And the answer is....
45%. A good rule of thumb is that at NO time should the max draft be aft of 50% back from the luff. The lighter the air, the farther aft the draft can be-up to a maximum of about 45%.
Mainsails will have a designed draft of between 38-45% depending on the boat and intended use for the main (racing boats might have light air and heavy air mains-the light air main would have about 45% and the heavy air main might be around 38%). "AP" or cruising mains will be in the 40-45% area.
For headsails, same applies: A light/med genoa should be about 45%.A # 3-or heavy air jib will be about 33-35%, and hvy #1 or #2 will be somewhere in between.
One of the biggest reasons, for those who care, is that as the wind builds and the boat heels, the tendency for weather helm (pressure) increases. Something which contributes to weather helm is having the draft in the sailplan aft relative to the CG (pivot point or keel)-this makes the boat want to "weathervane"-i.e. bow towards the wind. Pushing the draft forward in heavier air sails helps reduce this tendency, since if the CE of the sailplan were truly fwd of the CG, the boat would in fact have lee helm. The reality is that when you add heeling, mast rake, weight placement, and all the other factors which can affect weather helm, moving the draft towards the 30-35% area simply mitigates the amount of helm you would have if you added all of those factors combined with a draft aft sail.
In light air, when sometimes you can't get ANY weather helm (which in light air helps you point higher), having the draft aft helps "push" the bow up towards the wind-so you point higher. The limitation is about 45-48%-any efficient airfoil must have the max draft (chord depth) forward of the midpoint so the exit will have a clean, straight enough run and air can exit the sail/wing/airfoil at the highest speed..
Did this make any sense at all? Hope so. I actually took the test, too, and missed the security around military boats question-it was pretty good though..
Cheers all!
S
