Prop shaft noise

[JOHNBOY]

Just fitted a new propeller, after much consideration we decided on a lower pitch. We now have much more torque... no vibration...BUT NOW WE'VE GOT NOISE!!!! Its so noisy I can't brave it over 2000rpm....... The sound is like a Irregular Rattle / Grumble.
Volvo system , glassed in tube water lubricated bearing. Am I getting close?
Please Please Any suggestions.

 

[longjohnsilver]

Is it a larger diameter, if so less clearance from the hull (min recommended is 15% of diameter) would cause excess noise.

 

[Guest]

Johnboy, sounds like a classic case of cavitaion to me. Is it a real RUMBLING sound? Another symptom would be on flat calm water the boat appears to surge and then slow only lightly. If it does I'm affraid youv'e selected the wrong prop pitch/dia.

 

[JOHNBOY]

Same size 15 inch with 40mm clearence. But the noise is amplifide through the tiller.

 

[JOHNBOY]

Peter thanks mate. Originally we thought we had cavitation so I was advised to go for less pitch. Was 15x11 now 15x10 no space for a larger dia.
Jonhboy.

 

[kgi]

i know it sounds stupid but how was the prop taken off? if it was removed with a prop extractor and a V large hammer i would check the engine mounts as i do know of a boat that tore a mount like that.........just a thought cheers keith

 

[vyv_cox]

You can usually differentiate cavitation by selecting forward from reverse and applying revs, say to about 1500. Cavitation noise commonly appears almost as soon as forward rotation starts, then declines as the prop starts to bite. It may come back at higher revs but there is always a lower speed where it does not happen.

My thought is that your prop shaft is knocking on the stern tube. Maybe the increased power is pushing the engine further forward on its mounts and this results in contact in the stern gear. Try checking that the static shaft is central in the tube, maybe by feeling outside the rubber tube that attaches the gland but in my case I can take this off, with the boat in the water, and the inflow is easily controlled with the bilge pump. Many prop shafts go through their first critical at about 2000 engine revs, try increasing revs through the noise and the knocking may decline. This tends to confirm the theory. Try a search of this BB on Aquadrive and you will find lots of stuff on my own problems of this type.

Just in case, check that your coupling bolts are all tight and the shaft is rotating without displacement. My big problems of your type were caused by a faulty coupling that gave the shaft a displacement amplitude of about 0.5 mm.

 

[Sinbad1]

JB, why did you assume you had cavitation and change the prop???

Cavitation is caused by pressure differential between the front and back of the prop blade. You can reduce this pressure by changing to a lesser pitched prop or by increasing the blade area.

With this type of propblem you have to start with the basics, altho as stated by a reader above I would suspect that the shaft is touching the stern tube or more likely, that the shaft is bent.

Tie her up to the dock, put her in gear and watch the shaft by the gland. Is the gland wobbling? It shouldn't be. If there is any wobble it can be a number of things, but best to go back to basics.

First, is the shaft straight. (Shaft goes to engineering shop and they check and straighten it)

If shaft straight, then check wear on cutless bearing.

If cutless ok, then you need to check that the two half couplings (end of shaft plate that connects to similar plate on gearbox output shaft) are perfectly mated. Use feeler guage once disconnected. It maybe that the rear mounts on the engine have gone soft. This will 'drop' the shaft harder onto the forward face of the cutless and cause a rumble.

All sounds a bit frightening but its not. Start by disconnecting your shaft/ gearbox coupling and use the feeler guage. You can also run engine and gearbox and use a screwdriver like a stethoscope and try and locate the precise point of the noise.

 

[JOHNBOY]

Thanks guy's BUT I have alliminated the obvious for the simple reason before launching everything ran true and smooth even at full tilt, now the only difference is, its in the water........ And the alingment has been checked agian. However!!! there was about .05mm play in cutless b. Must be OK?

 

[jfkal]

Seems like cavitation and wrong dimensioned prop.

 

[vyv_cox]

Alignment does change in the water and varies with power input. It's worth watching the stern gland and holding the rubber tube while someone increases the revs.

 

[Sinbad1]

I would assume that your Buchanan is wood and thus subject to shape change once in the water. I think that it is unlikely to be cavitation and much more likely to be misalignment.

Nigel Calder recommends that boat sits in water for a few days before aligning shaft to allow boat to take up proper shape! Suggest you disconnect shaft coupling and get out the feeler guages. This will give you a starting point.

But why did you change the prop?

 

[JOHNBOY]

Good subject hey; I changed the prop because we could never reach anything like engine revs max 2500 in flat water. 18hp penta revs to 3200. Therefore poor boat speed, max 5kns, 3 in chop, we sail at 6kns +. Buchanan Bonito is glass composite long keel stiff as they come proverbial b sht house. and anyway I checked the alignment my self. Shaft runs smooth only a bit of engine wobble. There is a plus, odley we have less vibration (throughout the boat) than with the old prop.
I am almost convinced I'm am cavitating?