True wind and apparent wind
The true wind is the wind you feel standing still. On a moving boat, your own motion creates a headwind equal to the boat’s speed. The sails feel the apparent wind, the vector sum of the two. It is always shifted toward the bow compared with the true wind, and stronger when sailing upwind, weaker when sailing downwind.
The formulas
With true wind speed TWS, boat speed BS and true wind angle TWA measured from the bow:
AWS = √(TWS² + BS² + 2 × TWS × BS × cos TWA) AWA = arccos((TWS × cos TWA + BS) ÷ AWS)
Examples with 15 knots of true wind and 6 knots of boat speed
| True wind angle | Apparent wind speed | Apparent wind angle |
|---|---|---|
| 45° (close-hauled) | 19.7 kn | 33° |
| 90° (beam reach) | 16.2 kn | 68° |
| 135° (broad reach) | 11.6 kn | 113° |
| 180° (dead run) | 9.0 kn | 180° |
On a beam reach the apparent wind moves 22° forward and gets about 1 knot stronger. Upwind, crews feel almost 20 knots and may reef, while downwind the same true wind feels like a gentle 9 knots.
Why it matters
- Sail trim follows the apparent wind, which is why wind instruments and telltales show it.
- Fast boats such as catamarans, foiling dinghies and iceboats can sail faster than the wind because the apparent wind keeps moving forward as they accelerate.
- Heavy weather: turning from a run to a beat increases the apparent wind sharply, often catching sailors by surprise.
Tips
- Enter the true wind angle relative to the bow, from 0 to 180°, on either tack.
- Boat speed through the water is the right input for sail trim; speed over ground matters for navigation with current.
- Instruments compute true wind from apparent wind, boat speed and heading; small calibration errors there show up as true wind shifting when you tack.