: What is the rough industry standard for the dimensions of the cross-section
: of the cockpit coaming? I am thinking of 3/4" from the underside of
: the lip to the deck, and also 3/4" from the outside of the vertical
: portion to the outer edge of the lip. In other words, the groove that the
: skirt edge fits into is a 3/4" square. Is that about right?

Yup--that should work pretty well. If you're considering the use of a sea sock, a bit bigger would be better. 3/4" wide by 1" tall will work a little better with sprayskirts that have thick rubber rands.

The shape of the outside of the lip will have as big an impact on skirt retention as anything. A sharp lip will hang on tighter (less chance of it popping loose), but also be harder to remove (more dangerous for beginners).

Shawn