Calculating hull speed and what it means for your insurance
Hull speed is the point beyond which a displacement boat cannot usefully go any faster: 2,43 times the square root of the waterline length in metres.
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A displacement hull pushes a bow wave along in front of it. The faster it goes, the bigger that wave gets, until the boat is caught in its own trough. From that point on every extra revolution burns fuel without adding any speed. The formula behind it is old and simple: 2,43 times the square root of the waterline length in metres gives the hull speed in knots.
For insurance purposes the hull form matters more than top speed. Displacement boats, semi-displacement boats and planing boats show different damage patterns: displacement boats run aground, planing boats slam hard in a seaway and damage the hull and the engine bearers. If you insure your boat as a displacement boat but habitually run it on the plane, you depart from the details given on your proposal form.
In practical terms: state the hull form and the engine power the way the boat is actually run. With us it is engine power that feeds into the premium, not theoretical top speed, and our policy conditions set no fixed speed limit.
Frequently asked questions
Does the formula apply to every boat?
Only to displacement boats. Semi-displacement hulls exceed the figure slightly, planing hulls by a wide margin, as they lift out of the water and get clear of their own bow wave.
Do I have to state the top speed on the proposal form?
We ask for engine power, not for top speed. Our policy conditions contain no speed limit.
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