What D/L measures
The displacement to length ratio (D/L) says how heavy a boat is for its waterline length. Two boats of the same length can differ in weight by a factor of three: a light racer floats on top of the water, a heavy cruiser sits deep in it. D/L puts that difference into one number, which tells you a lot about how a boat behaves: how quickly it accelerates, whether it can surf, how much it pitches, and how much load it can carry before it is affected.
The formula
D/L compares displacement with a cube of the waterline length (LWL), because the volume of a hull scales with the cube of its length.
- Imperial:
D/L = (Disp (lb) ÷ 2240) ÷ (0.01 × LWL (ft))³, with displacement in long tons of 2240 lb. - Metric:
D/L = 27.87 × Disp (kg) ÷ LWL (m)³, the same formula with the conversions folded into the constant.
Because of the long tons and hundredths of a foot, the result is not a pure number, which is why the metric version needs the 27.87 constant to give the familiar values.
How to read it
- Under 100: ultralight. Sport boats and offshore racers that plane readily.
- 100 to 200: light. Modern performance cruisers and most recent production boats.
- 200 to 275: moderate. Classic cruiser-racers and many cruising designs of the 1980s and 1990s.
- 275 to 350: heavy. Traditional bluewater cruisers.
- Over 350: very heavy. Long-keeled cruisers, motorsailers and working-boat derivatives.
These bands are for monohulls. A multihull has a much lower D/L for the same character, so for catamarans and trimarans the calculator only shows where the value sits among other multihulls.
Limitations
D/L uses the waterline length at rest. Boats with long overhangs gain waterline when heeled, so their effective D/L under sail is lower than the number suggests. The displacement is usually the builder's light-ship figure; loaded for cruising, a light boat's D/L can rise by a quarter or more, because the same gear weighs more against a light hull. Over the past decades boats have become lighter for their length, so a value that was moderate in 1980 reads as heavy today. Combine D/L with SA/D for a picture of performance, and with the comfort ratio for motion at sea.
Sources
The ratio and the bands follow the conventions used by Ted Brewer and sailboatdata.com; this calculator reproduces sailboatdata's published D/L values to within 0.2%. See our sources and methodology.