Hull speed or displacement speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough dimension increases as well. When hull speed is exceeded, a vessel in displacement mode will appear to be climbing up the back of its bow wave. From a technical perspective, at hull speed the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag. Ship drag for a displacement hull increases smoothly with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed. The concept of hull speed is not used in modern naval architecture, where considerations of speed/length ratio or Froude number are considered more helpful.
Background
As a ship moves in the water, it creates standing waves that oppose its movement. This effect increases dramatically in full-formed hulls at a Froude number of about 0.35 because of the rapid increase of resistance from the transverse wave train. When the Froude number grows to ~0.40, the wave-making resistance increases further from the divergent wave train. This trend of increase in wave-making resistance continues up to a Froude number of ~0.45, and peaks at a Froude number of ~0.50. This very sharp rise in resistance at speed/length ratio around 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. This led to the concept of hull speed.
Empirical calculation and speed/length ratio
Hull speed can be calculated by the following formula: where If the length of waterline is given in metres and desired hull speed in knots, the coefficient is 2.43 kn·m−½. The constant may be given as 1.34 to 1.51 knot·ft−½ in imperial units, or 4.50 to 5.07 km·h−1·m−½ in metric units, or 1.25 to 1.41 m·s−1·m−½ in SI units. The ratio of speed to is often called the "speed/length ratio", even though it is a ratio of speed to the square root of length.
Hull design implications
Wave-making resistance depends on the proportions and shape of the hull: many modern displacement designs can exceed their hull speed even without planing. These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly used by canoes, competitive rowing boats, catamarans, fast ferries. For example, racing kayaks can exceed hull speed by more than 100% even though they do not plane. Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing. Ultra light displacement boats are designed to plane and thereby circumvent the limitations of hull speed. Semi-displacement hulls are usually intermediate between these two extremes.
