Apparent Wind vs. True Wind

[Brian D]

I had a great sail this weekend in the Pacific Ocean. The feeling of maintaining a good boat speed and seeing nothing but horizon is hard to express. I even had a Catalina 34 pass me but could not shake me off because I was almost matching his boat speed. What a great weekend.

But now I think someone with a good knowledge and background of AW vs TW should help us not so understanding people as to what AW really is.

I know from experience that the closer I get to sailing into the wind, the wind velocity appears to increase. And the slower my boat goes, the wind seems to shift and die down.

To help some of novice sailors, lets generate a discussion of Apparent Wind vs. True Wind.

 

[JeffM]

True wind is pretty obvious ... the wind as it's felt and measured from a stationary point. Apparent wind is the combination of the true wind and the wind generated by your boat's motion ... the wind you you feel, the wind the arrow points at, the wind that crosses your sails.

Think of these two examples. If you traveled downwind at 10 knots in a 10 knot breeze, you would feel 0 knots of apparent wind. If you move at 10 knots at 90 degrees to a true of 10 knots, you will feel about 14 knots of apparent wind coming at 45 degrees off your bow (A sq + B sq = C sq).

The bottom line is that your boat's motion always shifts the apparent wind forward, and your sails react to the wind that crosses them, not the true wind. This is one of the reasons that many catamarans do not point very high. Their high speed moves the apparent wind further forward, and at a beam reach their apparent wind may be close hauled.

There's a start to the discussion

Jeff

 

[Tim R.]

A note on bow waves. Smaller boats can take advantage of a bow wave from a larger boats to keep up with them. There is a Tanzer 22 that I race against and if he can get on my stern going downwind he can usually keep up with me. His boat fits perfectly in my bow wave. Good thing I can outrun him on the upwind leg at the start!

 

[sailingdog]

Another problem with apparent and true wind is that the difference between the two can cause you serious problems, especially on a multihull.

For instance, say you are sailing nearly dead-downwind in 20 knots of wind, and your boat is making 15 knots. The apparent wind is only 5 knots... and you've got a lot of sail up for the relatively light winds. Almost every boat can have full sail up in 5 knots of wind. You get to a point where you have to turn 100˚ and head upwind... now, you've got full sails up in 20 knots of wind... and while it is a bit much it is manageable... then the boat starts moving upwind on a close reach, and you're doing 15 knots upwind.... and all of a sudden, the apparent wind is now 30 knots... and you're way overcanvassed—and you capsize.

Going from nearly dead downwind to upwind on a close reach made the apparent wind jump from five knots to almost six times as much.... You do need to keep an eye on how much sail you've got up when going downwind, because if you do need to turn up... you want to make sure that you don't have too much sail to do so safely.

Also, be aware that the true wind your instruments give you are only an approximation. The wind instrument only gives apparent wind with any real accuracy.

For instance: Say you're sailing a course of 0˚ with the wind out of the east at 10 knots, and your boat is moving at 10 knots... The knotmeter on your boat reads STW as 10 knots. The apparent wind is about 14.15 knots at 45˚. (10^2 + 10^2)^.5=10*sqrt (2)=14.15 knots. The wind instrument calculates the true wind to be 10 knots at 90˚, or something fairly close to that, based on the knotmeter reading and the apparent wind sensed by the wind instrument.

Now, say your sailing a course of 0˚ with the wind out of the east at 10 knots and your boat is moving at 10 knots, but with the help of a northernly current moving at 5 knots. The knot meter on the boat now reads STW as 5 knots. The apparent wind is still only 14.15 knots at 45˚, since the boat is still moving north at 10 knots with a 10 knot easterly wind. However, the wind instrument now calculates the true wind to be 11.17 knots at 63.4˚... which is obviously incorrect.

For the wind instruments to calculate true wind accurately, they'd have to use SOG rather than STW. However, none of the ones made to date, AFAIK, use SOG—speed over ground, rather than the less accurate STW—Speed through water.

One other point, which affects multihulls more than monohulls, is that your VMG is often better if you fall off a few degrees, since the increase in speed may well more than make up for the extra distance sailed. This is especially true in faster multihulls, where falling off from 40˚ to 50˚ off the wind may increase the boat speed by 30-40%.

 

[Franklin]

This is one of the reasons that many catamarans do not point very high.
Jeff

and the other is that it doesn't have a keel to generate lift windward...but that's off topic

Brian, I think you slowed down because you were pointing too high and the waves were slowing you down. Normally in light winds, you go faster into the wind because the apparent wind is greater as you've noticed. At a point though, you point too high and the sails force is too much sideways instead of forward.

Waves have a great part of slowing boats down. That's why sailing offshore it's good if you do 125 miles in 24 hours. In winds where you might do 7 knots in protected waters you might be lucky to get 6.

 

[higgs]

I explain it to novices by pointing out that in a car, traveling at 60, the wind is always dead ahead of you and will be around 60 mph because of your forward motion. This is at least a starting point. The principal is the same on a sailboat, but much less pronounced.

 

[sailingdog]

Not when you're in a bad storm... It all depends on true wind speeds. If the true wind speed is 120 MPH... then the fact that you're moving 60 MPH will have an effect, but will not eliminate the true wind's effects on apparent wind. You're vastly oversimplifying it. Of course, if you're out driving with 120 MPH winds, you've got other problems...

I explain it to novices by pointing out that in a car, traveling at 60, the wind is always dead ahead of you and will be around 60 mph because of your forward motion. This is at least a starting point. The principal is the same on a sailboat, but much less pronounced.

