So long as everybody is on the same course, then correcting for the number of laps that different divisions do is not hard. This system is built in to TopYacht so it's really easy to do the results.

I've just looked at the NoR for last year's regatta. It's a little confusing but it looks like they have a division for cats (3) and a division for most of the dinghies (2), but these divisions will be divided into "slow" and "fast" and given a different number of laps to sail, but then their times will be multiplied up to get a time for 4 laps. This means the slow cats and fast cats will be racing a similar amount of time but can still get an overall result.

It doesn't look like the monos are scored against the cats - which is a relief. Racing multihulls against monohulls is a no-no.