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decisions...
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Posted: Thu Apr 14, 2011 3:43pm

Post Subject: Irritating Noise

Hello! I have a Thornycroft T95 engine with a PRM 120 gearbox fitted. The problem I have is that between 500 to 1100 revs I get this really irritating whine sometimes accompanied with a rattle. Every now and again it just goes away but if I put the boat in reverse and then back to forward it comes back. However, over 1100 revs it is sweet. I have had a look in the gearbox manual and it states that noise can be heard if the idle speed is too low (currently 5-600 revs). Can you advise? If it is the idle speed how do I adjust it? or could it be that a repair to the gearbox is needed? I have had the boat since last July and we have done over 120 hours with the problem no worse but still very irritating.

Tony-B...
Tony-B...

Posted: Thu Apr 14, 2011 6:11pm

Post Subject: Irritating Noise

If you locate the lever on the engine that the throttle cable is fitted to you will see two screws with lock nuts fitted at right angles to each other with the lever between. One should have locking wire on it - DO NOT TOUCH THIS ONE. The one you want is the one with the lever hard against it with the control set to idle. If you unlock the locknut and screw it in slightly it will raise the idles speed. Remember to re-lock the nut. However the usual noise associated with a low idle is more of a chatter. Given that you keep this noise up to 1100 RPM my suspicion is a singing prop. If the noise seems to alter with canal depth this would be my first suspicion. It might be the weed hatch anti-cavitation plate vibrating in the trunk. You will be able to localise this by using a long screwdriver as a stethoscope. Come back if you want more advice. Tony Brooks

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Posted: Fri Apr 15, 2011 8:46am

Post Subject: Irritating Noise

Tony Many thanks, I shall try the fixes you have suggested. James

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Posted: Fri Apr 15, 2011 8:53am

Post Subject: Irritating Noise

Tony Sorry, I should have asked: What is the anti-cavitation plate? Is it the cover plate or something else?

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Posted: Fri Apr 15, 2011 9:02am

Post Subject: Irritating Noise

Tony Apologies again, I have just found out what the anti-cavitation plate is and my boat does not have one! It has just a flat cover plate and appears to have been with the boat since new Could this be the problem?

Tony-B...
Tony-B...

Posted: Fri Apr 15, 2011 3:42pm

Post Subject: Irritating Noise

I very much doubt the lack of an anti-cavitation plate will be the cause but if this turns out to be cavitation then you may need to get one fitted. However you need an experienced engineer to listen to the noise before having one fitted to your cover. Tony Brooks

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Posted: Fri Apr 15, 2011 4:03pm

Post Subject: Irritating Noise

Tony Thank you! James

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Posted: Tue May 03, 2011 11:32am

Post Subject: Irritating Noise

Tony Have checked the anti-cavitation unit and all appears OK. Can you advise on resolving a singing prop?

Tony-B...
Tony-B...

Posted: Tue May 03, 2011 11:47am

Post Subject: Irritating Noise

The boat will have to come out so you may just as well ask the yard where that will happen. It involves grinding the prop edges and it may not work - we are into black arts here. If you want something to try with the boat in the water then the following is totally at your own risk, you may make the situation worse. The singing involves natural prop blade vibration frequencies so anything that alters either that vibration frequency or the vibrations caused by water passing over the prop may help. It will help even more if each blade is caused to vibrate at a different frequency. Get a large F shaped adjustable spanner and put it over the top of a blade with the jaws pointing to the hub. Give the blade a slight twist to kink it. Try the boat and if it does not work try another blade. Alternatively find a shallow river with a gravelly bottom and use the prop to do a bit of dredging. The impacts of the stones on the prop just might alter the/a blade's vibration frequency. Tony Brooks

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Posted: Wed May 04, 2011 1:14pm

Post Subject: Irritating Noise

Tony Many thanks for the advice!

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