Jeez, that picture looks a lot like the first race at the NA's in Annapolis (without the fifteen broaching boats behind Zephyr)!

There were only 3 or 4 broaching boats at the Mids... or at least that's what they told me in the john at o'Briens

Nope. I was there (and behind) and saw a whole bunch of broaches-some boats more than once. As far as I could tell, Zephyr didn't even start to roll.

What material is the mainsail made of on the boat pictured above?

It is North "Sharkskin", a polyesther cloth which has some dark polyesther threads.

Sharkskin was popular on J/105 which also require poly in their main. I believe North has discontinued that cloth because of quality problems.

If there are quality problems in the sharkskin, we haven't discovered any. North Sails in New Orleans continues to make the sharskin main and #3 for J/30s.