Understanding how Apparent Wind works and its impact on how you should efficiently sail the boat is crucial, and even though it may seem theoretical at first it’s well worth mastering this subject. You may get some “aha moments” while reading the theory as you would have seen the effects on the water already.
What is Apparent Wind?
The wind you experience while moving with the boat is called Apparent Wind. It’s the combination of the True Wind and the Boat speed Wind.
Apparent wind changes in wind gusts
So, what happens when your boat sailing close hauled suddenly is hit by a stronger breeze – a wind gust?
The first thing that happens is that the True Wind component increases, and hence the Apparent Wind also increases and changes direction. This means that your leeward telltales will suddenly start spinning, the boat will be heeling over from the stronger wind and it feels like you are getting a lift, although the True Wind still comes from the same direction as before.
When the boat is hit be the more powerful breeze it will either accelerate and eventually the True Wind direction will become more or less the same as it was before the gust, or if the boat has already reached its maximum speed it won’t accelerate further. Hence the way to respond to these gusts will vary if you’re in e.g. a small planing skiff or in a large keel boat.
and in lulls…
The opposite obviously applies when sailing into a lull (less wind). Suddenly the Apparent Wind changes direction and your windward telltales will start spinning around. It’s tempting to bear away, although if it’s a very short lull it may be better to keep your course waiting for the wind to fill in again. There may also be a temptation to tack as it feels like you’ve been headed. The decision whether to tack or not should then be based on where the best wind strength is and other tactical aspects such as what other boats are doing. Just be aware it’s not a real header you sailed into.
The boat will also slow down in the lighter breeze and eventually the Apparent Wind direction will be similar to before. Again, changes in boat types will react differently, where a large heavy boat will keep its boat speed much longer than a dinghy would.
Why fast boats don’t sail dead downwind
Let’s now look what happens when we sail downwind. See how the Boat Speed Wind and the True Wind are opposite resulting in a small force of Apparent Wind speed. At times at the end of a gust, or surfing down a wave, you may even sail faster than the wind and suddenly all power goes out of the sails.
For fast planing boats going dead downwind is rarely a good option as they will be so much faster reaching that the extra distance they will need to cover is more than well compensated for in better boat speed. By sailing at more of a reaching angle these boats will generate more apparent wind which in turn allows them to sail much faster.
Apparent Wind and current / tide
If you’re sailing in a current your boat speed component will also be affected by the moving water. We’ll look further into this in another article.
I’m sure there’s a lot more to be said about apparent wind, but this hopefully should serve as an introduction. As always we welcome questions and comments to the article.