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Engine Power For Boat Speed Calculator

Work out engine power for boat speed instantly with clear inputs, formula shown and shareable results.

About Engine Power For Boat Speed Calculator

The Admiralty coefficient method sizes power from displacement and speed: power scales with displacement to the two-thirds power times speed cubed, divided by a coefficient that depends on the speed-length ratio. It captures the sharp rise in resistance as a displacement hull approaches hull speed.

Formula

Admiralty coefficient

SHP = (displacement^(2/3) x V^3) / C, with C set by the speed-length ratio

Worked example

With the default values below, Engine Power For Boat Speed Calculator returns Engine power required of 1,646.02. Change any input above and every figure here is recalculated in your browser.

Inputs used

Displacement
9 t
Waterline length
11.5 m
Target speed
8 kt
Overall propulsive efficiency
55 %

Results

Engine power required
1,646.02 kW
Engine power required
2,207.35 hp
Speed to length ratio
1.302
Effective towing power
905.31 kW

Step by step

  1. 1Values used
  2. 2Admiralty coefficient
  3. 3Engine power required= 1,646.02 kW
  4. 4Engine power required= 2,207.35 hp
  5. 5Speed to length ratio= 1.302
  6. 6Effective towing power= 905.31 kW

Assumptions and limits

  • The result is derived only from the 4 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 3 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

Why does power rise so steeply near hull speed?

Wave-making resistance grows very rapidly once the hull's own wave system is as long as the hull, so the coefficient C drops and power demand climbs.

Is this accurate for planing boats?

No. Once planing, dynamic lift changes the physics and Savitsky-type methods should be used instead.

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