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.