Hi and welcome Guus.
I had a similar challenge with my 1966 Alberg 35
. It had been repowered with a 30 hp diesel
, replacing the original Atomic 4 petrol engine.... so, the prop shaft now is smaller dia than optimum for the 30 hp, the space for the prop is too small for the optimum prop dia.
I wanted to end up with a 3 blade
to push the hull
at hull speed
against the strong currents around here, and to push it off a lee shore in weather
. The 30 hp at specified 3,500 rpm is way overpowered for this hull and the smaller prop shaft. So I asked the shop to provide me a prop that would push me at hull speed
, at 2,500 rpm (the lower end of cruising rpm for the 30 hp).
Fortunately I had a skilled prop shop near the marina. I was nervous given the cost but he said not only could he do it but he would guarantee it for hull speed at 2,500. He bought in a prop, and in his forge, he "tuned" it for his calculated dimensions for my set-up. Sure enough he did it, said there is nothing magic about it, it is "just" math and skill in the forge. So now I have very good fuel consumption
, and plenty of reserve power for whatever , before I reach 3,500 rpm. Very pleased that I took this route
rather than all the advice I got around the marina.
BTW, ref disposing of your old props .... take a close look at them for any sign of pink in the metal ... this indicates old brittle metal and you won't be doing anybody a favor by selling them.
Fair winds, Peter