More technical stuff
I am back after 3 months of R&R (with my opinions)
but in this case it is fact.
If the system (luff attachment) were frictionless, it wouldn’t matter if you pulled up on the head or down on the foot, the tension in the luff at any one point would be the same given value unique to that station along the luff. This tension is independent of the direction of pull. Keep in mind; other things effect the static tension at a particular height station along the luff,(weight of the sail for one). By just hoisting the sail at the dock, the entire weight of the sail is equal to the tension along the luff at the head, where down at the foot is close to zero.
Most luff attachment systems have friction, the worst being a boltrope in a groove and the least being a bat-car type system. However, even if you had a bolt rope system and all you had was halyard tension to tighten the luff, you could luff up, then take more halyard tension OR you could crank it tighter then luff up, then resume heading. In both cases, unloading the friction from the luff, so as to let the tension equalize.
One problem racers have, and this was the basis for Briggs Cunningham to have designed this innovation, was that in the racing rules you can’t exceed a luff and foot length and that was designated by white bands on the mast and boom (so other racers could see and keep you honest) these bands were painted (on the mast) at the head, foot (tack) positions when the boat was measured, also on the boom at the clew. Now in light air this is ok, you are at your upper and lower bands maximizing sail area but not exceeding it by going past the white bands. But now the wind comes up and the draft is moving rearwards and you have to pull it back forward so you engage the Cunningham and do some tension WITHOUT exceeding the white luff bands. So this system lets you adjust the draft and flatness of the main and still maintain maximum sail area in light air. If you didn’t have a Cunningham setup, then in light air, the sail would have a shorter luff (inside the white bands),to maintain the proper shape and you would pay the penalty of less sail area in light air.
This setup is especially beneficial with stretchier material. Today’s exotic materials minimize stretch (and get tension) and the Cunningham system is less needed, however most boats can use them. My luff on the 46 is 54 feet and when going up in 20kts< do about 5 inches of Cunningham. This is with a new 5 batten, 10 oz Dacron main ,
BTW, Seth, I had that sail re-cut as per a post you made some time age, where you said that even though the sail is new, the sail maker might not have nailed the shape right. This was true in my case, and now I have a much flatter sail, more forward vector for a given heel (a different boat!)
, but I'm guessing that just tensioning the halyard will stretch the sail luff a bit, with greater stretch near the head/top of the sail as that's where the tension is being applied. So it will stretch there, and a bit less stretch down lower, until you apply cunningham tension, when it will increase the stretch on the lower part of the sail, so it's even from top to bottom. Just my guess--but I can see it happening (a bit) in my sails, so I'm sure it's true.....