Wind Pressure & Force on a Sail Calculator

How it works

q = ½ ρ V² (ρ air = 0.002377 slug/ft³, V in ft/s) = 0.00256 × V²(mph) psf; F = q × A × C; heeling moment ≈ F × 0.85 × CE height; sheet load ≈ area × V²(kt) × 0.00431 lb

The force a sail generates is the dynamic pressure of the wind — proportional to air density and the square of the apparent wind speed — multiplied by the sail area and a coefficient that depends on the sail's angle of attack and shape, around 1.0 to 1.5 for a well-trimmed sail. Because of the square law a rise from 15 to 20 knots adds nearly 80% to the force, which is why reefing feels sudden and why depowering (flattening, twisting, easing traveller) works by reducing the coefficient before you reduce the area. Applied at the height of the centre of effort the force creates the heeling moment that the keel's righting moment must balance; when they are equal at 30° of heel the boat is overpowered. The same pressure loads sheets and halyards, giving the numbers winch makers use in their sizing formulas.

Worked example

At 20 kt apparent (33.8 ft/s) the dynamic pressure is 1.35 psf; on 600 ft² with a coefficient of 1.2 that is 975 lb of force, and with the centre of effort 22 ft up about 18,300 ft·lb of heeling moment — 73% of a 25,000 ft·lb righting moment, so it is time for the first reef; the rig would overpower the boat at about 23 kt. The 350 ft² headsail loads its sheet with roughly 600 lb.

Frequently asked questions