I race in a mix of monohulls and multihulls more often than not. It doesn't really seem to be much of a problem. Sailors (especially thistles and isotopes) have simply learned that it doesn't pay to get caught up dueling with a boat in a different fleet. Generally speaking, you're much better off sailing away from a confrontation, even if you have right-of-way.

Most of my racing is around drop-marks, and race committee often starts multihulls first (so they get out and away from the monohulls) and frequently send the multi's an extra lap (trying to even out the racing time). Yes, the different boats do interact on the course, but that's just life. I have occasionally heard members of one fleet griping about having to avoid other fleet boats, but that's just sour grapes. Every boat (no matter what type) is entitled to be there and everybody has to abide by the same rules. If I don't like sailing with other boats, that's my problem, not theirs.

Most inter-fleet enounters seem to take place at the leeward mark -- you just have to take care not to get pinned under a boat that doesn't round up as fast as you want. One of the clubs I sail with used to have a separate leeward mark for catamarans. We did away with that, though, so that we could report finish data to US Sailing for the Portsmouth Handicap. We've been racing that way for two years now without any real problem.

Regards,
Eric