A high speed prop is always less efficient , a prop turning at 1600 to 1800 rpm
is a normal 3 bladed prop is 30 to 35 % efficient at 1000 to 1200 a prop is up to 50 % efficient , a prop on an outboard revving 2400 and more is only 25 % efficient.
This is one of the 2 reasons for me to choose slow revving props for the electric propulsion
If you take the 20 HP diesels you have in mind minus 10 % loss leaves 18 Hp at the propshaft - 65 % leaves 6.3 hp as propulsion
If you would use a racing
folding prop you loose another 5 % in efficiency or 5.8 is left. All these percentages are not exact but jus to give an idea of the losses that occur between the crankshaft and the actual output.
With an electric motor
of 10 Kw or 13.3 Hp out put at the shaft and low revving 3 bladed prop the actual propulsion output will be around 6.7 and the torque is there from o rpm so the effect and acceleration will be higher that with the 20 hp diesel.
On Green Motion we use props with a .71 blade area and the loss is around 46 % or even more efficient. these where designed for us and can only be used because they come out of the water
when sailing otherwise the added resistance would be killing for the speed