Mmmm, very interesting. The theory I have running around in my mind right now is that as the boat pitches and accelerates/decellerates over/through waves causing the sail to move back and forth through the wind, the apparent wind moves forward and aft very quickly, say through 10-20 degrees for every wave that is encountered.

So you need to be able to change the sheeting angle of your sail to obtain the maximum thrust from this ocsillating apparent wind. As the boat pitches backwards the apparent wind shifts aft a bit and as the boat pitches forward the apparent wind shifts forward a bit.

Changing the sheeting angle is very difficult, or impossible, to do fast enough on many boats. So another technique needs to be developed to allow the sail to operate efficiently at all these different angles of attack very quickly. Dinghies do it by pumping and "bouncing" the crews body weight through waves, although it is illegal, as this moves the rig towards the wind and counteracts the apparent wind moving aft as the boat/sail is pitching aft.

Cats don't tend to operate the main much upwind so a different technique has evolved, big head mainsails with the right amount of twist, not too much, not too little. With the right amount of twist, the sail will "on average" operate more efficiently as different sections of sail at different heights change between being highly efficient and then inefficient. So while one section at the top may be operating poorly, a section at the bottom may be operating highly efficiently. Then when the boat pitches the other way the top will start operating highly efficiently while the bottom operate poorly. But on average, a correctly twisted sail will operate better than an over-twisted or under-twisted sail. This all depends on the conditions ofcourse, if there are no waves, more of the sail can be in the best position more of the time.

The first principle to get straight is that the apparent wind oscillates/rotates across the sail as the boat moves through waves. A cross section of a sail works best (produces best thrust) at one angle of attack. Twisted sails have many angles of attack to what is referred to as the steady wind direction. All of these sections up and down the sail at sometime during the oscillation of the apparent wind, due to pitching, will be at their most efficient angle of attack to produce thrust. This means that the centre of thrust is actually moving up and down the sail with each pitch. Over time, this thrust will add up to be more than is produced buy a sail that is setup with too much or too little twist since both these scenarios have a section of sail that will rarely operate at its most efficient angle of attack.

This might explain why it is a common indication of a correctly twisted main that the top leech telltale should be flowing about 50% of the time. The top section of sail is only operating at its best angle of attack to produce thrust about <50% of the time (telltales will stream slightly either side of best angle of attack though). While the top section is operating poorly, the other sections of sail are operating efficiently. Imagine the apparent wind angle as it moves up and down your sail. It will tend to slowly move forward as you go down the sail and will tend to move aft as you go up the sail.

I might be a nerd and crunch some numbers later to see how much the apparent wind will rotate at the head of a pitching sail. That will give an indication of whether or not my theory is correct.

How this relates to square head mains may be that the sail area is distributed more evenly up and down the sail so the instantaneous thrust is pretty much the same during the tiny oscillations.

Be careful with my terminology, I have used rotate, move and oscillate interchangeably with reference to the apparent wind angle throughout the post depending on the scenario I'm trying to explain.

Excuse my long sentences, I wanted to get it out quickly before I forgot it.

Nick.