Sailing Apparent Wind Knots Calculator

Sailing wind speed in knots

A sailboat doesn't just travel at the wind speed. What actually matters is VMG (Velocity Made Good), the component of the boat's velocity projected onto the direction of the wind or the next mark. Sailing close-hauled, boats point 30–40° off the true wind, and the apparent wind speed equals the vector sum of true wind and boat speed. Head downwind to 180° and the apparent wind subtracts boat speed instead. An ILCA dinghy (Laser) cruises at 4–6 knots in a 12-knot breeze. A foiling AC75 from the America's Cup, on the other hand, tops 50+ knots in 20-knot wind by sailing faster than the wind itself.

Applications

In America's Cup tactics, skippers optimize VMG to lay the next gate. The ORC and IRC handicap rules rely on polar diagrams that convert wind speed and angle into expected boat speed. You'll also see it in Olympic fleet racing (ILCA, 470, Nacra 17), in match racing tactics, and in recreational cruising, where it helps pick headsail size by Beaufort.

FAQ

Why use knots instead of km/h? One knot is one nautical mile per hour, which lines up with minutes of latitude. Since every chart and GPS works in knots, wind and boat speed end up sharing the same unit.

What is VMG? It's the component of boat speed projected onto the line to the wind, or to the mark. Point too high and you lose speed; bear away and you gain speed but stretch out the route. The sweet spot for VMG sits somewhere in between.

How does a foiling boat sail faster than the wind? It pulls energy from the apparent wind. As the boat accelerates, the apparent wind shifts forward and grows stronger, which lets the foil and wing keep generating thrust well beyond the true wind speed.