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post #1 of 23 Old 03-26-2007 Thread Starter
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tacking angle

does wind velosity, light air to medium air change the tacking angle when on a beat ?
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post #2 of 23 Old 03-26-2007
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here's a tricky question...yes and no..

Wind/air velocity change the amount of lift/pull/push on the sail. The faster the air passes thru the sail, the lower the depression created (according to Bernoulli's principle), the higher the pull, the higher you can point, (due to the reduction in the vectors), however, as the wind goes up you have to change sail shape, changing camber (bag), in order to prevent it from stalling / flaping, so you go back to where you were.

So yes, theoretically you should be able to reduce pointing angle, due to increase in relative airflow velocity, but in reallity you can't because of sail shape and boat speed.

Think of point angle as angle of attack on a wing of a plane, which is the angle created by relative airflow and incidence angle.

If you do its almost negligenciable.

NO is the answer in real life. If you want to reduce point angle you have to reduced the angle of attack, which can only be done by trimming sail, during construction of the boat, which dictated where and how sail travelers are located, and how much camber (bag) does your sail have.

Then, there is heel, as the wind picks up, the boat heels more, and by doing so, it increases wet surface on leeward, increasing drag, the sail inclines, looses efficiency due to decreased frontal area...bla bla bla....forget it...

NO

Last edited by Giulietta; 03-26-2007 at 02:22 PM.
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post #3 of 23 Old 03-26-2007
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If a racer like Gui said that it must be true. I specially liked the bla bla part of explanation. Realy good !
in theory there is a small difference. Look at the polar charts for my boat (Beneteau Oceanis 423):
http://www.finot.com/bateaux/batprod...polaire423.htm

You can see that the best VMG is achieced on different angles for different amounts of wind. But a difference is so small that probably Gui's advice was the best possible.
Here is relevant data for my particular boat:
wind in knots 4 6 8 10 12 14 16 20 25 30 35
Up.Vs2.794.105.115.816.206.396.506.646.796.876.91
Up.Bt47.046.144.843.742.741.440.439.940.741.843.5Up.
Vmg1.902.843.634.204.564.804.955.095.155.125.01

So: One should tack differently in stronger wind - but in real life - look at the tell tales (or how are they called in English) and the sails.
If I singlehand I just hit the tack button on my autopilot (set to 90deg.), then I adjust the sails for beat, then I fine tune the course. If I am in the mood I trimm the sails a bit and of course handsteer to follow the slight wind shifts.

Last edited by tomaz_423; 03-26-2007 at 03:06 PM.
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post #4 of 23 Old 03-26-2007
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Tomaz...he's asking about pointing angle, not speed.
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post #5 of 23 Old 03-26-2007
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Table is not readable well. The tack angle would be: 2*(90-Up.Bt)
For example:

In 4 knots of wind you tack 86 degrees while
in 20 knots you tack 100 degrees.

In theory. For this boat. And with no waves, and new sails and clean hull, ... each boat is different. But on my boat it is always close to 90 (while on a good performance racer it is much less).
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post #6 of 23 Old 03-26-2007
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In light air, as your boat accelerates out of a tack, you pull the apparent wind forward, and may have to bear off a little to keep the wind angle wide enough. Or, you could delay trimming those last few clicks on the winch until you start accelerating.

Same thing in heavier air, but less change in apparent wind angle since your speed is less in relation to the velocity of the true wind.
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post #7 of 23 Old 03-26-2007
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in reply to: Tomaz...he's asking about pointing angle, not speed.

I know. But the whole idea of tacking is to get max VMG. You want to have pointing angle that gives you max VMG.
On my boat I can point 30deg. and make no speed or can point 80 deg. and make no upwind gain.
Somewhere in between the two is the best pointing angle to get you upwind fastest.

Last edited by tomaz_423; 03-26-2007 at 03:33 PM.
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post #8 of 23 Old 03-26-2007
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Ahhhh I am suffering of "boom trauma"...not going to discuss

I still believe the real answer is no...sorry
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post #9 of 23 Old 03-26-2007
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Quote:
Originally Posted by Giulietta
Ahhhh I am suffering of "boom trauma"...not going to discuss

I still believe the real answer is no...sorry
Now I fell almost sorry - The last thing I wanted to do is to open the "boom wound".
My post really supports your post -only you said it so much better (I really liked the bla bla).
My boat is more of a cruiser and does not point that good, so there might be some more difference. It is really small. and not noticable for practical purposes.
The theory tells me to tack 86deg. in 4 knots of wind - but in 4 knots I would run the engine if I needed to go upwind.
Then the theory tells me to tack 100 deg. in 20 knots of wind - but at 20 knots we have waves and I start thinking bout the first reef and search a compromise between safety, VMG and comfort (if my wife is with me comfort comes before VMG). In 2o knots I just enjoy and forget about silly theory the designers have put on the web..
In most cases my tack is very close to 90 deg.
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post #10 of 23 Old 03-26-2007
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Tomaz - I think Giu is of the racing breed, then VMG should be PBO (Performance Better than Opponents)
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