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Displacement Calculator

Calculate boat displacement and hull speed from waterline length and beam. Estimate vessel weight and theoretical speed for sail and power boats.

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Inputs that matter
Waterline Length, Beam, Draft, Block Coefficient (Cb)
Output to expect
Displacement Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Hull Speed

Displacement hull speed is the theoretical maximum efficient speed before planing, based on waterline length creating a bow wave the hull cannot escape.

Hull speed (knots) ≈ 1.34 × √LWL (feet)

Displacement Estimation

Displacement approximates from waterline length, beam, and draft using prismatic coefficient for hull type. Sailboats typically 0.45–0.55; power cruisers 0.60–0.70.

Disp (lb) ≈ LWL × beam × draft × Cp × 64 (salt water)

Speed-Length Ratio

Actual speed divided by square root of LWL indicates how hard the hull is pushed beyond displacement mode. Ratio above 1.34 suggests planing or excessive wave-making.

S/L ratio = speed (knots) / √LWL (ft)

Updated: July 2026

Example Scenarios

Hull speed = 1.34 × √26 ≈ 6.8 knots. Comfortable cruising at 5.5 knots (S/L 1.08). Pushing to 7.5 knots wastes fuel and creates excessive wake.

LWL 28 ft, beam 11 ft, draft 5 ft, Cp 0.50: disp ≈ 28 × 11 × 5 × 0.50 × 64 ≈ 49,280 lb (~22,350 kg) loaded cruising weight.

LWL 22 ft, hull speed 6.3 knots. At 25 HP pushing 18 knots the boat planes — S/L ratio 3.8 indicates planing mode, not displacement calculation range.

Common Mistakes to Avoid

Using LOA instead of LWL for hull speed

Hull speed depends on waterline length, not overall length. A 35 ft boat with 28 ft LWL has hull speed based on 28 ft (√28 × 1.34 = 7.1 knots), not 35 ft.

Applying displacement hull speed to planing powerboats

Planing hulls break free of bow wave at planing speed. Hull speed formula applies below planing threshold only — high-speed powerboats operate in different regime.

FAQ

The speed at which wavelength of bow wave equals waterline length. Displacement hulls resist exceeding ~1.34 × √LWL knots efficiently because the hull traps itself in its own wave.

Yes — surfing, planing dinghies, and lightweight catamarans exceed hull speed. The 1.34 formula applies to traditional displacement monohulls in flat water.

Actual displacement is measured by weighing the boat or calculating from immersion (Archimedes principle). Calculators estimate from dimensions for preliminary design.

Added weight increases immersion (more displacement) and typically increases LWL slightly. Heavier boats sit lower but hull speed formula uses loaded waterline length.

Cp compares hull volume to a rectangular block of LWL × beam × draft. Fine racing hulls ~0.45; full-bodied cruisers ~0.55–0.65. Higher Cp means fuller hull and more displacement.

About Displacement Calculator

Displacement and hull speed are fundamental naval architecture metrics. This calculator estimates boat displacement from dimensions and computes theoretical hull speed from waterline length for displacement hulls.