engines are always low rpm because of higher effiency.
To give you some numbers
Your average car diesel
or 3000 rpm yanmar
gets about 40-45% effiency
A medium speed 4 stroke
diesel engine (500-800 RPM) gets up to 48-49% efficiency
The large low speed 2 stroke
diesel running about 30-120RPM gets up to 51% on its own and up to 57% using exhaust
gas to generate steam and drive a steam turbine..
As for prop speed, best RPM for high efficiency depends on cruising speed and prop diameter. Best efficiency is usual obtained at around 100 RPM .
I know of some commercial fishing
vessels that have recently been converted to a smaller engine and a slower running bigger prop this gives them a slightly lower max speed but increased pulling power a low speeds and higher efficiency.
Furthermore a low speed engine lasts longer between overhauls.
However on a sailboat,
Do you really want extra weight?
Do you really want a big prop? (a lot more drag when sailing)
On a small boat wont the extra weight out balance the 5% higher efficiency from the engine and propeller
My trintella 1 still has the original SABB 10hp 200kg engine and I'm happy with it.
But most people wouldn't appreciate it. I just love the sound.