Work out the wind the boat actually feels. Your own speed adds a headwind to the true wind, so the apparent wind is stronger and further forward when you sail upwind, and weaker and further aft when you run.
When you want to know what the masthead instruments should read for a given true wind, or the reverse: why the wind feels so much lighter after you bear away. It also explains reefing decisions. A boat beating at 6 kn in 15 kn of true wind is carrying sail in about 20 kn of apparent wind; turn downwind and the same breeze feels like single figures.
True wind and the headwind from the boat’s motion are added as vectors. Angles are measured from the bow, 0° dead ahead and 180° dead astern.
AWS = √(TWS² + BSP² + 2 × TWS × BSP × cos TWA)
AWA = atan2(TWS × sin TWA, TWS × cos TWA + BSP)
Beam reach. 12 kn true at 90°, boat at 6 kn: AWS = 13.4 kn at 63.4°. The wind has moved well forward of the beam.
Close-hauled. 15 kn true at 45°, 6 kn: 19.7 kn at 32.6°. Reef for the apparent wind, not the forecast.
Broad reach. 12 kn true at 150°, 7 kn: 6.9 kn at 119.5°. It feels like a gentle breeze, which is why boats get caught over-canvassed when they round up.
| True wind angle | AWS (kn) | AWA (°) |
|---|---|---|
| 40° | 20.0 | 28.9 |
| 60° | 18.7 | 43.9 |
| 90° | 16.2 | 68.2 |
| 120° | 13.1 | 96.6 |
| 150° | 10.3 | 133.0 |
| 180° | 9.0 | 180.0 |
The angle is the same either side; the calculator reports it from the bow without a side.
That covers one side of the boat; the other side mirrors it.
Not on this page. Use the boat speed and the apparent figures in reverse with the same vector method.