “V” shape to its bottom, or hull. This type of hull is designed to push through the water, moving, or displacing the water in a proportion equal to the weight of the boat. Planing hulls, on the other hand, are designed to glide along the water’s surface, displacing considerably less water in its passage.
The displacement hull can be described as floating in the water, even when underway. A planing hull, however, rides on top of the water. Thus, a displacement hull will generate considerable wake or waves astern at displacement speed, while a planing hull generates much less of a wake. Because of the reduced friction in moving atop the water rather than through it, a boat with a planing hull will be weigh considerably less, and is significantly faster than a displacement-hulled craft of the same length.
A vessel with a displacement hull will ordinarily have a larger engine than will a planing hull. The resistance of the water against the deeper hull obviously requires more power to overcome. Planing hulls, in their ability to displace less water, require far less power to achieve greater speed when riding over the water rather than through it.
Displacement-hulled boats are ordinarily heavy vessels with large engines. Trawlers are one very good example of a boat with a displacement hull. Since a boat with a displacement hull rides lower in the water, bow spray can be not only uncomfortable, but copious. The high bow of a trawler-type hull prevents some bow spray from reaching passengers, and interior spaces. The more modern flared bow of some displacement hulls achieves the same thing, with the deck at the bow flaring out over the water.
A distinct disadvantage of the displacement hull is its tendency to wallow in almost any kind of sea, even at displacement speed. Though more stable at speed, a displacement hull is still subject to the motion of the water because the hull is sitting deeper in the water. Displacement speed, sometimes known as cruising speed, is, in essence, the most fuel efficient speed for a specific weight and length of boat.
Displacement hulls are designed to carry significantly heavier loads, and to safely handle heavier seas and higher winds. Planing hulls, because of their shallower hull structure are not designed for heavy seas, becoming less controllable when bouncing from wave to wave. A floating hull is, quite obviously, far easier to maneuver than an airborne hull.
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