Boat Displacement Calculator

Hull speed is derived from the displacement-hull speed/length rule, 1.34 times the square root of waterline length, which models the physics of the bow-and-stern wave pattern a hull generates as it moves — a pattern whose wavelength is set by length, not by beam or draft. Beam and draft do change displacement and waterline area, but in this formula they don't enter the hull speed calculation at all.

Hull speed is the approximate point at which a displacement-mode hull is trying to climb over its own bow wave, and pushing past it requires a steep, inefficient jump in power for each additional knot of speed. It's a soft physical limit rather than a hard wall — many trawlers and sailboats simply never try to exceed it, cruising instead at 80-90% of hull speed for fuel efficiency.

Hull type sets the block coefficient, which represents how much of the length-times-beam-times-draft 'box' the actual underwater hull volume fills. A full-bodied displacement cruiser hull (coefficient 0.55) carries substantially more underwater volume — and therefore more displacement weight — than a planing hull (coefficient 0.35) with the same outer length, beam, and draft, because planing hulls are shaped with flatter, shallower sections designed to lift onto the water's surface rather than push through it.

Yes — the hull speed formula describes the wave-making limit for a displacement-mode hull pushing through the water, but a planing hull with enough power rises up and skims across the surface instead, escaping that wave-making limit entirely. This calculator's hull speed figure is most meaningful for displacement and semi-displacement hulls; a true planing boat's top speed is governed by power-to-weight ratio, not this formula.

The block coefficient this calculator uses is calibrated against a boat's canoe body — the main hull shape — not a fin keel bolted or molded beneath it. A keeled sailboat's total draft can be 2-3x its canoe-body depth, and feeding that full figure into a formula calibrated for canoe-body draft inflates the computed underwater 'box' volume by roughly that same factor. Use the depth of the hull's main body below the waterline instead, excluding the keel, for a realistic displacement estimate.