Propeller advice Swan 44 75HP engine

Questar is a 1975 Swan 44 and was re-engined with a Volvo Penta D2-75 by a previous owner.  Unfortunately the 16"diameter * 12" pitch 3-blade propeller seems to be unchanged from the original much-smaller engine, so the big engine revs away without much forward motion!

I want to install a new fixed-blade propeller, but there is very limited hull clearance for a larger prop.  I could just about squeeze in an 18" diameter prop if I grind a bit of the hull away.  Even an 18" propeller seems very small and presumably will require a very course pitch to generate enough power.

Shaft is 28mm diameter.  Gearbox is 2.27:1

Does anyone have any similar experience / advice / solutions?

Dear Simon

original spec for S&S Swan 44 was a prop of 17". Remember clearence should be at least 10% of the prop diameter, otherwise you are going to have a lot of vibration (and noise)

Fair winds!

matteo (38/067 Only You)

Dear Simon
It appears the choices for this installation were based on guesswork, as the present propeller absorbs about 20 hp at full rpm.
If you want to use the full power of this engine the propeller diameter needs to be at least 21" unless you choose four blades or three very wide blades.
The shaft diameter 28 mm is smaller than original, and needs to be found out which shaft material was used here, it may restrict the power.
One way to create space for a bigger diameter is to angle the strut further aft and extend it, but the shaft length should not exceed abt 1.5 m.
If you are content with using less engine power - about half of the present maximum would be sensible - a smaller propeller diameter would be sufficient.
Kind regards
Lars