Wave-making resistance

Wave-making resistance is a form of drag that affects surface watercraft, such as boats and ships, and reflects the energy required to push the water out of the way of the hull.

For small displacement hulls, such as sailboats or rowboats, wave-making resistance is the major source of the marine vessel drag.

A salient property of water waves is dispersiveness; i.e., the greater the wavelength, the faster it moves.

Thus, the waterline length of the ship directly affects the magnitude of the wave-making resistance.

Thus, the magnitude of the wave-making resistance is a function of the speed of the ship in relation to its length at the waterline.

A simple way of considering wave-making resistance is to look at the hull in relation to bow and stern waves.

When the vessel exceeds a "speed–length ratio" (speed in knots divided by square root of length in feet) of 0.94, it starts to outrun most of its bow wave, the hull actually settles slightly in the water as it is now only supported by two wave peaks.

The hull is now starting to climb its own bow wave, and resistance begins to increase at a very high rate.

While it is possible to drive a displacement hull faster than a speed-length ratio of 1.34, it is prohibitively expensive to do so.

The disadvantage of this is that planing is only practical on smaller vessels, with high power-to-weight ratios, such as motorboats.

A hull with a blunt bow has to push the water away very quickly to pass through, and this high acceleration requires large amounts of energy.

The bulb alters the waves generated by the hull, by changing the pressure distribution ahead of the bow.

Because of the nature of its destructive interference with the bow wave, there is a limited range of vessel speeds over which it is effective.

A bulbous bow must be properly designed to mitigate the wave-making resistance of a particular hull over a particular range of speeds.

Since semi-displacement and planing hulls generate a significant amount of lift in operation, they are capable of breaking the barrier of the wave propagation speed and operating in realms of much lower drag, but to do this they must be capable of first pushing past that speed, which requires significant power.

[1] The planing hull will rise up clearing its stern off the water and its trim will be high.

[2] A qualitative interpretation of the wave resistance plot is that a displacement hull resonates with a wave that has a crest near its bow and a trough near its stern, because the water is pushed away at the bow and pulled back at the stern.

A planing hull simply pushed down on the water under it, so it resonates with a wave that has a trough under it.