Apparent wind is the wind you feel on a moving boat: the true wind combined with the wind made by the boat’s own movement. Except on a dead run it comes from further forward than the true wind, and it is stronger sailing upwind and lighter sailing downwind.
When a boat moves, she makes a wind of her own. Motoring at five knots in a flat calm, you would feel a five-knot breeze from straight ahead. Under sail, that breeze combines with the wind blowing over the water, the true wind, and the result is the wind you feel on board. This is the apparent wind, and it is the wind the sails work in.
Because the boat’s own wind always comes from ahead, the apparent wind comes from further forward than the true wind, except on a dead run, when the two come from the same direction. Its strength depends on your point of sail:
- Sailing upwind, the boat’s own wind adds to the true wind, so the apparent wind is stronger than the true wind.
- Sailing downwind, the boat’s speed takes away from the true wind, so the apparent wind is lighter than the true wind. On a dead run it comes from the same direction as the true wind, reduced by the boat’s speed.
This has practical effects. A run downwind can feel pleasant and gentle, but when you turn round to sail home the wind may feel much stronger and the boat heel much more. Decide whether to reef before you turn upwind, not after. And because the apparent wind moves forward as the boat speeds up, you often need to trim the sails in a little as she gathers speed.
Changes in the true wind also show up in the apparent wind. In a gust, the true wind strengthens faster than the boat can accelerate, so the apparent wind moves aft for a moment. In a lull, it moves forward. Experienced helmsmen use these small shifts to steer upwind.
The wind indicator at the masthead shows the apparent wind, and so does the AWA (apparent wind angle) on most yacht instruments. The angles usually given for the points of sail, and the TWA (true wind angle) the simulator’s lessons use, are measured from the true wind.
More in the book: Chapter 3, How sailing works