Do I have a singing prop or a more serious problem?

[Baggywrinkle]

At the end of last season everything sounded reassuringly normal and ‘boat engine’ like, chugging along with no odd noises coming from anything – it just worked and sounded fine.

Over winter I took the prop off to replace the anode and then put it back on (standard alloy 2 blade prop, Volvo MS25S Saildrive ) - Everything seemed tight and secure, rotated freely with no play evident or any fouling of prop/anode. I removed some barnacle growth from the prop and saildrive, cleaned it all off and primed and painted the leg and prop with the appropriate paint and stuck the rubber seal back on the hull. It did look really nice, with smooth prop surfaces and perfect edges. The prop must have been relatively new with no corrosion or pitting anywhere.

Anyways, it went back in the water and last weekend we went for our first cruise. To my horror at 1600rpm the boat started to produce a high pitched whine, rising to a crescendo at 1750rpm and disappearing again at 1900rpm. In reverse it produced no noise at any speed. Sailing in neutral, the prop spins and you can hear the saildrive turning over normally. The engine seemed fine, no loss of power, no laboring, no vibrations, no grinding or graunching noises. The saildrive oil is clear as crystal, forward/reverse selection is fine. When throttling back, as the revs drop through 1750, the noise returns briefly and then disappears again.

Removing the companionway steps and the access panels in the two rear cabins made the noise more difficult to hear over the engine, but putting them back on, the whine is very noticeable – it seems to be coming from the hull in the vicinity of the saildrive and is resonating in the two rear cabins. Putting my ear near the saildrive housing, the noise seems to be associated with the hull, not the saildrive - nothing untoward seems to be transmitted through the housing. It sounds almost like high-pitched drilling of metal resonating through the hull in the vacinity of the prop.

After 6-7 hours use the noise isn’t getting any worse or any better.
Does anyone recognise these symptoms and have any ideas about what it is and how to make it go away? 1750rpm is my preferred cruising speed

 

[iainbluk]

prop whine

Its not unusual for the prop to make that sort of noise at certain rpm's, just a bit unfortunate that it happens to be in your cruising range. The fix is to file the front edge of the blades ever so slightly which changes the resonance of the blades. My 3 blade Volvo prop on a M25s does the same, but at 1200 rpm fortunately.
If you google the problem I think you will find the article.

 

[vyv_cox]

I cannot imagine why a prop that has never whined before should suddenly start. Conversely, i was speaking to the owner of a Volvo gearbox recently, his has recently begun whining. He has been told it is an internal seal that is failing. I cannot imagine what this might be, but I would ask a Volvo dealer for advice.

 

[Baggywrinkle]

Barnacle growth was removed from the prop and it was lightly sanded and repainted. As the boat is new to me I've only run it for 30 trouble free hours before the prop was cleaned, and 6 hours afterwards (with whine).

 

[Baggywrinkle]

Thanks for the link, it may be the problem .... if it gets worse it'll be off to the Volvo dealer.

Problem is I have no experience of failing engine/transmission bits in boats so can't really judge.

 

[brians]

It sounds exactly like a singing prop I had, exact same revs etc. However mine was a brand new VP 2 blade supplied and fitted by a VP main dealer. VP replaced it free of charge. Why it would happen with a 'used' prop I cannot uinderstand.

 

[Baggywrinkle]

Here are the before and after pics ... could my scraping, sanding and painting create a singing prop?

 

[sarabande]

if it is not the gearbox, then I suggest that your hard work in polishing and painting the prop is the culprit.

If you make a really smooth round end to the trailing edge of the prop, then tiny alternating eddies are produced; these build up at varying shaft speeds to make the singing sound.

To stop the eddies forming, you need to chamfer the trailing edge, and add a chamfered finish over the last 3 - 5mm on the suction side of the blade.

Change the trailing edge geometry just a fraction, and the resonating eddies disappear as they do not have a chance to form in the clean flow of water off the prop.

 

[Baggywrinkle]

Will give it a go, shame I've not got any dive gear.

I assume a prop can be removed and re-fitted underwater.

 

[Tranona]

Can't really help with the cause of the noise - but it is unlikely to be the shape or finish of the prop blades. That is a standard Volvo prop and does not normally sing. Would not start messing around with trying to reshape the blade tips as you might on a bronze prop as you will destroy the coating and the aluminium will corrode unless you spend a lot of effort recoating it.

 

[iainbluk]

prop singing

Having read Sarabande's post I think she is right, its the trailing edge that needs to be changed slightly to rmove the whining. If it was the gearbox, surely the noise would be apparent throughout the rev range?

 

[longjohnsilver]

Will give it a go, shame I've not got any dive gear.

I assume a prop can be removed and re-fitted underwater.

My props sing when cleaned. There's no way I would want to remove them underwater even after 30 years of diving, recipe for disaster and loss of prop.

 

[SAMYL]

Having read Sarabande's post I think she is right, its the trailing edge that needs to be changed slightly to rmove the whining. If it was the gearbox, surely the noise would be apparent throughout the rev range?

Quite correct, the trailing edge has to be chamfered, not the leading edge.

A prop can start to sing for many reasons even after years of uneventful use.

 

[seabright]

Mines the same

Sounds like a singing prop to me. Proof of the pudding, as it were, is whether the singing starts to disappear as your prop gets fouled. I had the same situation after I cleaned my prop for application of prop shield. If you run your boat in gear tied alongside does the gearbox make any noise? If not then I would relax. Make sure your lines don't slip....that would be a little embarrasing.....

 

[Baggywrinkle]

Back down there in a week or so .... will investigate further.

Thanks for all the replies.