The concept of apparent wind is largely unknown to non-sailors but if you sail a boat it’s a fundamental fact of life: Apparent wind is what you sail in.
Apparent wind is the wind you experience when the boat is moving – it’s the true wind modified by the boats motion. A 15-knot breeze coming at you from 45 degrees off your bow when you’re stationary becomes a 20-knot breeze at about 35 degrees off your bow when you’re moving forward at around 6 knots. The boat speed adds to the true wind speed, and modifies its angle of approach.
Conversely, when the wind is from behind its speed is reduced by the speed of the boat. A 15-knot breeze from dead astern is an apparent wind of just 9 knots when the boat is moving at 6 knots.
When you’re sailing you don’t really think about apparent wind – it’s the wind you’re sailing in and that’s that. However, there is a time when you really need to consider the effects of apparent wind and that’s when you change from a course off the wind to a course on the wind. If you’re running downwind in 20 knots true wind it will feel like a perfectly manageable 14 or 15 knots apparent. But as you slow and turn onto the wind things will become a bit more of a handful, unless you’ve anticipated it.
Apparent wind – the wind you sail in.