 

[Roger Long]

It's not directly applicable to your question but this article covers some related points.

http://www.pointseast.com/template.shtml?id=EEuApppyplMkENKsEG&style=story

The technique described is quite a bit harder to apply on a smaller vessel where things happen faster and there is more turbulence from nearby land and the waves are larger relative to the vessel. Understanding the principles however will improve your helmsmanship to windward.

 

[sailingdog]

Roger-

Great story... I remember reading that in Points East...

 

[Tim R.]

Nice article. Roger, you need to come out racing on Wed. nights at CYC. You can race your boat or you can crew on mine. We need more sailors with your experience out there. It is a very relaxed atmosphere and we always have dinner with all the crews afterwards to talk tactics and do some bragging. Maine Sail is a regular and he can tell you how much fun it is.

 

[Roger Long]

Nice article. Roger, you need to come out racing on Wed. nights at CYC.

Thanks.

You want to know something weird though? Despite my broad experience with things nautical, I have not been in a single sailboat race since being co-captain of the sailing team in high school. No intent to avoid it. I was just always moving in different circles and it was only a couple years ago that I realized that it was true.

I found myself "racing" against the closing of stores when I needed a fuel pump and wrote about my reaction here:

http://files.openomy.com/public/RLMA/FallCruise.pdf

Still, I appreciate the invitation. I'll take you up on it if I'm around but current plans are to be sliding past Cushing Island shortly after my son's high school graduation next spring, not to be seen again in these waters until I've circumnavigated Newfoundland and seen some of Labrador.

 

[Brian D]

Great story. Gonna have to read it again to get a better understanding of what I read. LOL Thanks, Roger.

 

[Ariel]

See the attached for the calculations

http://www.sailingusa.info/true_wind_calculator.htm

 

[ITMaster]

Don't Forget the Effects

Of Lift and Drag not only on the sails, but on the keels of those of us that have them. I think we often overlook the physics on the keel and how this can either work to enhance the effect of True and Apparent wind or totally negate their forces .

Roger, thanks for sharing your article about Lunch in Bermuda, I enjoyed that a lot.

 

[Franklin]

Yep, the die-hard racers here know about falling off and then turning it up, but only the real good ones know how to do it correctly and it shows. It doesn't matter what boat the good captains are on, they always outperform the other boats by a big big margin when going upwind. It is amazing what a good helmsman can do.

In the Regatta De Amigos, a J109 (Surprise) was step for step with a J44 (Kenia) the entire 630 miles except for a couple of shifts as you can see here (http://charthorizon.com/races/2008_galveston_veracruz/htdocs/#). I heard that Surprise brought in a helmsman from England and we all know those Island people have sailing in their blood

A J44 has 8.5' of waterline on the 109 so it should have left it in the dust.

 

[Brian D]

So, Roger. I reread your article and I must ask; basically you were snaking your way windward by falling off a little to regain speed then heading up to point higher then falling off again, and so on and so on. Is that a correct assessment?

 

[Roger Long]

Yes. The basic technique is to head up as slowly and gently as possible and then quickly, almost abruptly, fall off slightly at the first sign of loss of drive. Same thing with tell tales, up slowly till the streamer breaks, then a slight quick nudge to re-establish the optimum flow.

Falling off to reach a higher speed and then coming up very slowly so that the vessel slows to it's best speed made good to windward instead of trying to accelerate up to it will result in the boat settling down at a slightly higher speed. This is easier to do in a big gaff rigger with it's highly twisted sails but the same principle applies to the boats we sail. It's just takes a lot of finesse to make it work.

I'm sure we'll be talking about this more.

 

[Joseph Shirley]

A lot of these answers have the right idea but might be confusing to you. Try this:

Draw a vertical arrow with the head up (that represents your boat speed). Then draw a line 45 degrees from that head, twice as long, and put an arrowhead on it so that the two arrowheads are touching (thats the true wind speed and direction). Next connect the tails of the two arrows making a new arrow with its head at the tail of your first arrow. That arrow is your apparent wind.

Now do it some more with different arrows of boat speed and course and you can see the relationship of true wind to apparent wind on all headings.

Speed to weather is a little different to explain.

If you draw an X Y graph and place a curve in the upper right quadrant that looks like the upper right half of a heart, you would have a partial speed curve. one that represents speed to windward. the wind is coming down X and your max speed to windward is at the horizontal tangent to the curve and you can represent your boat speed with an arrow from the xy cross to that tangent. You will notice that if you sail higher you go slower, (your arrow is shorter) and if you sail further off the wind The arrow gets longer and you actually go faster; but you are making less speed to windward.

A speed curve like that differs for every boat and for every wind speed so you have to learn where that point is for your boat in each wind speed. That's one of the things that separates winners from loosers.

Good luck

Joe S

 

[Bill Roosa]

VMG button

I use the GPS and wind instruments to calculate the velocity made good. Yes, I read manuals. Once I have the desired destination in the GPS I can reset it to give the course a new start point. Then hit the VMG button on the wind instriment and steer till the number is maximum(ized). Re-trim the sails for that course and wind and repeat. After 3 cycles the boat is pretty much making as much speed toward the destination as is possible. It accounts for wind, current strength and direction but will not keep you from running aground. I tried this on several occasions (tired of beating into the wind and out of beer) and the increase in speed "getting there" was significant.
You still have to consider the shore line location and effects on wind here in the Chesapeake Bay so it is still possible to get into trouble.
I have a Ray 300 GPS and ST 60 wind.

 

[newly anonymous]

an easier way to visualize apparent wind:

acceleration always moves the apparent wind forward. deceleration always moves the apparent wind aft.

once you understand that, you've got it, whether you're racing dinks at a collegiate level or motorsailing with blown out sails.