My opinion would be to do two fleets. Don't do anything with corrected time at all. Give the Perpetual trophy to the largest fleet.

Any kind of corrected time is bull in a race like this.

Then the only question is if you start the F18's and N20's at the same time. My vote would be to start the F18's a half-hour before the N20's. That gives everyone an opportunity to be first to the beach.