Hobie Class legal, yes - at least I haven't heard there is a corresponding rule change proposed.

F18 legal? Not sure - you'll have to pull out the tape measure and check. The new rule was raised in late '06 or early '07 (I'm told) to address a trend in fatter and fatter heads on mains. You very well may not have an issue. This sort of thing may have eventually self-limited through trial by the fleet, but a rule was passed anyway. The intent, like most F18 rules, is to keep some grip on development and forced obsolescence.