| >How does twist help the need for reefing?
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|If the top of the wing/sail is twisted to produce very little or no lift, it will add little or no heeling moment. The bottom will produce the lift for sailing. So this is similar to reefing. But with the added drag from the unused sail/wing area.

Sure, but, why add the complexity if twist, when one can just ease the angle of attack?

| This is more or less what any sailor does when easing sheet tension. However, easing sheet tension generally adds camber though, which is the opposite to what one wants in a strong breeze. On the foresail, there is the option to change the angle of the sheet in this situation, to make the bottom part tight and flat, and the top part twisted and spilling wind or even flogging. For the main, tightening the outhaul can flatten the bottom of the sail to get a similar effect.

Sure, but are we not talking about Robs wing sail? Which has no fore sail? This is a schooner or una rig, right? Does one not generally ease said sail by decreasing the angle of attack?

| In any situation though, if the boat is steered in a direction which increases the angle of attack significantly, it will heel over anyway. A wing with a tail should solve that situation, since the tail works to keep the apparent wind at a constant angle of attack.

I don't think Rob's wing sail is expecting to have a tail. But yeah, a tail makes sense on a solid wing sail.

| Another problematic situation is accelerating to a speed where the lift is enough to heel the boat over. This is where the sailor has to change his input in some way, even with a wing with a tail. Perhaps by changing the angle of the tail, or by decreasing angle of attack in the top (twisting the flap and/or wing).

Should the lift not drop as you gain speed?
Of course one can change the angle of attack. Is that not easier than changing twist? And if so, I am back to why have twist control?

| I read somewhere that the AC50 cats could twist the top enough to produce negative lift, to actually get righting moment from the top of the wing.

The bell shaped wing guys are big on this. For flying machines, it allows the control of yaw while turning without a vertical tail (flying wings). Al Bowers calls it the bird wing solution, because that's how birds fly. No vertical tail.

For a boat though, I still do not understand why the added complexity is desirable. Yes, a small thrust at the top of the mast is super neat. But I am not sure how it works when I am expecting the mast to flex when gusting, and it stands rigid due to aerodynamic loading. That seems like it removes a safety feature.

Thanks for being patient with a sailing noob Björn