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Old 12-03-2013, 17:40   #1
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Sail Trim - True vs. Apparent Wind

I have read that for all points of sail between Close Hauled and a Beam Reach, optimum sail trim occurs when the Luff of the Mainsail is pointed directly into the wind.

Is that apparent wind (as indicated by the masthead fly) or true wind?
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Old 12-03-2013, 17:44   #2
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Re: Sail Trim - True vs. Apparent Wind

That's going to be apparent wind. Optimum sail trim is a bit more complicated than that, but this is a good place to start.
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Old 12-03-2013, 17:46   #3
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Quote:
Originally Posted by OldFrog75 View Post
I have read that for all points of sail between Close Hauled and a Beam Reach, optimum sail trim occurs when the Luff of the Mainsail is pointed directly into the wind.

Is that apparent wind (as indicated by the masthead fly) or true wind?
No there must be an angle of attack.

All wind you and your sail experience on board is apparent wind, there is in reality no other. True wind is merely a mathematical calculation of the apparent wind with the speed of the boat through the water at zero

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Old 23-03-2013, 12:09   #4
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Re: Sail Trim - True vs. Apparent Wind

It's always apparent wind. Unless you are sitting still, then they are the same.
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Old 23-03-2013, 13:10   #5
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Re: Sail Trim - True vs. Apparent Wind

Apparent, for sure.

The reason why you don't do this (with the mainsail) deeper than a beam reach (even on the rare rigs which permit it) is that the lift vector would be trying to pull the boat to leeward, rather than downwind.

[ It's an interesting question though, and it explains why it pays to ease the luff tension when sailing deeper, so that the sail shape is deeper and the luff is more aligned with the apparent wind. You're trying to maintain 'attached flow' across the sail to the deepest course possible... because you can always get more 'lift' out of a certain sized sail than 'drag'

But to optimise lift, you have to get the wind flowing nicely onto both sides of the luff, using the alignment you describe]

With spinnakers, the luff is still nearly in line with the apparent wind even when broad reaching and running, if the cut of the spinnaker is optimal for the course.
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Old 23-03-2013, 13:19   #6
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Re: Sail Trim - True vs. Apparent Wind

I just trim the headsail for the appropriate spot based on heading (just before luffing). Then adjust the main down until it is "just before luffing", also watching the boat speed. On some points of sail it seems the boat is faster with just a hint of mainsail luff starting.
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Old 23-03-2013, 14:33   #7
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Re: Sail Trim - True vs. Apparent Wind

I adjust till my tell tails are straight and then kick back.

I only pay more attempt than this when some other boat thinks they are going to pass me.
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Old 23-03-2013, 14:55   #8
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Re: Sail Trim - True vs. Apparent Wind

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I adjust till my tell tails are straight and then kick back.

I only pay more attempt than this when some other boat thinks they are going to pass me.
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Old 23-03-2013, 14:57   #9
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Re: Sail Trim - True vs. Apparent Wind

Bear in mind as well that the apparent wind changes with height, as the true windspeed increases but the boatspeed stays the same; hence sail twist. This effect lessens as windspeed increases due to the boatspeed forming a smaller part of the apparent wind, hence more kicker (vang) in higher wind.

Recommend this RYA Sail Trim Handbook - for Cruisers assuming you can get hold of it in the US.
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Old 23-03-2013, 15:20   #10
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Re: Sail Trim - True vs. Apparent Wind

You'll find that fast sailors pay as much attention to the leech as the luff when close hauled. The leech should not "hook" to windward (making a bag of the sail) or fall off to leeward destroying the lift. If you sight up from under the boom, the battens should be reasonably parallel to the boom.

If you have some telltales on the leech, they should stream and not "hook" around either side of the sail. This shows that you have "attached" air flow across the whole width of the sail. A good thing.
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Old 23-03-2013, 20:04   #11
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Re: Sail Trim - True vs. Apparent Wind

It's extremely difficult to know the true wind aboard a sailing vessel. And if you did, the next instant you'd be wrong again.
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Old 23-03-2013, 20:29   #12
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Re: Sail Trim - True vs. Apparent Wind

+5

Learn to use telltales.

Learning to Sail with the Tell Tales
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Old 23-03-2013, 20:48   #13
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Re: Sail Trim - True vs. Apparent Wind

Actually when you look up the main you don't want the "battens" to be parallel to the boom...you want the top batten to be parallel. The other battens will all be more open.
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Old 23-03-2013, 22:40   #14
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Re: Sail Trim - True vs. Apparent Wind

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Actually when you look up the main you don't want the "battens" to be parallel to the boom...you want the top batten to be parallel. The other battens will all be more open.
That can't be right. Twist is always the other way.

The secret to trim is to adjust the sails to suit the conditions. Takes many years of practice. Books and text are useless. Going sailing is the only way. Going with experienced people, racers perhaps, is the best.
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Old 23-03-2013, 22:45   #15
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Re: Sail Trim - True vs. Apparent Wind

Maybe my English could be better or your eyes are wonky but when under the boom looking up the top batten should be in line with the boom. The other battens are not in line but proggresively are more open(away from the boom leeward)
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