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Displacement Hull Wave Speed from Waterline Length

Estimate the conventional displacement-hull wave speed from loaded waterline length, using five length and five speed units with reverse entry.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

A 25-foot waterline gives an estimated wave speed of 6.7 knots with the conventional 1.34 coefficient.

Estimated wave speed (knots) = 1.34 × √waterline length (feet).

Worked example

Enter these known values and leave the other values blank.

Waterline length
7.62 m
Hull speed
12.408 km/h

Assumptions and limitations

  • Use length along the loaded waterline rather than the boat’s overall length. The formula uses feet and knots internally; unit controls convert other displayed units.
  • The 1.34 coefficient is a conventional rounded deep-water wave-speed relationship for displacement vessels. It is an estimate, not a guaranteed top speed or a hard speed limit.
  • Planing, hydrofoils, slender hulls, shallow water, available power, loading, sea state and current can change actual performance. This tool does not model those effects.
  • Both waterline length and estimated speed must be positive. Inverting the formula recovers the length consistent with this estimate, rather than measuring a boat.

Common questions

Can a boat exceed this estimated hull speed?

Yes. This is a displacement-wave benchmark, not a universal speed limit. Hull form and propulsion affect actual performance.

Should I enter overall boat length?

Use the loaded waterline length. Overhangs outside the waterline do not belong in this relationship.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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