What apparent wind is
Apparent wind is the wind you feel on a moving boat: the true wind combined with the wind made by the boat's own motion, which always blows from dead ahead at the boat's speed. American Sailing describes it as the wind that fills the sails and drives trim decisions, and Physics Today notes that a sail's lift and drag are worked out relative to it. When the boat is stopped, apparent wind and true wind are the same.
How it works
The calculator adds the two winds as vectors. Taking the bow as straight ahead, with true wind speed T at angle A off the bow and boat speed B:
- Apparent wind speed = √(T² + B² + 2TB cos A)
- Apparent wind angle = atan2(T sin A, B + T cos A)
The default inputs reproduce Sail Train's example: a beam reach in 10 knots of true wind at 6 knots of boat speed gives about 12 knots of apparent wind from about 60 degrees, which is why the sails need trimming in as the boat speeds up.
The model ignores leeway, current, heel and the difference in wind speed between the water and the masthead, so treat the result as the textbook figure, not an instrument reading. Basic wind instruments show apparent wind, and showing true wind needs a boat-speed input, as Sail Train explains.
What to notice
- Upwind, the apparent wind comes from farther forward than the true wind (Wikipedia), and the calculator shows it stronger than the true wind as well.
- Downwind, the apparent wind can be lighter than the true wind, which is why running can feel calmer than it is.
- Fast boats drag the apparent wind forward. Wikipedia notes that iceboats have so little drag that their apparent wind stays close to their heading, so they sail with sails sheeted in on most points of sail.
See points of sail and sail trim for how this changes what you do with the sails.