Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Hull speed

The speeds of most boats (and of some surface-swimming animals) are limited by the fact that they make a wave due to their motion through the water. A fast motor-powered boat can go faster and faster, until it is going at the same speed as the waves it creates. It may then be unable to go any faster, because it cannot climb over the wave crest that builds up in front of it. Increasing the power to the propeller may not help at all. Putting more energy into the waves doesn't make them go any faster, it just makes them taller and more energetic, and that much more difficult to climb over.

A water wave, unlike many other types of wave, has a speed that depends on its shape: a broader wave moves faster. The shape of the wave made by a boat tends to mold itself to the shape of the boat's hull, so a boat with a longer hull makes a broader wave that moves faster. The maximum speed of a boat whose speed is limited by this effect is therefore closely related to the length of its hull, and the maximum speed is called the hull speed. Small racing boats ("cigarette boats") are not just long and skinny to make them more streamlined - they are also long so that their hull speeds will be high.

Last Update: 2009-06-21