Hull Speed Calculator

Classic displacement hull speed from waterline length. Enter LWL and an optional speed-length ratio (1.34 default). Returns hull speed, Froude number, and hull-type classification.

Classic S/L

1.34ratio

Semi-displacement

1.34–2.5S/L

Planing

2.5+S/L

1 knot

1.852km/h

Units Switches inputs and speed units

Waterline length (LWL) (ft) Length of hull actually in the water

Speed-length ratio (S/L) 1.34 = classic displacement; higher = semi/planing

Hull speed

7.34kn

Displacement hull

S/L ratio used

1.34ratio

Froude number

0.40Fr

Hull type

Displacement

Metric

13.59km/h

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Tip Hull speed is not a hard limit. It's the speed above which wave-making resistance rises sharply on a conventional displacement hull. Modern slender hulls, multihulls, and planing boats cruise well above it.

Worked example

30 ft LWL cruising sailboat, classic S/L = 1.34.

1. Square root of LWL (ft) √30 = 5.477 2. Hull speed (knots) 1.34 × 5.477 = 7.34 kn 3. Metric conversion 7.34 × 1.852 = 13.59 km/h 4. Froude number check LWL_m = 30 × 0.3048 = 9.144 m V_mps = 7.34 × 0.5144 = 3.78 m/s Fr = 3.78 / √(9.81 × 9.144) Fr = 0.40 → displacement regime

Speed-length ratio reference

Hull type	Typical S/L	Description
Displacement	1.10–1.34	Efficient at low speed, buoyancy-supported, wave-making resistance rises sharply near classic hull speed.
Semi-displacement	1.34–2.5	Extra power and favorable hull form exceed classic displacement limits without full planing.
Planing	2.5+	Dynamic lift at speed reduces wetted area; operates outside the displacement-wave regime.

Common mistakes

Using LOA instead of LWL. Boats with overhangs (classic designs, some cruising sailboats) have LWL well below LOA. Use waterline length actually in the water at normal loading.
Treating hull speed as a cap. It's an inflection in the resistance curve, not a wall. A modern light-displacement hull can cruise at S/L 1.5–1.8 with reasonable power.
Ignoring loading condition. A heavily loaded cruiser sits deeper, trims differently, and effectively has longer LWL but more displacement. Recompute after major loading changes.
Comparing across hull types with S/L alone. Froude number normalizes for scale — a better tool for comparing boats of different sizes.

Warn Hull speed alone tells you nothing about stability, righting moment, or ability to handle heavy weather. It's a resistance reference, not a seaworthiness rating. Passage planning must use vessel-specific data.

FAQ

What is hull speed for a boat?

The traditional theoretical maximum efficient speed for a displacement boat. Estimated from waterline length because longer boats generate longer wave patterns and can travel faster before wave-making resistance climbs sharply.

Can a boat go faster than hull speed?

Yes. Hull speed is a rule-of-thumb for displacement behavior, not an absolute barrier. Boats exceed it with enough power, light enough displacement, slender enough hull, or a shape capable of semi-planing or planing.

Why does waterline length matter more than overall length?

Waterline length is what interacts directly with the water to create the dominant wave system. Overall length may include bow or stern overhangs that contribute little to displacement-wave behavior.

What is the theoretical basis for hull speed?

Wave-making resistance. As speed increases, the wavelength in the hull's wake grows until it approaches waterline length and the boat must climb its own bow wave. Froude number expresses that relationship dimensionlessly.

How does hull shape affect hull speed?

Hull shape affects drag, trim, wake formation, and the boat's ability to transition into higher-speed regimes. A narrow lightweight hull may exceed classic displacement limits more easily than a broad heavy hull at the same LWL.

Sources

SNAME Principles of Naval Architecture NA Displacement-hull theory primers MIT OCW — marine hydrodynamics notes ITTC Recommended resistance procedures

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