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Go Back   SailNet Community > General Interest > General Discussion (sailing related)
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  #1  
Old 01-15-2002
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Sail trim for rookies

I try to read everything I can about performance sail trim, but most books and articles are far too technical, and presuppose that I was raised at a yacht club from age 2. I''ve been sailing about six months, 45 years old, so I''m on the fast track!

I''m starting to "get it", and some things are just intuitive, but I have a couple of questions. The boat is 35'', fractional rig, new dacron main, new Kevlar 155, dacron #3, new asym. chute/w sock.

Last Sunday saw very light winds on SD Bay, maybe 2-5 kts, with large patches of nothing.
This was the first time I tried to sail the boat agressively in very light winds. My setup was a very loose foot on the main, the main topping lift slack with highly tensioned leech, all battens pointing to windward. Lots of bag in the sail. Traveler was slightly to leeward. No backstay tension.
The genoa was sheeted slightly looser than on windier days. The foot of the genoa is right on the deck, so I have no way of tensioning the leech with a different sheeting angle.
The boat moved quite well when close hauled, but almost stopped a few times while reaching, with sheets eased a little more.
The problem is the wind was so light and variable, I really couldn''t tell when I had it right.

1. Does this setup sound OK for very light winds?

2. The only way I could gybe the Asym. was to douse it with the sock, pull it over the headstay and reset it on the new side. I did an OK job of blanketing the asym. with the main when setting/resetting. The bunched up sock seems to prevent it from being gybed while set. I believe I''ve seen sprit boats gybe asyms. without dousing them. What am I doing wrong?

Thanks for any responses,

Art
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Old 01-15-2002
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Sail trim for rookies

.... with regards gybing the asymetric in light winds:
Use light weight sheets, and low friction blocks etc.
The Method I use for light flukey winds and BIG asymetricals:
Bring the mainsail to center line so that most of the air flow goes to the A-spinn and keeps it full.
Go close (not all the way) to dead downwind and let the sheet go forward until the A-kite is fully in front of the bow.
Slowly turn to the new tack until the clew is well away from the centerline on the new tack .... and then start to tension the new sheet when the clew is far enough from the center line that the a-spinn will fill with stability. (Caution: If you pull in on the new sheet while the clew is near the centerline the Spinnaker will become very unstable and may fly itself around the forestay.)
Preliminarily trim the A-spinnaker, Release the main and trim it, then retrim the A-spinn.
You can reduce the spinnaker''s sail area by partially ''furling'' it with a spinnaker sock when you first start doing this. After some practice, you wont need or want the spinnaker sock at all.
Hope this helps.
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Old 01-15-2002
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Sail trim for rookies

Thanks RichH,

At the risk of sounding completely stupid (learning how to sail is good for ones'' humility), are you saying the clew rounds the headstay forward of the luff?


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Old 01-15-2002
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Sail trim for rookies

Ahoy aasault, yeah sometimes even aft of the luff depending how much rum you''ve had!!Big Red the Pirate
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Old 01-16-2002
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Sail trim for rookies

Rich is right about light air sheets. Most race boats on the Chesapeake carry kevlar ''dental floss'' for light air sailing. In really light stuff there is not always enough air to support a spinacker no less a really large assymetrical chute. You some times can do better sailing a hot reaching angle flying a very light, flat cut genoa.

On your upwind set up, you might actually do better bringing the traveller well above the centerline and going for lots of twist. This is very conditions dependent but here on the Chesapeake you can get outrageous gradient wind phenominas and so twist becomes critical.

Jeff
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Old 01-19-2002
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Sail trim for rookies

aasault:

In REALLY light air, keep your halyards tight and trim more than you think you should (and don''t use a very big genoa). In very light conditions, the air hasn''t got enough energy to get around the baggy shape you describe and still propel the boat. It seems counter-intuitive, but extremely light air requires the same kind of trim as heavy air.
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Old 01-19-2002
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Sail trim for rookies

Thansk HHJ,

You''re right, it is counter intuitive. Somehow I get the airplane wing concept stuck in my head, where a high aspect ratio and deep section create a very powerful foil, similar to a high performance sail plane.

