Yanmar/Shaft resonance and

[seaesta2]

I have a 34 foot Alberg 34 displacement yacht. 6.5ton. Underwater it is a bit like
a folkboat. A new Yanmar 3GMD is installed under the cockpit and main companionway conventionally Using a flexible plastic coupling to a 2.6 to 1 Yanmar Box. Shaft and gearbox are in line. The shaft is 1inch and about 3ft long to a 3 blade 16.25 by 9 inch pitch propeller.(Prop has been mechanically resized from 17 by 12). When we got the boat it had a resonance at about 1300RPM which was easily missed. On removing the propeller to have it resized due to fitting a 3GMD
reconditioned engine it was found that the propeller had been originally
machined for a 1.125inch shaft and a nylon sleeve had been fitted.
A 316 stainless sleeve was fitted but this has now caused worse
resonances. We get a resonance from about 1000 to 2000 RPM. There is a very small gap and then another resonance takes over from 2000 to 2500. So we are
restricted to going at either 2knots or 6knots because it is difficult to avoid the resonances which cause the shaft system to ring.

HEELP! Is there anything that can be done about these resonances without
replacing the prop?

 

[steve_cronin]

I wouldn't have thought that there was.

Previously you had a combination that effectively included an acoustic insulator in the nylon bush. Now you have a single unit in the rigidly connected prop and shaft - rather like a tuning fork. You could try one of the internal flexible shaft bearings like a Halyard not for centering purposes but to de-tune the shaft but these tend to be expensive, more so than a new prop.

However if you were to change the prop how can you pre-determine with any degree of accuracy, when the harmonic frequencies will be reached?

If it is important to you (& severe vibration will cause damage) then a shaft bearing may be the only solution without getting involved in endless trial & error experiences and a large bill for multiple haul-outs.

Steve Cronin

 

[jfkal]

Seems to be you have introduced some im-balance in the whole system. Try to work your way from gearbox to tp prop and see whether evetthing is properly aligned. I suspect your new piece of 316 ingtroducing the trouble.

 

[Chris_Stannard]

I am not sure why you have gone to a 9" pitch. I have a 8.25 ton displacment boat with a Yanmar engine which revs up to 3700. It was fitted with an 18 x 13 but when I changed to a folding prop Yanmar advised a 17" x 13" pitch. My reduction ratio is also 2.6 to 1. The pitch seems very fine for the prop and speed.

Firstly, if the prop was mechanically altered, by which I take it you mean the baldes were cut down and the pitch changed by twisting, there are two likely problems. The pitch may not be the same on all blades or the prop may now be unbalanced. It may be a combination of the two. The way you can check this is to trail the prop and sail at best speed to see if it still vibrates with the gear in neutral. If this is the case you might try having the prop rebalanced but I would suggest you talk to Yanmar about what sort of prop you should have.

If you decide it is not the prop then you might have to look at the gear box and engine and the mounts. One of the problems is that the performance of these parts of the system do not react in the same way with no load, as they do when loaded but you might be able to find some traces of the viabration with the engine in neutral.

Hope this helps

Chris Stannard

 

[Sinbad1]

Hmmm...the question states 'reasonance' and the replies refer to 'vibration'.

If the problem is just one of reasonance then it is likely that the prop is the cause and that it is 'singing'. This is easily removed by slightly reshaping the blade edges on the prop.

If its a vibration that is not removed by curing the 'singing' then start with the basics. Get the prop computer balanced. Check to see the cutless bearing is ok, then get the shaft checked for true and restraightened if necessary. Its surprising what a lobster pot line around the prop can do to the shaft!