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Madmouse said..
I have never understood how catamarans which don't plane can achieve speeds much higher than hull speed.
Hull speed is the point where the drag created by the bow and stern waves rises sharply, and because of their very slender hulls cats don't make bow and stern waves that are significant in size. The drag of the waves does rise, but it's so low that it can't be discerned. When they get more power in a gust they just accelerate.
Where cats lose out a bit is that their wetted surface remains fairly high because of the deep and narrow hulls and the fact that they don't really plane well like dinghies, so they don't lift out of the water and reduce their wetted surface at high speeds.
It's funny that people traditionally think that cats (and windsurfers) are downwind flyers whereas these days it's often the other way around - the cats and boards have less drag so they can carry efficient sails upwind and go well, but they can lose out downwind because the monos can carry bigger kites and be more powered up.
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twodogs1969 said..
I may very well be wrong but isn't hull speed really only relevant when motoring.
To windward most if not all displacement boats will go nowhere near hullspeed but on a reach and a moderate breeze will do well over hullspeed.
The maximum speed I have recorded on my boat is 15.08 knts twice hullspeed. But granted it was surfing. ??
One of the funny things about the Mottle is that it was originally thought of as a non-IOR rule downwind flyer, and compared to contemporary boats like the S&S 34 or Dunco 34 it really was quick downwind in a breeze.
Problem is that it's also a good cruiser so these days many of them are loaded down with cruising gear and not sailed fast, so many people don't realise how well they can go. The Cavalier 32 has a similar 'problem'.