Art
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Old 02-14-2002
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Sail trim for rookies

Ahoy, Art. Keep the wing concept stuck in your head. The sail and the wing operate on the same principle, the Bornoulli effect. Notice, as existence, that some planes (planes on the air) use sails, and some boats use wings.
May I muse?
I attach the following:
RichH: ...light weight sheets....
(Jeff agreed. I concurr.)
JeffH: You sometimes do better
sailing a hot reaching angle
flying a light, flat cut
genoa.
JeffH: ...you can get outrageous
gradient phenominas and so
twist becomes critical.
I muse:
The bag concept holds for light air. The bag, though constructed of light material, is heavy. It is matter. It takes a weight greater than its own weight to alter its inertia. Some weight must hold it up and press it into shape before it can be an air foil per Bernoulli. Then, airflow over it must produce lift.
It isn''t that there is something wrong with the concept assymetrical spinnaker. In certain conditions, they are excellent.
It is, just as we always underestimate the impact of an increase in wind speed, we always underestimate the impact of the decrease as the wind speed deminishes.
All that running around and bobbing up and down and rolling to and fro is disturbing both the air and the water. IN VERY LIGHT AIR, EFFICIENCY IS PREMIUM. As another indicated, you need the same shape main (and Jeff about the Genoa) that you need for heavier air. Very light air is very light because it has very little weight. It has very little weight because it is either not dense (warm air), or it has little velocity (cold air). Strong systems are highly energetic, but that are also, highly stable. Very light systems have little energy, and little weight, and are therefore easily disturbed.
Also, if I bubble a part of the sail and it begins to bellow, it will have to life dead sail to expand.
Next, the wind strikes a convex surface at the edges first. This tends to straighten the convex, bringing it square to the wind, increasing the area perpendicular to the wind. If I have a flat sail,trimed square to the wind in a calm, the first air will hit it flush, billow the sail easily, and, if the sheets do not cause drag, will push the sail into position and shape quickly.
If you are racing in very light air, accelerators have advantage. It is making hay while the sun shines. I don''t like assymetricals in very light air.
Prior to conclusion: (Jeff will not like this, I am sure, but I found it useful.)
Recall that if the center of effort of the sails is forward of the center of later resistance of the hull, the boat will wind-vane down by the bow - now the larger push - causing lee helm. Recall that heeling places the drive of the sails outside the centerline of the boat, adding turning moment, as the heel changes the underwater shape of the boat, summing to a tendency to head up.
I like to add a slight forward bend in the mast, moving the center of effort forward just a little. I balace that against heel. I strive for the heeling effect (turn up) to balance the pushing off effect of moving the coe forward, producing no helm,eliminating that drag.
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Old 02-14-2002
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Sail trim for rookies

In summary: Why fight the assp? It''s difficulty offsets its gain. Be still and quiet and let the sails and wind interact. Keep your sails flat. If they are flat enough, you can almost sheet them as though you were close reaching. If necessary, have your crew hold the sheets in their hands to keep their weight off the sails. Keep your boat where it has the least amount of wetted surface and the least amount of rudder. React quickly but quietly and gently to the puffs. Acceleration is key. A body, once set in motion, tends to stay in motion unless acted on by an outside source. Momentum. Also, an increase in closing speed, wind and sails, produces an increase in relative wind. Because the air is light, winds aloft are exaggerated. Strange to see twist in sail in such winds as these. You don''t want a parachute you have to fill up. You want a board you can stick up in the air. Oh, well, just a muse.
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Old 02-14-2002
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Sail trim for rookies

First of all - I have to get my insane jealousy out. "I was out on I was on SD bay..." I was on my boat today too. Of course it was 28 degress out, and she''s on the hard and shrinkwrapped for two more months...

In Bethewaite''s "Performance Sailing" he gives a pretty decent overview of the light air, and why things change suddenly around six knots of true wind (give or take).

Basically it ties around turbulent vs. laminar flow of the air over the water. Below 6kts the airflow is laminar. With the laminar wind flow, the air closest to the surface (water) moves the slowest, and the furthest from the friction it moves faster. Think of a leaf in a slowly moving stream - at the bottom it barely moves at all but further up there is more current. In extremely light air the net effect is the same, and most of the sails drive comes from the top of the sails. There will also be an apparent wind difference from the top of the mat down to deck level. So, according to Bethewaite you add twist to the sails in extremely light air to take advantage of the fact that there is more air & wind motion higher up and the apparent anbgle of that wind can be somewhat different that it seems down below.

Over 6 kts and you get turbulent flow. All of a sudden the boundary layer near the water dissapears and the wind velocity is fairly regularly distributed from the water level on up. Drive is distributed across the sail, from the foot up instead of mostly at the top. So lest twist can get more power to the foot, until you have to start dumping air off again in higher wids with twist.

All good in theory of course. In practice, we still haven''t mastered light wind with my crew, and we pretty much got spanked every time the wind dropped below eight knots. I''m not claiming expertise, just relaying what I''ve read...
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