Whistling Engine

[Tim G]

Our boat is 18 months old. When we first took it over there was a low vibrating whistle at ~1400 revs. This was quite loud and obviously very annoying. When the engine revs were increased or decreased the whistle went away. It sounded mechanical as though something was rubbing. Talking to various people the diagnosis was thought to be a whistling propeller.

 

After a month or so the propeller was changed and the whistle reduced although it was still audible but far less intrusive. Very occasionally though it was louder and we always had that nagging doubt that perhaps some long term damage was being caused.

 

Very recently the boat has been in dry dock for some welding work (the weedhatch was modified and the rudder shortened). We took the opportunity to black the bottom at the same time but nothing else was done to the engine or gearbox. Disappointingly the whistle has now returned and although intermittent it can be as loud and intrusive as it was in the beginning. It seems worse when the engine is under load i.e. pulling away from stationary although it will also do it when gently cruising at ~1200 - 1400 revs. The engine has now done >500 hours. I've been told the noise is just a gearbox whine and all boats do it although I have never heard it on any other boat.

 

Does anyone have any suggestions as to what it could be and more importantly what if anything can be done to eliminate it?

[Yoda]

Does anyone have any suggestions as to what it could be and more importantly what if anything can be done to eliminate it?

 

Could it be the exhaust system? This will resonate at its natural frequency which will be different on every boat.

It could also be called organ piping, depending on what is actually causing the problem.

[Tim G]

Could it be the exhaust system? This will resonate at its natural frequency which will be different on every boat.

It could also be called organ piping, depending on what is actually causing the problem.

 

That's an interesting possibility although I wonder if it would reduce when the prop was changed only to return several hundred hours later. Certainly it isn't possible to pinpoint exactly where the noise is coming from so the next time we're on the boat I'll have a close look at the exhaust. Possibly the note will change if I partially block the exhaust outlet. Thank you.

[Guest]

A little bit more info would be useful. Does it only make the noise when in gear, does it only do it in forward gear? What engine /gearbox is it? is there a thrust bearing on propshaft? I heard one of these making a dreadful noise many years ago. Also air intakes on engines can make strange noises.

[John Orentas]

Could be the cavitation plate, some builders make these too lightly constructed, sometimes made worse if they are in contact with the swim plate.

 

* The cavitation plate is the rectangular horizontal plate that 'fills' the weed-hatch aperture to prevent unwanted turbulence. Sometimes they are fixed to the weed-hatch cover and other types drop into the hatch and must be lifted separately but they must be clamped down in some way, both types can resonate in sympathy with the prop. and if touching the sides of the hatch that can make a lot of noise.

 

If it is the latter type try running without it. If the former try to check that there is a distinct gap all round when it is fitted, look for signs of rubbing. If the entire thing looks flimsy, brace it up a bit to the hatch top.

[Yoda]

A little bit more info would be useful. Does it only make the noise when in gear, does it only do it in forward gear? What engine /gearbox is it? is there a thrust bearing on propshaft? I heard one of these making a dreadful noise many years ago. Also air intakes on engines can make strange noises.

 

All of the above and also the following.

Is the engine/ouputshaft/gearbox in line and fully bolted down? Is the output shaft showing signs of swash?

[bottle]

swash (swsh, swôsh)n.1. a. A splash of water or other liquid hitting a solid surface.b. The sound of such a splash.2. a. A narrow channel through which tides flow.b. A bar over which waves wash freely.3. See uprush.4. a. Swagger or bluster.b. A swaggering or blustering person.

 

Sorry how does this fit in with the output shaft, is there another meaning?

[Yoda]

swash (swsh, swôsh)n.1. a. A splash of water or other liquid hitting a solid surface.b. The sound of such a splash.2. a. A narrow channel through which tides flow.b. A bar over which waves wash freely.3. See uprush.4. a. Swagger or bluster.b. A swaggering or blustering person.

 

Sorry how does this fit in with the output shaft, is there another meaning?

