I am always trying to get my boat "Brillig" to go that bit faster but am up to hull speed as you can see from the attached pics. Brillig is 18foot on the waterline and weighs a ton loaded
I have a 3" bore by 4" stroke Tiny Power engine directly driving a 18" dia by 24" pitch prop. Steam comes from a 24" high by 18" VFT boiler which has about 18 square feet of heat transfer area.
Fuel is "bio Coal" ie old fence palings soaked in second hand chip fat which I get for free!
I think I can get about 5 HP from this combination. 2.5 hp gets me hull speed. Anything above this is wasted.