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Hull Speed Calculator

Work out hull speed instantly with clear inputs, formula shown and shareable results.

About Hull Speed Calculator

Hull speed is 1.34 times the square root of waterline length in feet, the speed at which the wave the hull makes has a length equal to the hull itself. A displacement hull trying to exceed it must climb its own bow wave, so resistance rises very steeply — the reason long hulls are fast hulls.

Formula

Hull speed

V (kt) = 1.34 x sqrt(LWL in ft); Froude number = V / sqrt(g x LWL)

Worked example

With the default values below, Hull Speed Calculator returns Hull speed of 8.23. Change any input above and every figure here is recalculated in your browser.

Inputs used

Length on waterline
11.5 m
Displacement
9 t
Speed being considered
7 kt

Results

Hull speed
8.23 kt
Speed to length ratio
1.140
Froude number
0.3391
Regime at the stated speed
Semi-displacement / transition

Step by step

  1. 1Values used
  2. 2Hull speed
  3. 3Hull speed= 8.23 kt
  4. 4Speed to length ratio= 1.140
  5. 5Froude number= 0.3391
  6. 6Regime at the stated speed= Semi-displacement / transition

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Marine results are planning estimates only. Verify against the vessel's approved stability booklet, loading manual, class society rules and applicable maritime regulations before loading or sailing.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Marine results are planning estimates only. Verify against the vessel's approved stability booklet, loading manual, class society rules and applicable maritime regulations before loading or sailing.

Frequently Asked Questions

Can a boat exceed hull speed?

Yes, if it has the power and the hull form to plane or semi-plane. Heavy displacement hulls effectively cannot.

Why 1.34?

It comes from deep-water wave speed: a wave whose length equals the waterline travels at 1.34 x sqrt(length in feet) knots.

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