For reference purposes I wanted to publish a table of hull speed versus ship’s water line length. As a rule of thumb, “hull speed” typically establishes the maximum economical cruise speed of a displacement hull as a result of wave making. Hull speed is a speed to length ratio where
Speed in knots / square root of the waterline length in feet = 1.34
I am also including the speed where the ratio equals 2 because this is typically an effective limit on maximum speed.
Like most rules of thumb, there are exceptions, but in most cases, the hull is at least to some degree “planing.”
- Length Hull Speed Speed to Length=2
- 64 10.72 16
- 81 12.06 18
- 100 13.4 20
- 121 14.74 22
- 144 16.08 24
- 169 17.42 26
- 196 18.76 28
- 225 20.1 30
- 256 21.44 32
- 289 22.78 34
- 324 24.12 36
- 361 25.46 38
- 400 26.8 40
Not that it matters to the Coast Guard, but when ships get really large, wave making is not much of a problem. For a ship (like an aircraft carrier) that has a waterline length of say 1,000 feet, hull speed is 42.4 knots and they would not reach a Speed to Length ratio of 2 until they are going 63.2 knots.