Hull speed

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Hull speed, sometimes referred to as displacement speed, was a common rule of thumb used to provide the approximate maximum speed possible of the hull. It is generally described as the 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, it is expressed as:

$v \approx 1.34 \times \sqrt{LWL}$

where: "LWL" is the length of the waterline in feet, and " $v$ " is the speed of the vessel in knots

It should be noted that Speed Length Ratio is not usually used with metric (SI) units.

Hull speed is typically not a term used by naval architects (they use, instead, a specific speed/length ratio or Froude number for the hull in question) but is often used by amateur builders of displacement hulls, such as small sailboats and rowboats.

[edit] 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 maxima 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 1.50 was probably insurmountable in early sailing ships and became an apparent barrier.

It must be pointed out that these values will change dramatically as the general proportions and shape of the hull is changed. Modern displacement vessels can easily exceed their Hull Speed.