Learn · 7 min read
Apparent wind explained
Your sail never feels the wind that is blowing. It feels a mix of that wind and the breeze made by the boat's own speed. Understanding this mix explains half of all trimming.
Stand on a dock on a calm day, then ride a bicycle. Suddenly there is a breeze in your face, even though no wind is blowing. A moving boat has exactly the same effect, and its sail feels the result.
Sailors use two names for this:
- True wind: the wind you would feel standing still, for example on a dock or an anchored boat. Its speed is written TWS (true wind speed) and its direction relative to the bow TWA (true wind angle).
- Apparent wind: the wind you feel on the moving boat. Its speed is AWS (apparent wind speed) and its angle AWA (apparent wind angle).
The sail, the telltales, the wind instruments on the mast and the flag on the backstay all respond to apparent wind. Only the weather forecast talks about true wind.
The wind triangle
Apparent wind is simply true wind plus the headwind created by the boat’s speed. Both have a speed and a direction, so they add like arrows. Drawn together they form the wind triangle (industry term: wind vector diagram).
- One arrow is the true wind.
- A second arrow, pointing from bow to stern, is the headwind from boat speed. Its length equals boat speed.
- The arrow that closes the triangle is the apparent wind.
The simulator draws this triangle in the top-right gauge: grey for true wind, green for the boat-speed headwind, blue for the apparent wind.
For readers who like the maths, with boat speed V:
- AWS = √(TWS² + V² + 2 · TWS · V · cos TWA)
- AWA = atan2(TWS · sin TWA, TWS · cos TWA + V)
Example: true wind 12 knots at 90° (straight across the boat), boat speed 8 knots. Apparent wind speed = √(12² + 8²) = 14.4 knots, and the angle = atan(12 ÷ 8) = 56°. The wind feels stronger and comes from 34° further forward than it really does.
The same wind, every direction
The table shows the simulator’s 40 ft monohull in 12 knots of true wind, with the sail kept at its best trim.
| True wind angle | Boat speed | Apparent wind angle | Apparent wind speed |
|---|---|---|---|
| 45° (close-hauled) | 6.4 kn | 30° | 17.2 kn |
| 60° (close reach) | 7.3 kn | 38° | 16.9 kn |
| 90° (beam reach) | 8.0 kn | 56° | 14.4 kn |
| 120° (broad reach) | 7.5 kn | 82° | 10.5 kn |
| 135° | 6.9 kn | 100° | 8.6 kn |
| 150° | 6.3 kn | 124° | 7.3 kn |
| 170° (run) | 5.9 kn | 161° | 6.3 kn |
Two patterns stand out:
- Apparent wind is always further forward than true wind (unless the boat is stopped). The faster the boat, the bigger the shift.
- Upwind, apparent wind is stronger than true wind; downwind, it is weaker. Close-hauled, this boat feels 17.2 knots from a 12-knot breeze. Running, it feels only 6.3 knots.
This is why sailing upwind feels wild and wet, and why sailing downwind in the same breeze feels warm and calm. Nothing changed in the weather; only the apparent wind did.
Why it matters for trim
Because the sail feels apparent wind, its angle of attack depends on boat speed. That has some practical consequences.
As the boat speeds up, sheet in
When the boat accelerates, the apparent wind moves forward. The angle between the wind and the sail shrinks, so a sail that was perfectly trimmed at low speed starts to luff. You have to sheet in as the boat gets going.
In the simulator, switch off Auto-trim, set a trim, and send a gust. Watch the apparent wind arrow swing as the boat speeds up and slows down.
A gust shifts the wind aft, then forward again
When a gust arrives, true wind speed jumps but the boat has not sped up yet. For a moment the true wind arrow is longer, so the apparent wind moves aft (toward the stern) and grows. Sailors feel this as a “lift” and a surge of power. As the boat picks up speed, the apparent wind swings forward again.
Fast boats sail in a “headwind”
The faster a boat is compared with the wind, the further forward its apparent wind sits. On a beam reach in 12 knots, the simulator’s 44 ft catamaran reaches 8.3 knots and feels the wind from 55° instead of 90°.
Very fast boats, such as foiling racers and ice yachts, go so fast that their apparent wind comes from almost straight ahead on every course. They sail with their sails sheeted in tight even when the true wind is behind them.
Apparent wind and choosing a course
Apparent wind also explains why sailing dead downwind is often slow. On a run, the boat’s own speed cancels part of the true wind, so the sail feels a weak breeze. It is also stalled on purpose: with the wind from behind, the sail works like a parachute, pushed by drag rather than pulled by lift.
Turn up a little toward a broad reach and two things happen. The apparent wind grows, because the boat is no longer running away from all of it, and it moves forward enough for the sail to make lift again. In the table above, the monohull is faster at 135° (6.9 knots) than at 170° (5.9 knots), even though 170° points more directly downwind.
That is why many boats sail downwind in a zigzag of broad reaches, turning the stern through the wind between them (a gybe), instead of going straight. The simulator’s polar diagram chart shows this trade-off: it marks the angle that gets you downwind fastest overall, called the best downwind VMG (velocity made good: the part of your speed that takes you straight toward or away from the wind).
The same logic works upwind. Pointing very close to the wind looks direct, but the boat slows and the apparent wind closes up even further. Bearing away a few degrees often gains more speed than it loses in angle. The polar diagram marks the best upwind VMG angle too.
A close-hauled example
Close-hauled at 45° to a 12-knot true wind, the simulator’s monohull sails at 6.4 knots. Put those numbers into the formula and the apparent wind comes out at about 17 knots from 30° off the bow. The boat is pointing 45° from the true wind, but the sail sees a wind only 30° off the bow, and 40% stronger than the breeze actually blowing. That narrow, strong apparent wind is why the sail is sheeted in tight upwind, and why spray flies.
Common confusions
“The wind instrument says 17 knots, but the forecast said 12.” Both are right. The instrument measures apparent wind; the forecast gives true wind. Most modern instruments calculate true wind from apparent wind and boat speed.
“I turned downwind and the wind died.” It did not. You started moving with it, so less of it reaches you.
“My sail was perfect a minute ago.” If the boat has sped up or slowed down, the apparent wind has moved, and the trim has to follow.
Key takeaways
- The sail always feels apparent wind: true wind plus the headwind from boat speed.
- Apparent wind is further forward than true wind, stronger upwind and weaker downwind.
- When the boat accelerates, sheet in; when it slows, ease.
- Gusts swing the apparent wind aft for a moment, then forward again as speed builds.
Next: learn to read what the air is doing on your own sail with Reading telltales and trimming the sail.
The simulator uses a simplified physics model: airflow around the sail, stall and luffing, hull resistance, heel and round-ups. It shows the right trends for learning; it does not predict exact numbers for a real boat. Numbers quoted on this page come from the simulator's 40 ft monohull unless stated otherwise.