Apparent wind is not what you may suffer after too many apparent sprouts with your Christmas dinner this week. The apparent wind of which I speak and have spoken of in the past is an important aspect of sailing that needs to be fully grasped. 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 are running dead downwind at 6 knots in 15 knots of true wind and you then round up onto a close hauled course, the apparent wind goes from 9 knots to over 20 knots quite quickly. It can come as quite a shock! You need to be ready for it.