Not a SIB but I followed a dinghy back in the other day which I am sure had a 15hp Yamaha on the back and he was doing about 18 knots pacing him at a steady distance, and that was with an adult and 2 kids on board. I suppose it was about 3.5m long didn't have a tape measure handy.
Wished he would hurry up as it was bloody tipping down with rain but I thought overtaking him in a narrow channel might sink him and cause some whinging