True and Apparent wind

[Don Guillette]

Ross (Perryville, MD) brought up the subject of apparent and true wind on another thread and I didn't want this item to get lost there.Previously I wrote about 10 sentences on the subject and my good friend Alan came back and said the following in 1 sentence. He said "if the boat is stationary the wind we feel is true but if your moving the wind we feel is apparent".I can't say anything in one sentence!! So I add, we sail with apparent wind except when sailing dead down wind. Then we sail with true wind.

 

[Ross]

Don, that is apparently true but

Only as to wind direction, wind speed True minus boat speed equals wind speed Apparent. A friend and I were sailing a canoe in just enough breeze to ripple the water. he smoked a pipe and we kept up with the smoke down wind.

 

[Seattle Scott]

Ross, not upwind it don't

Think about situation where true is 15knts and boat is beating upwind at 8 knots. My guess is apparent will be closer to 18 knts rather than 7 knots.

 

[John Nantz]

See Polar Diagram

If you look at a polar diagram (boat speed vs wind speed) there is only one point where there is true wind and that's at the exact center. Everything else on the diagram is apparent wind.And, technically, a point has no area to it so it's a hypothetical spot with no dimension.HOWEVER, if the boat is on a body of water with a current then the above doesn't apply. How's that for some thought?Time to leave.

 

[Ross]

Up wind it adds

down wind it subtracts absolutely. Fifteen kts wind plus 8 kts. boat speed equals 23 kts apparent. when you get to points off the wind then you get into vecters and that is not one we can get very far with here.Edit to add . Just read John's post, with absolutely no wind but with a three kts. current you could broad reach across the stream as though there were a 3 kts. speed.

 

[Stu Jackson]

It's a combination of

direction AND speed. The answer is a simple vector diagram for any direction of true wind and the boat.Stu

 

[Ross]

Oh Oh! the seriousness mongers are getting

involved, lets keep this one fun. Your windex by what ever name you have one will always give you apparent wind direction. If you have an anemometer that will give wind speed and it doesn't care about the fine details of apparent and true, it just likes wind where ever it comes from.A couple of years ago we were trying to sail up river in about five kts straight outa the north add to that about 1 kt. current and we were sailing close hauled east and west and not gaining on the river at all. The bridge was about a 1/4 mile up river and we had a lovely afternoon tacking back and forth across the river without going anywhere.

 

[Seattle Scott]

Ouch, John don't do that

"And, technically, a point has no area to it so it's a hypothetical spot with no dimension."

 

[Ross]

Scott, Does that mean that if

you are at the center of the polar chart that you are nowhere?

 

[Clyde]

To be technically correct! ;D

True wind speed only happens when you are not moving! If you are sailing DDW, the true wind speed is the apparent wind speed plus the boat speed. Technically the boat speed is assumed to be the true ground speed. To get true ground speed you need to know the current vector, which isn't normally known or measured while sailing. If you are sailing DDW against a current equal to your boat speed, then the apparent wind speed is the true wind speed because your ground speed is zero. You can tell when this happens because you aren't moving!If you have a calculator that can do calculations using polar coordinates, the polar equation is:True Wind Vector (Wind Speed, Angle off the bow) = Apparent Wind Vector (Wind Speed, Angle off the bow) - Boat Speed Vector (Boat Speed,0 degrees).The Boat Speed Vector is the boat speed in knots with 0 degrees angle off the bow. The Apparent Wind Vector is the wind speed the boat is measuring in knots and the wind angle measured off the bow. If you don't have a calculator, then you can do it by hand. The trigonometric equation is:True Wind Speed = [{Apparent Wind Speed x cosine (90 - Apparent Wind Angle)}^2 + {Apparent Wind Speed x sine (90 - Apparent Wind Angle) - Boat Speed}^2]^1/2True Wind Angle off the bow = Arctangent [Apparent Wind Speed x sine (90 - Apparent Wind Angle) - Boat Speed / Apparent Wind Speed x cosine ( 90 - Apparent Wind Angle)]I have a calculator that does math using polar coordinates, so it's easy for me to do the math, I just plug in the two vectors in polar coordinates and hit the minus button. Fair Winds,Clyde

 

[Ross]

Clyde , I must say that was )

a windy explaination. But as best I could tell bang on correct. By the time you finish that calculation the wind will have shifted.

 

[Ross]

John, what a delightful devise

I have a windvane that does that. They're great.

