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Displacement Hull Speed Calculator

Estimate a displacement boat's theoretical maximum hull speed from its waterline length, using the classic 1.34√LWL rule of thumb.

Hull speed
7.08 knots
Hull speed (mph)
8.14 mph
Hull speed (km/h)
13.11 km/h

How it works

A traditional displacement hull (one that pushes through the water rather than planing over it) can't easily exceed a speed governed by its own bow-and-stern wave train: hull speed (knots) = 1.34 × √(waterline length in feet). Past that speed-length ratio of about 1.34, the wave the hull generates becomes longer than the hull itself, the stern squats into the trough behind it, and pushing faster costs disproportionately more power for very little extra speed.

The 1.34 constant is calibrated to waterline length measured in feet, so an entered metric length is converted to feet internally before applying the formula, then the result is converted to mph and km/h for reference (1 knot = 1.15078 mph = 1.852 km/h exactly, from the international nautical mile).

This is a rule of thumb, not an exact physical law — it describes traditional displacement sailboats and trawlers reasonably well, but says nothing about hulls designed to plane (most powerboats, dinghies at speed) which can and routinely do exceed it.

FAQ

Can a boat go faster than its hull speed?

A displacement hull can be pushed past hull speed, but only by adding power at a sharply worsening return — resistance rises steeply as the hull tries to climb over its own bow wave. A hull specifically shaped to plane (rise up and skim the surface rather than push through it) escapes this limit entirely once it has enough power and speed to get up on top of the water, which is why planing powerboats can far exceed the hull-speed number this calculator gives for their length.

Is the 1.34 constant exact?

No — naval architects generally treat it as a useful rule of thumb rather than a precise law, and some prefer a slightly different constant or a fuller hull-resistance curve for serious design work. It's a widely used approximation across the sailing and boating world, not an exact hydrodynamic constant, so treat the result as a reasonable estimate rather than a guaranteed ceiling.

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