Maker Label Studio

Hull Speed Calculator

Estimate displacement hull speed from waterline length and passage time over a chosen distance.

Inputs

Use the vessel's loaded waterline length for the condition you want to estimate.

LWL is the length at the waterline, not overall length.
Passage time is calculated at the estimated hull speed.
Self-tests not run.

Results

Complete the inputs to calculate a passage estimate.

Estimated displacement hull speed

N/A

Results update as inputs change.

Hull speed N/A
Speed in mph N/A
Speed in km/h N/A
Passage time N/A

Method and reference

Cited category: Transport & Maritime. Displacement hull speed uses hull speed (knots) = 1.34 x sqrt(LWL in feet); metric approximation = 2.43 x sqrt(LWL in meters).

Froude-based semi-displacement note: The 1.34 coefficient is a rule of thumb near Froude number 0.40 for displacement hulls. Semi-displacement and planing hulls may exceed this estimate depending on power, hull form, trim, loading, and sea state.

About the Hull Speed Calculator

Sailors, naval architecture students, boat buyers, and passage planners use a hull speed calculator to estimate displacement speed from waterline length. Enter length at the waterline to calculate theoretical hull speed in knots and convert it into trip-time estimates. It gives a useful benchmark for displacement boats while recognizing that hull shape, power, and sea state still matter.

How it works

  1. Enter the boat's length at the waterline, not only overall length.
  2. Calculate hull speed using the displacement speed-length relationship.
  3. Convert knots to miles per hour or estimate passage time if needed.
  4. Adjust planning assumptions for current, sea state, sail trim, and engine power.

Frequently asked questions

What is the hull speed formula?

A common displacement estimate is 1.34 times the square root of waterline length in feet, giving speed in knots. The coefficient is a rule of thumb, not a universal physical limit.

Is hull speed a hard maximum speed?

No. Some boats exceed the traditional estimate through planing, surfing, very slender hulls, high power, or favorable conditions. For many displacement hulls, resistance rises sharply near that range.

Should I use LOA or LWL?

Use length at the waterline. Overall length can include overhangs or appendages that do not determine the main wave-making relationship.

Why do longer displacement boats go faster?

Longer waterline length increases the wavelength associated with efficient displacement motion. That is why two similar displacement hulls can have different comfortable cruising speeds.

Can hull speed estimate arrival time?

It can provide a rough upper benchmark, but passage planning should use realistic average speed after current, tide, wind, sea state, traffic, and safety margins.

References