I dont think that is is correct to say that a square top have more laminar flow. I always believed that you had either laminar or turbulent flow over the sail? But what you say about pressure leaks is the same explanation I have heard earlier explaining why square tops are more efficient.

Now, I believed the extra drag caused by an oversheated sail was mainly due to early separation and an increased tip vortex. I did not believe that pressure leaking across from windward was a major contributor to this even if it's part of the "big picture" when taking into account circulation?

If Glenn says 250-300mm of twist, on an A cat. Assuming that he designs the sail with about 8 degrees twist and that it have a 8000mm leech, the buildt in twist is 640mm (I dont know how much buildt in twist he uses, but I think 8% should be about right for a sloop rigged cat). That must mean that he sheets the main quite hard to achieve 250-300mm.

A tight leech is paramount to achieve height while beating. The pictures of Jennifer and Matt must have been taken while rounding the windward mark, or else they would not have been in front.


I would say that a square top is easier to trim and sail well with compared to a pinhead, as the squaretop reacts better to mainsheet and downhaul (just an opinion).
Without knowing how to read the sail (telltales+boat's course+what gear you are in+knowing what you want to do+knowing what shape you want) you are equally slow whatever kind of mainsail you have

An oversheated main is slow, but can give you some extra height if you need it. Take care not to hook the leech..