What are the best ways to read a Windex?
Mark Matthews responds:
The Windex—the swiveling wind indicator that many sailors mount on the masthead of their boats—is really just a reference point that lets you know where the wind is coming from relative to the heading of your boat. It confirms visually what you should eventually be able to sense otherwise.
The best way to utilize a wind indicator (Windex is actually a trade name) is to look up at it from time to time and relate its direction to your boat's heading. Basically a wind indicator is designed so that it points into the wind. Remember that the wind at the masthead is likely to be slightly stronger and fractionally different in angle than the wind you're feeling on the deck, so what the indicator tells you shouldn't be taken as absolute, just relative. Also, once your boat is under way, the arrow atop your mast is giving you an apparent wind direction reading, not a true-wind direction. (The difference here is important, and a good explanation of it can be found in this article: Understanding Apparent Wind by Steve Colgate.)
If your boat points well upwind, you'll be able to sail close to the angle of the wind indicator. If your boat sails better on a slightly lower course, keep in mind that you shouldn't try to get your course too close the direction of the wind arrow when sailing upwind or the boat will slow down and the sails will eventually stall. After all, the wind arrow up top is just a reference.
You can also benefit by using the wind indicator on the boats around you to see what the wind is doing there and to determine if you're sailing in another vessel's wind shadow.
I hope this helps answer your query.