Here's one fly in the ointment on a combined F16/F18 class. If both fleets are small (less than 5 boats each) probably not a problem, but if either fleet has 10 boats of their own, do not be offended if they don't want you in their way at the start or mark roundings. Many is the time I have been "trapped" in a bad possition at the start, by a boat I was not racing against! Then there are the mark roundings, port/starborad crossings, etc...all of which can be fouled by having another boat, not in your class, in your way. If that boat is your direct competition, well, that's racing. But if that boat is from another class entirely and is costing you time or worse, it is not good to have them in your way.

The best case would be to have so many of both clases that each gets it's own start and the two never interfere with each other but that may be a way off yet.

Here's a thought...start the F18's then start the F16's one minute behind them. That should give the F18's time to clear the line but not so far ahead that the F16's can't catch up. In the end, you simply subtract 1 minute from the F16 finish times to see how comparable the speed is.