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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Topping Lift and Sail Shape
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Thread: Topping Lift and Sail Shape Reply to Thread
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Topic Review (Newest First)
02-24-2011 11:25 AM
jackdale
Quote:
Originally Posted by Skipper995 View Post
You will see a speed decrease as the boom is raised and a speed increase as it is lowered.
If the leech is being choked off, then the boom needs to be raised. This is actually easier with a rigid (spring loaded) boom vang. Hardening the topping lift may interfere with the roach.
02-24-2011 11:06 AM
Skipper995 jackdale, your diagrams for air foils for aircraft are very appropriate for aircraft but they do not cover ultra light air movement nor are they intended to. No aircraft designer is interested in the lift/drag qualities of a foil in 1 - 5 knots of wind for obvious reasons but as sailors we are sometimes required to deal with just such conditions.
My only suggestion is to go and try it out. Run an actual experiment to demonstrate it for yourself. The knotmeter is the final judge. Allow the boat to settle into a course and speed that is stable. Then simply adjust the boom height and give the boat some time to readjust and note the boat speed. Then do it again to verify your readings. You will see a speed decrease as the boom is raised and a speed increase as it is lowered.
02-24-2011 08:05 AM
sailingdog Yes, but in either case, you want to keep the leech of teh sail open...without adjusting the topping lift to support the boom, you can't do that.
Quote:
Originally Posted by aaronwindward View Post
There's evidently some disagreement here among authorities, with at least two schools of thought. I'm still learning; my general understanding was similar to jackdale's, but perhaps things are not so simple...

Dedekam's Sailing & Rig Tuning says that in very light air (2-5 knots) your traveller should by high and pretty much everything else should be really loose, bagging out the sail and maintaining high twist to open the leech.

Mainsail Trimming by Marks disagrees, stating the sail should be (quoting) "flattish" by having the outhaul and backstay both half-on. This is for the same reason Skipper995 gave, to maintain air attachment, whatever that means. (Dedekam did not mention this effect at all; I'm guessing it hypothetically has something to do with formation of a boundary layer?) Marks agrees about the open leech, though.

So the advisibility or importance of flatness is not really clear to me. I think really what is needed here is experiment. SF Bay summer weather is still a ways off, so I think there may be some opportunity yet.
02-24-2011 06:12 AM
aaronwindward There's evidently some disagreement here among authorities, with at least two schools of thought. I'm still learning; my general understanding was similar to jackdale's, but perhaps things are not so simple...

Dedekam's Sailing & Rig Tuning says that in very light air (2-5 knots) your traveller should by high and pretty much everything else should be really loose, bagging out the sail and maintaining high twist to open the leech.

Mainsail Trimming by Marks disagrees, stating the sail should be (quoting) "flattish" by having the outhaul and backstay both half-on. This is for the same reason Skipper995 gave, to maintain air attachment, whatever that means. (Dedekam did not mention this effect at all; I'm guessing it hypothetically has something to do with formation of a boundary layer?) Marks agrees about the open leech, though.

So the advisibility or importance of flatness is not really clear to me. I think really what is needed here is experiment. SF Bay summer weather is still a ways off, so I think there may be some opportunity yet.
02-24-2011 12:20 AM
Faster
Quote:
Originally Posted by Liquorice View Post
I agree with Skipper995!
Below 2 or 3 knots wind the sail needs to be flattened or you can't keep the flow attached.
I'm not arguing flat vs full...what I'm trying to say is that in real light air, the full weight of the boom alone will physically pull the clew down (gravity sucks...) and this will cause the leech to turn hard to 'windward', closing off the leech. This creates a very non uniform camber that cannot promote good attached flow - it is, as I said, like having full flaps on an airplane wing.

Whether you're of the 'flat' or 'full' mindset - you want a smooth uniform curve and a clean exit...

In light air do you crank on the leech line on the genny to induce a 'hook' in the leech??? Same thing.
02-23-2011 11:44 PM
Liquorice I agree with Skipper995!
Below 2 or 3 knots wind the sail needs to be flattened or you can't keep the flow attached.
02-23-2011 12:27 PM
jackdale Full sail for light air.

Flatter sail for heavier air. I will flatten a sail prior to reefing.

Compare that to airplane wings. Short Take Off and Landing planes have deeper camber, while high speed fighters have much thinner wings.

02-23-2011 12:11 PM
sailingdog In very light conditions, a flat main won't generate any lift...and you won't move... also, not supporting the boom will close the leech of the sail, as Faster pointed out, which was my reasoning for supporting the boom with the topping lift...to allow the sail to have a fuller shape to help generate lift and to keep the leech open. Yes, flat sails help with attached flow, but you have to balance keeping the flow attached with sail shape to maximize lift. A flat sail generates almost no lift.

Quote:
Originally Posted by Skipper995 View Post
I have to disagree with sailingdog on one issue however. In very very light conditions the main needs to be made as flat as possible to try and squeeze as much power out of the breeze as possible. In these conditions air flow has a very hard time staying attached to the sail. Detached air flow means loss of efficiency and power. A flat sail allows the breeze to maintain nearly complete flow attachment for best sailing performance.
02-23-2011 11:53 AM
Skipper995 Faster, I hear you and have a suggestion. Next time you're out in ultra light conditions try flattening the sail and give the boat time to establish a steady speed. Then without changing anything else apply your topping lift as you suggest and watch the boat speed. I guarantee it will drop as the boom lifts.
02-23-2011 01:20 AM
Faster
Quote:
Originally Posted by Skipper995 View Post

I have to disagree with sailingdog on one issue however. In very very light conditions the main needs to be made as flat as possible to try and squeeze as much power out of the breeze as possible. In these conditions air flow has a very hard time staying attached to the sail. Detached air flow means loss of efficiency and power. A flat sail allows the breeze to maintain nearly complete flow attachment for best sailing performance.
... the problem is, though, that the weight of the boom will close the leech of the sail off and you'll lose that attached flow because, essentially 'the flaps are on'. The topping lift's support will allow the leech to maintain its proper shape without the force of a reasonable breeze.

Today's rigid vangs can often provide the same function.
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