True vs Apparent Wind

Although most of my boating time is spent in a kayak, I am fairly comfortable piloting small sailboats, and have crewed on a few larger sailing vessels. However, most of my sailing know-how comes via experience. For example, I can use the sail’s telltales but I do not know the science behind what I am doing.

A friend who is a very good sailor often explains things to me referencing “true vs apparent wind”. For example, we are watching a sailboat race and I comment on a boat pulling an especially effective maneuver to win. My friend explains: “…excellent choice. <mumble>, <hand wave>…true vs apparent wind”.

Can someone explain true vs apparent wind? (without the explanation being too long winded :D)


Thing called “Google”. Totally awesome.

The OP is obviously looking for somebody to explain it to him, not rudely shove Google links at him. For pity’s sake.

And I wasn’t looking for your feedback. And the links are succinct.

Yep, I’ve read the explanation using boat speed/etc on the sailingusa site, as well as a few others. It’s comprehension that I lack, not a source. I last had trig 35 years ago and have not had much use for it until now. Thanks anyway.

For the issue in the OP, and the potential complexity, the links are pretty succinct. The reality of the issue is that math comes into play. You say you don’t understand the science… you seek an explanation that is not long-winded… and I have to say that the links provided do a pretty good job of meeting the conflicting desires posted in the OP.

Apologies for the “Google” comment.

I think I can explain it pretty succinctly:

Have you ever stuck your head out the window of a moving car? The air itself my be still, but you’re experiencing a 60mph headwind. The same is true on a sailboat. Any forward motion is going to produce some feeling of headwind just from the motion of the boat. Now if the air is in motion as well (ie if you were sitting still, you would feel the wind blow) that is the true wind. Those two aspects combine to form apparent wind.

I’ll give you two examples. On a sailboat sailing directly downwind (the wind is blowing from behind) you may feel NO apparent wind. The wind blowing from behind is cancelled out by the headwind you feel from the boat moving. On a sailboat sailing perpendicular to the wind, you have true wind coming in from, say, the right and headwind coming in from the front. Your Apparent wind will be somewhere in between.

From one of my links:

Here is an easy description of true wind and apparent wind. Put you hand outside the window of your car traveling at 60 miles per hour on a still day and your hand will feel a 60 mile per hour wind. That’s apparent wind yet the true wind is zero. What if the car was driving into a 20 mile per hour head wind? Your hand would feel 80 mph. Or if the wind was blowing from behind at 20 mph, your hand would feel 40mph.

OK, I can see that what I need is to review the trig behind the situation. I guess my OP should have just asked if there was a trig-free version.

As far as understanding the science, what I actually meant by “know” was more of an awareness thing.

I can do many things on a sailboat, I just am not thinking about the science behind them.

And thanks, USCDiver for the examples. They point out to me further that I’ll need the math to get beyond 0, 90, and 180 degree situations.:smiley:

Wouldn’t a vessel sailing before a ten knot wind,say moving at seven knots cause the people on board to experience only a three knot apparent wind as they themselves are moving in the same direction as the wind whereas somebody on a raft lying still in the water would experience the actual ten knot effect of the wind?

But I’m only guessing.

^Well, the thing is (IME) it is a rare thing to be sailing in the same direction as the wind.

That’s exactly right. Often when sailing downwind (or very near to it) in light winds you will feel like you aren’t moving if you don’t look at the water because you may have a very minimal apparent wind

Well, if the wind is blowing in the direction you want to go, you won’t have much choice! and you’re lucky!

Never happens! (IME):smiley:

I’m going to take a crack at it.

Apparent vs. true wind doesn’t matter much when the wind is at your back. It matters a lot when you’re sailing upwind.

Apparent wind is not an imaginary thing. Your sails actually work with it. Starting from zero speed, your boat can sail (let’s say) 40 degrees off the actual wind. Once you start moving, though, your forward motion generates its own wind, and not in the true wind’s direction. The combination of the two is faster than the true wind, and its direction is between the true direction and the direction of travel. So, you can turn closer into the wind.

Sailing upwind is seldom a straight line. As your speed increases, you turn a bit into the wind until your sail luff and flutter. Then you turn slightly off the wind, to get the sails working. The cycle starts again. Turn slightly into the wind until you start to stall, fall off to regain power, and so on. Your path is a series of shallow scallops.

The wind direction does shift continuously, but not as much as it seems to the sailboat.

Eh, maybe I helped. Maybe not.

Break it, and you’ll know. :wink:

Or deep scallops in my case.:smiley:

Thanks, AskNott. Between what you and others posted, and some trig review (thanks go out to my 18 yr old daughter), I’ve got a better understanding.

BTW, my daughter thinks I was quizzing her knowledge of trigonometry, and she feels great that I gave her review high marks!