Propeller Noise! New Engine, Old Prop

[Paolo]

A new Volvo 55HP engine has been fitted on my Swanson 38.

The old fixed blade propeller has been checked by a specialist, found 100% suitable and reinstalled, with a new Packless Sealing System shaft seal.

All is OK but...

The propeller makes a loud swishing noise that was not there before, and that is unacceptable. Is it cavitation? If so, why not before?

The mechanic who has installed the engine, quite competently, is so far at a loss to understand why and how to rectify the problem?

Can anyone advice, please?

Paolo
Sydney Australia

[captain465]

Was the propeller shaft length changed? If the propellers position in relation to the hull was changed, you might be experiencing cavitation.

[Boracay]

Could you give us some more information about the previous and the new installation.

Things like propeller size, rpms, exhaust before and afters. That sort of thing.

[Paolo]

The previous engine was a Mercedes 36HP - Marinized by Solè in Spain in 1984

The new engine is a Volvo D2-55HP

The propeller is three blade, the same propeller fitted before, checked by a specialist, not worked-upon, only re-fitted.

The exhaust discharges from the same above waterline outlet as before.

The rpm at which the engine run when experiencing the unprecedented propeller noise was in the same range (1500-2200) as with the previous configuration.

I must add that the Volvo, in a newly soundproofed engine room, is remarkably silent.

Thank you!

[r.furborough]

Sounds like you are overpowering your propeller and what you can hear is cavitation at those rpm's, now aparently more severe after soundproofing your engine compartment. Also a ~20HP increase in HP without propeller replacement or rework without a transmission ratio change will exacerbate this problem.

[Jolly Roger]

r,furborough took the words right out of my pen. Try a prop with a courser pitch, and/or a larger diameter. Lower revs - same speed. Might be a question of experimentation.

[donradcliffe]

more importantly, what was the gear ratio before and after the repower? The same 1500-220 rpm on the engine could be a considerable increase on prop rotation speed.

[CAELESTIS]

Check to see how many rpms you pull at WOT.

You have cavitation.
You need more pitch.
A Maxprop would give you more speed while sailing and have adjustable pitch.

[TOM]

I have just been thru this whole repower thing and ended up having to change my prop. The prop calculation sites on the internet are remarkable accurate as long as you enter the correct information.

The guys at Austral props in Sydney are excellent and will tell you exactely what size prop you need.My guess is that your new gearbox has a different ratio then your old one .Also check if your old motor have the same rotation as your new Volvo

www.byamee.com

[osirissail]

No criticism intended TOM, but if the boat is in the water and if when put in forward gear the boat takes off backwards (as a result of reversed engine rotation) that might be very noticable. If the boat is still out of the water - then absolutely double check that the engine matches the propeller. It seems rather humorous to imagine them putting the boat in forward, goosing the throttle, and the thing takes off backwards. Stranger things have happened.