Two speed numbers for multihulls
Catamarans and trimarans are not limited by hull speed the way monohulls are, so their performance depends mostly on how much sail they carry for their weight. Multihull sailors use two numbers to compare that potential: the Bruce number and Kelsall's KSP. Neither is a precise prediction; both are handy for ranking boats against each other.
The Bruce number
Named after Edmond Bruce of the Amateur Yacht Research Society, it is a power-to-weight ratio:
BN = √SA (ft²) ÷ ∛Disp (lb). The formula mixes square feet and pounds, so it is not a pure number;
in metric it needs a constant: BN = 2.521 × √SA (m²) ÷ ∛Disp (kg). Sail area is the mainsail plus the
100% foretriangle, without overlapping or downwind sails.
- Under 1.0: slow. Derek Harvey treats 1.0 as the dividing line between slow and fast multihulls.
- 1.0 to 1.3: moderate. Chris White notes that boats below 1.3 are slow in light winds.
- Over 1.3: fast, with good light-air performance.
KSP
Derek Kelsall's sail performance number adds waterline length, because a longer hull carries the same power to a
higher speed: KSP = 0.5 × √(LWL (ft) × SA (ft²) ÷ Disp (lb)), or in metric
KSP = 2.0 × √(LWL (m) × SA (m²) ÷ Disp (kg)). A rough reading: KSP × 10 is the boat speed in knots
you can expect in 10 knots of wind, so 0.6 means about 6 knots and 0.7 about 7 knots.
Limitations
Both numbers use published light displacement. Cruising multihulls are sensitive to load, and a catamaran full of water, fuel, stores and crew can weigh 20 to 30% more, which lowers both numbers noticeably. Neither considers hull shape, the drag of daggerboards or keels, windage or the quality of the sails, and neither says anything about stability or safety. They compare multihulls with each other; a monohull's Bruce number is not comparable.
Our catalogue has KSP figures for many multihulls, taken from sailboatdata.com. They use the builder's reported sail area, which can include a larger headsail than the 100% foretriangle, so a KSP calculated here from a measured sail plan can come out lower than the catalogue figure on the yacht page.
Sources
Bruce number: Chris White, The Cruising Multihull, and Derek Harvey, Multihulls for Cruising and Racing. KSP: the formula as published in sailboatdata.com's calculations help, which this calculator reproduces to within half a percent; we have not been able to check it against Derek Kelsall's own publications. See our sources and methodology, and the SA/D calculator.