 

Swash is a deformation by rotation of a coupling or plate that is part of an assembly. Swash type pumps can be used to vary the amount of fluid compressed but also maintain the pumped pressure at all times.

 

On an output shaft, the swash in the flexible output coupling will allow for any misaliegnment between two shafts.

[bottle]

 

Swash is a deformation by rotation of a coupling or plate that is part of an assembly. Swash type pumps can be used to vary the amount of fluid compressed but also maintain the pumped pressure at all times.

 

On an output shaft, the swash in the flexible output coupling will allow for any misaliegnment between two shafts.

 

Thank you, there is another meaning then, it is nice to learn something new.

[Tim G]

A little bit more info would be useful. Does it only make the noise when in gear, does it only do it in forward gear? What engine /gearbox is it? is there a thrust bearing on propshaft? I heard one of these making a dreadful noise many years ago. Also air intakes on engines can make strange noises.

 

 

OK, sorry, I should have given a bit more detail. The boat only makes the noise when it is in gear and moving forwards. It does not make it in reverse or if it's tied up on the bank and in gear at 1200 - 1400. The engine is a 2.2 litre Barrus Shire (model: 4TNV88-D) with a PRM gearbox (model: 150D2). There is just a stern gland and greaser on the prop shaft. Thanks for your help. I'll investigate the air intake next time I'm on the boat.

 

Could be the cavitation plate, some builders make these too lightly constructed, sometimes made worse if they are in contact with the swim plate.

 

* The cavitation plate is the rectangular horizontal plate that 'fills' the weed-hatch aperture to prevent unwanted turbulence. Sometimes they are fixed to the weed-hatch cover and other types drop into the hatch and must be lifted separately but they must be clamped down in some way, both types can resonate in sympathy with the prop. and if touching the sides of the hatch that can make a lot of noise.

 

If it is the latter type try running without it. If the former try to check that there is a distinct gap all round when it is fitted, look for signs of rubbing. If the entire thing looks flimsy, brace it up a bit to the hatch top.

 

 

An interesting one this as the work that has recently been done was to relocate the weedhatch as it was not over the propellor and also to fit a better cavitation plate as the previous one only covered about half the hole in the counter. The new plate is a perfect fit and of a very solid construction. It is attached to the hatch top making it difficult to run the boat without it. However, since the noise only occurs as relatively low revs it may be possible to loosen the hatch and lift it to see if the note changes. Many thanks for your thoughts.

[Tim G]

All of the above and also the following.

Is the engine/ouputshaft/gearbox in line and fully bolted down? Is the output shaft showing signs of swash?

 

Alignment is one of the things we are worried about since this could do long term damage. I have been assured that the alignment is correct although it hasn't been checked. The engine also appears to be fully bolted down.

 

We are going to check the alignment but since I discovered this forum only two days ago I thought it would useful to raise the topic in the hope someone could say 'That happened on my boat and it was ---'. Certainly I have been very impressed with the replies I have received and thank you and everyone else who has contributed. It goes us some useful suggestions as to what to investigate.

 

I don't think there is any sign of swash although I must confess to not really know what to look for. I presume that it is wear caused by uneven rotation. If this is so then I don't think there is as there is very little leakage from the stern gland. The grease is sufficient to prevent any leaks and the bolts only require very occasional tightening.

[John Orentas]

I did not suggest that you should try to run the prop with the weed hatch removed, don't try it, that is the easiest way to sink your boat. You worry me a bit when you say the cavitation plate is a perfect fit.

 

What you did not make clear, does the noise occur only when the prop is turning or does it happen also when in neutral.

[Tim G]

I did not suggest that you should try to run the prop with the weed hatch removed, don't try it, that is the easiest way to sink your boat. You worry me a bit when you say the cavitation plate is a perfect fit.

 

What you did not make clear, does the noise occur only when the prop is turning or does it happen also when in neutral.

 

 

Sorry John, the noise only occurs when the prop is turning and the boat is moving forward. It does not happen in reverse or if the boat is run in gear and tied up.

