Calculating hull speed and what it means for your insurance

Calculating hull speed means finding the point beyond which a displacement boat cannot usefully go

any faster,

and what sets it is 2,43 times the square root of the waterline length in metres.

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. Premiums for motor boats usually follow engine power rather than theoretical top speed, and hull policies rarely set a fixed speed limit.

Worked out for your type of boat

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