Hull Speed Calculator

Hull speed is the theoretical top speed of a displacement hull, set by the wave the hull itself makes. As the boat speeds up, its bow and stern waves lengthen until they span the whole waterline, and the boat settles into the trough between them, which sharply resists going faster.

It uses the traditional relation speed in knots equals 1.34 times the square root of the waterline length in feet. So a 25 foot waterline gives 1.34 times 5, which is 6.7 knots. Longer waterlines raise the ceiling, but only with the square root, so the gains taper off.

The knots figure is converted directly, multiplied by 1.150779 for statute miles per hour and by 1.852 for kilometres per hour. At 6.7 knots that is 7.71 mph and 12.41 km/h, shown rounded to two decimals in the result cards.

Enter the waterline length (LWL), the part of the hull actually touching the water, not the length overall (LOA). Only the wetted length makes the wave that governs the speed limit, and bow overhangs and swim platforms do not count.

Yes. Light planing powerboats and multihulls climb over their own bow wave and leave hull speed behind, and a heavy displacement boat can be pushed past it with a lot of extra power for very little gain. The formula marks an efficiency knee, not an absolute barrier.

The 1.34 coefficient is the long-standing default, but naval architects use anywhere from about 1.1 for a full, heavy hull to 1.5 or more for a slim, easily driven one. This tool fixes the coefficient at 1.34, so read the answer as a representative estimate rather than a figure tuned to your exact hull.

No. It depends only on waterline length. Displacement, sail area, engine power, sea state and hull fineness all shift real-world speed, so use this as a baseline and expect actual performance to vary around it.

The waterline length is entered in feet. If your figure is in metres, multiply by 3.28084 first, since the 1.34 coefficient is calibrated for feet and knots and will not give the right answer with metres.

No. The calculation runs entirely in your browser as you type, so your boat's dimensions stay on your device and nothing is uploaded.