 

[Clyde]

It's just a push of the button! ;D

You can get wind instruments that will give you true wind speed and direction, if you have them communicating with your other instruments. Raymarine has the ST60 plus which links the wind, the boat speed, and GPS instruments. It also does the Speed Over Ground (SOG) calculation using the GPS data link.Fair Winds,Clyde

 

[John Nantz]

VMG

The good part about this Datamarine Link system is it has VMG - Velocity Made Good. Really handy item. Gives the boat speed vector toward the direction the wind is coming from with no calculations required by the helmsman.Datamarine (a Massachusetts [if I spelled it correctly] company) came out with this in 1988 and I think it was pretty much ahead of it's time. The plastics they used held up better than the competiton. This helm pod is now 18 years old and still looks quite new. The other things this the 5000 can do is time (up and down), like countdown to the start, and same for distance (great for going through fog to catch waypoints).Down below at the nav station there is a repeater (550) which duplicates and displays all the info at the helm so one can be below doing whatever and, say, keep an eye on the depth to make sure the "crew" (okay, helmsman - er, wife. Err,... make that read 'Admiral'!) isn't doing anything out of the ordinary.All this doesn't eliminate the Windex though - got to have one of those too. The underside of the Windex has reflector tape so with a small (Windex) light aimed "up" one can see which way the thing is pointing at night.If you ask a pilot what they Windex shows they should tell you "it shows the direction the wind last blew". If you can't tell, I like Datamarine. It's really good stuff. Too bad they had to be bought out.

 

[Seattle Scott]

Hey John, where to you get

those stick on yellow arrows to put on your "where the winds at" thingy. "The center of the polar diagram does exist" but we all know that:"it's a hypothetical spot with no dimension"Glad we cleared that up. Just having some fun here.

 

[John Nantz]

Which Wind Thingy???

Scott - which wind thingy do you mean? The Datamarine yellow wind arrow is what it came with and I think the whatcha-ma-call-it is made of yellow plastic or at least coated yellow. It came that way. The other wind item (thingy??? - did I get this right?) is the Windex but it's not yellow. The underside has either white or red or both reflector tape.Can't we keep this apparent wind thread going a bit more???

 

[Seattle Scott]

So how close doe she go?

My normal day sail is in about 10 knts true. Beating I usually am headed at about 35 to 40 degrees apparent and go about 30 -35 degrees apparent when true is hitting in the 15-18 knt range. My sails are in fair to bad shape. Does that sound about right, or am I lying through my staysail?

 

[John Nantz]

Normally about 30-degrees

Normally, in the 10- to, say, 16-knot apparent range, the boat sails well right around 30 degrees apparent. This is typical but under some conditions, the speed is better when the wind angle is a tad more but almost never more than 35 degrees.The sweet spot is about 30 and more than 31 or 32 the boat tends to become less lively - you can actually feel it. In lighter winds it's best to increase the apparent wind angle and open up the gap as appropriate.With sails that have more draft there are a couple ways of looking at how to sail with the AWA (apparent wind angle). If one tries to push 30-degrees the boat will be sluggish and the sail will show signs of luffing due to the draft that shouldn't be there. So... you fall off a tad, get rid of the luffing, and the boat starts heeling more but probably only picks up a bit more speed. Any more increase in the AWA and the boat really starts heeling - especially in slight gusts - and sailing becomes frustrating. That's when it's time to make a trip to the loft for a recut, new sails, or seeing if they have any good used ones around - every once in a while they do.In the 15- to 18-kt range, and even into the low 20s, I still like to stay around 30-degrees but not because of speed. The boat seems to stand up better and is less affected by gusts. With couple sailing I go to a less-intensive work load style. The main traveler, however, gets let out a tad as that's easy to do.Datamarine made a Closehauled indicator that I always thought would be nice to have but never got. This instrument has the sweet spot angle spread out more so you can hit the numbers better but on a H-35 this is probably a little overkill.The picture is of the 5000 which is normally mounted at the helm - I removed it before winter and brought it inside to keep it from freezing as I think this protects the readouts. The picture was taken about three days ago when I was puting the heaters back onboard for the comming cold snap. Everybody was thinking that "winter" was over - were we wrong!

 

[Alan]

Don

I just got onto this post, you said, "So I add, we sail with apparent wind except when sailing dead down wind. Then we sail with true wind." You are talking about wind angle, I assume. Of course, true wind speed and apparent wind speed are not the same going DDW. Can you elaborate?

 

[Benny]

Clyde you are correct and

you have proven Alan's definition. As anyone knows when you are sailing downwind with a following wind of 20 knots and try to head up, things get mighty interesting as you realize that the true wind speed is on the plus side of 25 knots. I think Don is referring to the direction and force of the wind that drives the boat and not nesessarily to the mathematical relationship between the vectors. Which definition is better, I guess as long as you understand what you are working with the end result in sailing performance should be the same.