Why does tightening the downhaul flatten the main?
The best I’ve come up with is that tensioning an area of cloth pulls the threads farther apart, which draws the cloth perpendicular to the tension towards the tensioned area. Grab a piece of cotton cloth (like your T-shirt) and pull on it to see the effect.
As far as sail control goes, tensioning the luff primarily moves the position of the draft, in my understanding it does very little to reduce the draft. Someone once tried to tell me that a cunningham affects the sail differently than downhaul or halyard tension. I haven’t been able to figure out how that would be different myself.
With a bendy mast, draft can also be reduced somewhat. On modern high performance cats, the mast is slightly prebent by raking the diamond spreaders as far aft as allowed then tensioning the diamond wires. Now instead of bending the mast using the mainsheet, tighten the downhaul with your 8:1 or greater purchase. The downhaul pulls in a straight line through the cloth to the head of the sail decreasing that distance and bowing the middle of the mast farther forward. Since you are increasing the chord length (the sail is longer in the fore-aft dimension due to mast bend) for the same depth, the draft ratio decreases. This also frees the leech up high twisting off the sail since the mast tip is closer to the end of the boom.
The newer hi tech sail cloths and fully battened sails supposedly reduced the amount that the shape of the sail could be altered that some classes went back to dacron.
The outhaul is different, it does control draft in the lower third of the sail. This is easiest to see with a loose footed sail. As you loosen the outhaul the clew moves forward shortening the chord length making the sail cup more so it has a deeper draft.
-John C
