A N20 will do over 5 knots in 6-10 ft seas- on it's side.
With the mast pointing towards windward. (I had to get it that way, otherwise the wind on the tramp would turtle it.)
When we were on the lower hull, behind the tramp out of all the mayhem of the squall, I pulled the gps out of my vest, and we were doing 5+ knots, on it's side!
GT300-2010, first day near the end.