Calculate the heeling force on your sailboat and estimate its angle of heel and stability
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Download NowThe heeling moment is the turning force that leans a boat over, and understanding it is the difference between a yacht that carries sail confidently and one that is permanently on the edge of a broach. It is simply the wind's side force on the sails multiplied by the height at which that force acts, so it grows fast with both breeze and mast height.
The boat answers with a righting moment generated by its weight and stability, chiefly displacement times the metacentric height, and the two moments balance at an equilibrium angle of heel. A tender, low-GM boat heels a long way under a modest force; a stiff, heavy one stands up to far more before leaning the same amount.
Enter sail area, centre-of-effort height, wind speed, displacement and beam, and the calculator estimates the heeling moment, an approximate initial GM and the equilibrium heel angle, so you can see whether the boat is over- or under-canvassed for the breeze.
Wind Force = 0.00256 × V² × Sail Area; Heeling Moment = Force × CE Height; Righting = Displacement × GM × sin(heel)| Heel Angle | State | Feeling | Action |
|---|---|---|---|
| 0 - 10° | Stiff / under-powered | Flat, easy | Trim for power |
| 15 - 25° | Normal cruising | Balanced | Ideal working range |
| 25 - 35° | Over-powered | Heeled, loading rig | Reef or ease |
| Over 40° | Danger | Straining, hard to recover | Depower immediately |
Typical equilibrium heel bands under working sail.
Formula verified against standard naval-architecture references · Updated October 2026
It is the turning force that leans the boat over, equal to the wind's side force on the sails multiplied by the height of the centre of effort. It matters because the boat can only stay upright if its righting moment balances it, so the ratio of the two sets the angle of heel and how hard the rig and crew are working. Enter your sail, height, wind, weight and beam and the calculator estimates the moment and the resulting heel.
Cut the heeling moment at its two sources, sail force and its height. Reefing reduces area and usually lowers the centre of effort, feathering and traveller ease trim the effective force, and a smaller headsail shrinks both area and leverage. Long term, stiffness from low ballast and careful weight placement raises the righting side of the balance so the same sail heels the boat less.
For cruising, a steady fifteen to twenty-five degrees is normal and comfortable, and beyond about thirty to thirty-five the boat is over-powered, the rig is loaded and recovery from a knockdown becomes slower. Design intent varies: a stiff offshore boat carries more before leaning, a tender day-sailer less. The calculator's equilibrium angle tells you which side of that line a given breeze puts you.
GM is the boat's initial stiffness, and the righting moment that resists heeling is displacement times GM times the sine of heel. A larger GM means the same heeling moment produces a smaller angle, so a stiff, heavy-displacement, low-ballasted boat stands up to more sail than a tender one of the same weight. Low GM gives quick, large heel and a lazy, unresponsive boat that is slow to come back upright.
Heeling moment is the load; stability is the boat's ability to resist it. They meet in the balance between heeling and righting moments that sets the equilibrium angle, and full stability also covers large-angle behaviour, the righting curve and capsize recovery, which this small-angle estimate simplifies. Use it to gauge over- or under-canvassing, and the stability calculator for the complete picture.
Because the steady calculation understates the peak. Wind force grows with the square of speed, so a gust from 20 to 30 knots nearly doubles the heeling moment in an instant, and waves and the boat's rolling motion add to it, so the transient angle well exceeds the computed equilibrium. That is why prudent sailors carry reef points and depower settings against peaks, not the average breeze.
It is a sound first-pass using a simplified initial-GM model, useful for comparing rigs and judging sail-carrying ability, but the true righting behaviour depends on the full form and ballast, reflected in the stability curve, not just a small-angle GM. Treat the angle as indicative, and confirm serious questions of stiffness, capsize and recovery against a proper stability analysis.