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Hydrodynamics

The advance of a vessel through water is resisted by the water. This resistance can be broken down into several components, the main ones being the friction of the water on the hull and wave making resistance. To reduce resistance and therefore increase the speed for a given power, it is necessary to reduce the wetted surface and use submerged hull shapes that produce low amplitude waves. To do so, high-speed vessels are often more slender, with fewer or smaller appendages. The friction of the water is also reduced by regular maintenance of the hull to remove the sea creatures and algae that accumulate there. Antifouling paint is commonly used to assist in this. Advanced designs such as the bulbous bow assist in decreasing wave resistance.

A simple way of considering wave-making resistance is to look at the hull in relation to its wake. At speeds lower than the wave propagation speed, the wave rapidly dissipates to the sides. As the hull approaches the wave propagation speed, however, the wake at the bow begins to build up faster than it can dissipate, and so it grows in amplitude. Since the water is not able to "get out of the way of the hull fast enough", the hull, in essence, has to climb over or push through the bow wave. This results in an exponential increase in resistance with increasing speed.

This hull speed is found by the formula:

or, in metric units:

where L is the length of the waterline in feet or meters.

When the vessel exceeds a speed/length ratio of 0.94, it starts to outrun most of its bow wave, and the hull actually settles slightly in the water as it is now only supported by two wave peaks. As the vessel exceeds a speed/length ratio of 1.34, the hull speed, the wavelength is now longer than the hull, and the stern is no longer supported by the wake, causing the stern to squat, and the bow rise. 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. Most large vessels operate at speed/length ratios well below that level, at speed/length ratios of under 1.0.

For large projects with adequate funding, hydrodynamic resistance can be tested experimentally in a hull testing pool or using tools of computational fluid dynamics.

Vessels are also subject to ocean surface waves and sea swell as well as effects of wind and weather. These movements can be stressful for passengers and equipment, and must be controlled if possible. The rolling movement can be controlled, to an extent, by ballasting or by devices such as fin stabilizers. Pitching movement is more difficult to limit and can be dangerous if the bow submerges in the waves, a phenomenon called pounding. Sometimes, ships must change course or speed to stop violent rolling or pitching.