I appreciate what you say about running the boat with the weed hatch off. I was thinking of just loosening the hatch to see if the note changed.

I thought the cavitation plate was supposed to be a perfect fit so as to give as smooth a surface as possible on the underside of the counter and prevent air being drawn down into the propeller.

Tim

[Khayamanzi]

Bingo! I had exactly the same problem for months on my boat and know exactly what you mean, in fact I have a recording of the sound if only I could get it posted on here then you could see if it is the same. It's what is known as a singing prop. Mine only started after I had the hull re-blacked with 2 coats of thick bitumastic and this included doing the weed hatch throat plate. I tried everything and in the end dry docked the boat and had a brand new prop fitted as everyone was telling me it was to do with little 'nicks' being cut out from the blades causing cavitation. I had exactly the same propblem with the new prop! I then down-sized the prop ever so slightly and that cured the problem. Since then, I have met many people who have confirmed that the prblem was most likely caused as a result of slightly reducing the space between the top of the prop and the throat plate, (caused bythe bitumastic paint,) One guy said that he cured the problem on his boat by making a thicker rubber washer, (out of conveyor belt rubber,) to go round the weed hatch. This raised the throat plate slightly and allowed more space to get between - end result: less cavitation and absolutely no whining prop!

 

Edit to add link:

 

I have managed to use afile sharing site to upload the mp3 sound. Does this sound a bit like it?

Click here to listen.

Edited September 1, 2006 by Khayamanzi

[Yoda]

Bingo! I had exactly the same problem for months on my boat and know exactly what you mean, in fact I have a recording of the sound if only I could get it posted on here then you could see if it is the same. It's what is known as a singing prop. Mine only started after I had the hull re-blacked with 2 coats of thick bitumastic and this included doing the weed hatch throat plate. I tried everything and in the end dry docked the boat and had a brand new prop fitted as everyone was telling me it was to do with little 'nicks' being cut out from the blades causing cavitation. I had exactly the same propblem with the new prop! I then down-sized the prop ever so slightly and that cured the problem. Since then, I have met many people who have confirmed that the prblem was most likely caused as a result of slightly reducing the space between the top of the prop and the throat plate, (caused bythe bitumastic paint,) One guy said that he cured the problem on his boat by making a thicker rubber washer, (out of conveyor belt rubber,) to go round the weed hatch. This raised the throat plate slightly and allowed more space to get between - end result: less cavitation and absolutely no whining prop!

 

Edit to add link:

 

I have managed to use afile sharing site to upload the mp3 sound. Does this sound a bit like it?

Click here to listen.

 

Looking at the frequency responce curve of the uploaded file, the noise is very promenent. So to are the harmonics. This leads me to think that the prop is creating the vibration and the underwater boat structure is vibrating in sympathy. Any small change to the prop or metalwork underneath will change the resonant frequency up or down the scale. I suppose this is down to fine tuning. Certainly over a prolonged period of use there would be damage to the structure or propellor, so any change that reduces this resonants would be good.

[Tim G]

Bingo! I had exactly the same problem for months on my boat and know exactly what you mean, in fact I have a recording of the sound if only I could get it posted on here then you could see if it is the same. It's what is known as a singing prop. Mine only started after I had the hull re-blacked with 2 coats of thick bitumastic and this included doing the weed hatch throat plate. I tried everything and in the end dry docked the boat and had a brand new prop fitted as everyone was telling me it was to do with little 'nicks' being cut out from the blades causing cavitation. I had exactly the same propblem with the new prop! I then down-sized the prop ever so slightly and that cured the problem. Since then, I have met many people who have confirmed that the prblem was most likely caused as a result of slightly reducing the space between the top of the prop and the throat plate, (caused bythe bitumastic paint,) One guy said that he cured the problem on his boat by making a thicker rubber washer, (out of conveyor belt rubber,) to go round the weed hatch. This raised the throat plate slightly and allowed more space to get between - end result: less cavitation and absolutely no whining prop!

