Hull speed, sometimes referred to as displacement speed, is a rule of thumb used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design. It is usually described as a speed corresponding to a speed-length ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.
In English units, this may be expressed as:
where:
- "LWL" is the length of the waterline in feet, and
- "v" is the speed of the vessel in knots
The constant may be given as 1.34–1.51 knot·ft−½, or in SI units, 4.495–5.065 Km·hr–1.m-½ (where LWL is measured in metres and v is the speed in Kph)
The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio and Froude number are considered more helpful. It is still used by amateurs in relation to traditional displacement hulls.
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Background
Wave making resistance begins to increase dramatically in full-formed hulls at a Froude number of about 0.35, which corresponds to a speed-length ratio of slightly less than 1.20. This is due to a rapid increase of wave-making resistance due to the transverse wave train. At a Froude Number of 0.40 (speed-length ratio about 1.35) the wave-making resistance increases further due to the increase of the resistance caused by the divergent wave train which is added to the transverse wave train resistance. This rapid increase in wave-making resistance continues up to a Froude Number of about 0.45 (speed-length ratio about 1.50) and does not reach its maximum until a Froude number of about 0.50 (speed-length ratio about 1.70).
This very sharp rise in resistance at around a speed-length ratio of 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. On the other hand, these values change dramatically as the general proportions and shape of the hull are changed. Modern displacement designs that can easily exceed their 'hull speed' without planing include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. These benefits are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels based on such concepts.
Since the wave amplitude increases the energy transferred to the wave (like a laser), for a given hull length the wave drag can be very sensitive to the vessel's weight.
References
- A simple explanation of hull speed as it relates to heavy and light displacement hulls
- Hull speed chart for use with rowed boats
- On the subject of high speed monohulls, Daniel Savitsky, Professor Emeritus, Davidson Laboratory, Stevens Institute of Technology
- Low Drag Racing Shells
External links
Useful
Reminder
A knot = 1 nautical mile per hour
1 knot = 1.8520 km/hr
1 km/hr = 0.540 knots
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