On the subject of prop noise....

boatmike

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This should be fairly simple but isn't easy to find...
I have a 4 cylinder Vetus engine in a central location above the WL on a Snowgoose Cat driving a Sillette outdrive leg via a flexible coupling (no intermediate shaft)
I have a cyclic noise that sounds like resonance but is not in time with engine revs or prop revs. The best I can describe it is like "hunting" which you sometimes get with twin engines at about 1-2 second intervals. Rumph-rumph-rumph--- If you see what I mean!
I thought it might be an out of balance prop, but have just fitted a new one with slightly courser pitch.... still the same! I cant see anything out of line, the engine does not appear to move on its mounts, the leg itself is on a good mounting and not loose... Where the hell is it coming from? It doesn't seem to do any harm but is driving me bananas! I once had a boat where the cooker would resonate in a similar fashion at one RPM but changing the speed would stop it. This comes in at about 2500 RPM and just gets louder up to max throttle. Anyone got any ideas? /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif
 

Aja

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Does it happen with engine both in and out of gear?

At least you can identify if it is engine or drive train oriented. It could also be panel accoustic vibration. Any big unsuported bulkheads?

Donald
 

Pye_End

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Sounds like 'beats' - the interaction of the sounds from the 2 engines giving noise nodes and anti-nodes. Similar to wave patterns when they bounce off the sea wall - the waves get bigger due to their interaction.

Nothing wrong, just the way sound waves interact.
 

boatmike

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To answer both posts.
1. There is only one engine.
2. There are no large unsupported bulkheads, but it does sound like the structure is resonating, but not at a single frequency which you would expect.
3. It only happens with engine in gear and travelling at 4-7 knots. Below that speed nothing.
 

Pye_End

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I think I would still run with a 'beats' theory with a resonance with something.

However - 2 further thoughts - do you find that running with waves and running on a flat calm makes any difference to the noise/pattern?

The only other thought is do you have a wet exhaust system? If so the water comes out in batches as the pressure builds up in the wet box, and maybe this is affecting something?
 

boatmike

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Actually that does not sound so daft, and being a cat it has large flat decks, but these dont seem to be resonating, it is coming from lower down.... and it's not just at one frequency or it would go through that range as I alter the throttle opening...
 

boatmike

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Pulsating exhaust? Hmm... It is lying on the hull.... I will check! It's going to be something daft like that when I find it!
Oh and no... no difference with sea state....
 

whipper_snapper

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It certainly sounds like a beat. My best guess is that it is sound reflected back from the inside of the hulls on either side of the leg and beating with each other or with the leg.

And the solution would be ....ummm....dunno.
 

tobble

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I think irt is beating, but to achive this effect there must be two independant sources of noise that operate at slightly different frequencies. It will always sound like the hull/deck/bulkheads are the source because they are connected to the engine and sail drive, and are much more efficient radiators... unlikely that the beating is caused bya resonance, as a panel or other structure will generally only vibrate at the frequency(ies) being applied to it, but with different levels of response depending on if it's being excited near a resonance frequency or otherwise...

Is it possible that the prop is singing at a frequency slightly different to the engine through the range of revs?

p.s. is there anyway of posting sound clips on the forum? or videos even?
 
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