True vs Apparent Wind Calculator
Convert between true and apparent wind in either direction, with a vector diagram — and see why the wind forward of the beam is never what you feel.
Apparent wind always shifts forward
What you feel on deck is the vector sum of the true wind and the wind your own motion creates. Because the boat is always moving forward, that sum always lands forward of the true wind angle — never aft of it. It is why the masthead fly on a moving boat never quite agrees with the flag on the clubhouse.
Sailing upwind the two add, so it feels windier than it is. Bearing away, they increasingly subtract. This is the trap: on a run the boat feels calm and benign, and the wind you were handling perfectly well is still there — you have simply stopped feeling it. People carry too much sail downwind for exactly this reason and discover it when they round up.
The crossover is not at 90 degrees. Apparent wind only drops below true once you are meaningfully abaft the beam, and the faster the boat the deeper you must sail before it eases — a 10-knot boat in 14 knots of breeze has to get past about 111 degrees. That is why fast boats reach rather than run: they keep generating their own wind.
How to use
- Enter boat speed and heading relative to the wind.
- Enter either the true or the apparent wind.
- Read the other, with the angle.
- Check how much it shifts as boat speed changes.
Frequently asked questions
What is the difference between true and apparent wind?
True wind is the wind over the water; apparent wind is what you feel on a moving boat — the vector sum of the true wind and the wind created by your own motion. Instruments measure apparent; forecasts give true, which is why the two rarely agree.
Why does the apparent wind move forward?
Because your own motion adds a component from dead ahead. Sailing upwind, this shifts the apparent wind forward and increases its strength, which is why upwind sailing feels windier than downwind at the same true wind speed and why sails are trimmed to the apparent angle.
Why does downwind sailing feel so much calmer?
Because your boat speed subtracts from the true wind. Running downwind at 6 knots in 15 knots of true wind leaves 9 knots apparent, which feels benign — and is why boats get caught with too much sail up downwind and find out when they turn back.
How do fast boats sail faster than the wind?
By generating so much apparent wind from their own speed that they can sail at a deep true angle while their sails see a tight apparent angle. Foiling boats and iceboats exploit this to reach multiples of the true wind speed, which is counterintuitive and entirely consistent with the vector arithmetic.
Which should I use for sail trim?
Apparent, always — the sails only know what is actually flowing over them. True wind is for navigation, tactics and deciding which way to go, since it is the frame the racecourse and the forecast exist in.
Does apparent wind matter when anchored?
Not in the same way, since boat speed is zero and the two are identical. It matters again as soon as you are moving, including on a mooring in a strong tidal stream, where the boat's motion through the water is not zero even if it is stationary over the ground.
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