nope. my understanding is that trying to fully control the luff tension from the top can affect the twist. also, it takes less force to pull the luff downwards from the bottom than it does to try and pull from the top due to the friction created by the luff slides.

what makes sense to me (happy to be corrected) is to use the halyard to set the minimum luff tension, and then use the cunningham to move the draft around.

fishmonkey said..
nope. my understanding is that trying to fully control the luff tension from the top can affect the twist. also, it takes less force to pull the luff downwards from the bottom than it does to try and pull from the top due to the friction created by the luff slides.

what makes sense to me (happy to be corrected) is to use the halyard to set the minimum luff tension, and then use the cunningham to move the draft around.

Same with reefing. Use the halyard to place the new tack about 200mm above the fixed tack point and use the reefing line as the new Cunningham eye.

NowandZen said..
The video talks about moving the draft fore or aft by adjusting the cunningham tension - which therefore increases / decreases luff tension.
I don't understand the physics of how that works, you are pulling 90deg to the mainsail draft.
I thought you moved the draft fore or aft by adjusting the tension on the clew outhaul - tighter in strong winds and loose in light winds.
Can some of the more seasoned (ie: crusty ) sailors explain please?

because of the camber in headsails and mainsails, increasing the luff tension flattens the aft section of the sails and moves the draft forward. the outhaul tends to flatten the lower sections of the mainsail, but in doing so also tends to move the draft aft. so close-hauled in stronger winds you would ideally tension the luff and the outhaul --- you want to flatten the mainsail but also move the draft forward to increase forward drive.

from a physics standpoint, pulling straight up on the luff doesn't only create vertical forces in the sail. for starters, the leech is on an angle so you clearly have angular forces acting on the cloth as well.

i'm not yet crusty, but the salty sailors site has a really clear guide on camber and twist:

www.saltysailors.com/articles-how-to/understanding-sails.html

You also can't pull the sail up past the black band if tensioning the luff with the halyard. The luff has to remain within the p measurement (between black bands).