 

 

Andy, I think you've solved our problem. Thank you so much. Our sound is very similar to yours, just at a slightly different pitch. Certainly all the evidence is there as we put on two coats of blacking both on the swim and underneath the counter. The weedhatch has been moved forwards so it is directly over the prop and a new anticavitation plate fitted. The engineer actually bashed out a couple of dents in the prop which did concern me as perhaps it would have been better to leave it dented. As I mentioned earlier we had a different prop fitted very early on which reduced the whistle but didn't completely eliminate it. I don't think the new prop was much different to the old one in terms of size, pitch etc. and so the small changes made by blacking the bottom and modifying the weedhatch have found the new prop's resonance frequency.

 

I'll certainly pursue this line of thought and see about getting a different prop. I might even be able to fit it through the new weedhatch without taking the boat out of the water again.

 

Finally, thank you to everyone who have input their thoughts to my problem. I am so pleased to found this forum as we have been scratching our heads over this for quite some time and no-one else we have spoken to has been able to offer a rational explanation.

 

Tim

 

 

Edit to add link:

 

I have managed to use afile sharing site to upload the mp3 sound. Does this sound a bit like it?

Click here to listen.

[Khayamanzi]

Glad to have helped - do let us know what the outcome is as that will prove/disprove my theory and what others have been telling me!

 

Hope you get it sorted soon - I would certainly try some sort of rubber washer under the weedhatch cover to raise it slightly before going to the expense of a new prop - just wish I'd known this before I forked out.....

[Incentive]

Great!

Thanks............... I have a prop that sings, very annoying because it is at my favourite cruising revs, a bit like wheels that need ballancing on a car, go 68 mph and the steering wheel shakes, but 75mph and it's fine (honest officer - that's the only reason I was over the limit)

 

So, if I have understood correctly, it would be worth me attempting to raise the weed hatch a fraction with a suitable water tight washer?

 

I would be so glad to be rid of this noise. Cant wait now to give it a go.

 

Many thanks

 

Martin

[Tim G]

Glad to have helped - do let us know what the outcome is as that will prove/disprove my theory and what others have been telling me!

 

Hope you get it sorted soon - I would certainly try some sort of rubber washer under the weedhatch cover to raise it slightly before going to the expense of a new prop - just wish I'd known this before I forked out.....

 

The whistling prop has been sorted!! The solution turned out to be quite easy. The theory is that the whistle is caused by the propeller resonating at a certain frequency (i.e. engine speed) brought about by the propeller being 'too perfect'. The answer was to give each blade a little tweak with a large adjustable spanner. These tweaks must be small or the propeller could be put out of balance and cause vibration. We tweaked each blade by the same small amount and found that the whistle was much less. A second small tweak did the trick and it was gone. We then ran the boat through the whole range of engine speeds to check that the resonance point hadn't just moved but all was well. No whistle and no vibration either. I have since heard it said that a whistling prop will gradually cure itself as it becomes dented on rocks, shopping trolleys etc. I must admit I was rather worried that the problem could be cured by actually doing damage but it does seem to have worked. Thank you so much Andy for your reply which put us on the right track to a solution.

 

Great!

Thanks............... I have a prop that sings, very annoying because it is at my favourite cruising revs, a bit like wheels that need ballancing on a car, go 68 mph and the steering wheel shakes, but 75mph and it's fine (honest officer - that's the only reason I was over the limit)

 

So, if I have understood correctly, it would be worth me attempting to raise the weed hatch a fraction with a suitable water tight washer?

 

I would be so glad to be rid of this noise. Cant wait now to give it a go.

 

Many thanks

 

Martin

 

Hi Martin,

Please see my reply to Andy above re. our solution to the whistle. It may well be worth while trying a thicker weed hatch gasket first rather than 'modify' your prop. In our case the clamp on the weedhatch was such that we couldn't fit a thicker gasket. I said in one of my earlier entries above that the dents on our propeller had been smoothed out when the boat was in dry dock. This may well have been what caused the whistle. All we did was to put the dents back with an adjustable spanner. Good Luck, Tim