Topping Lift and Sail Shape

[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.

[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.

Originally Posted by 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.

[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.

[jackdale]

Originally Posted by Skipper995

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.