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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 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)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
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.
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.
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.
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.