I read a few words about Hickling Broad Sailing Club, which races what it calls Slipstream dinghies. I'd never heard of the class before, and I daresay that's because it's fairly localised. They also have the Norfolk Punt, which I'd thought was the biggest dinghy...
...but the Slipstream is 24'6"!!! And only four feet wide. So, enough to make the International Canoe look tame and moderate.
I suppose the Ultra 30 counts as a dinghy, too. What a pity they seem to have been and gone. The last I heard, a couple were mouldering in a field somewhere.