Low Oscillating resonance AD31P

chillee

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Anyone come across this before and know what the possible cause is. See the video I took (link below)
Video link
You can't hear the resonance on a mobile phone speaker (well not on mine). need headphones with a good bass. Or on my laptop I can just about hear it when I increase the revs in the video. But in real life it is very intrusive.
Basically it is a pulsating or oscillating deep/low noise. I can't hear it really below 2000 rpm. The noise seems to get louder with more revs but the oscillation period does not seem to increase.
Engine is all running fine.
Last weekend I had the noise on a run , on the way out and on the way back from New Town Creek.
Yesterday, I could not hear it at all on the way out, on the way back it materialised again, and was really very intrusive. From the video it does not sound too bad until you use headphones, then you can tell how much the resonance comes across over the top of the background engine noise.
Engine is a VP AD31P - with duoprop drive
 

Montemar

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I think you must mean the slow, low sound. This sounds like the beating effect caused by two sound sources generating sound at slightly different frequencies.
The sound generators would be your engine and the duoprop drive.
I would check the engine mountings to make sure they are tight and functioning properly. As you had a run with and one without the drone it suggests the duoprop. The effect of sea currents may have been different in the two directions.
Perhaps it is loose? It must be worth getting it looked at.
 

chillee

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I think you must mean the slow, low sound. This sounds like the beating effect caused by two sound sources generating sound at slightly different frequencies.
The sound generators would be your engine and the duoprop drive.
I would check the engine mountings to make sure they are tight and functioning properly. As you had a run with and one without the drone it suggests the duoprop. The effect of sea currents may have been different in the two directions.
Perhaps it is loose? It must be worth getting it looked at.
yes thats the noise I mean. hard to describe.
Engine mounts was one thought I was having too. Outdrive was not many hours ago taken off for service. But I recall before that I heard this noise, but it was not so bad and so did not pay too much attention.
 

chillee

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I think you must mean the slow, low sound. This sounds like the beating effect caused by two sound sources generating sound at slightly different frequencies.
The sound generators would be your engine and the duoprop drive.
I would check the engine mountings to make sure they are tight and functioning properly. As you had a run with and one without the drone it suggests the duoprop. The effect of sea currents may have been different in the two directions.
Perhaps it is loose? It must be worth getting it looked at.
out of interest why do you mention a difference in sea currents. How do you see this having an effect.
There was a difference actually. Weekend run was with tide, yesterday was mostly against tide.
 

BruceK

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That sounds so much like sympathetic resonance except the beat didnt change with revs however the tone did slightly. That would indicate the problem is on the engine and drive train itself. As per Montemar, check engine mountings, think back, were you crossing a headland or reef that caused the water to upwell and swirl etc changing loadings on engine and prop, but check things like belt alignment as well, and even possible surging from the fuel delivery system, air intake system etc. for dirty filters and the like.
 

chillee

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That sounds so much like sympathetic resonance except the beat didnt change with revs however the tone did slightly. That would indicate the problem is on the engine and drive train itself. As per Montemar, check engine mountings, think back, were you crossing a headland or reef that caused the water to upwell and swirl etc changing loadings on engine and prop, but check things like belt alignment as well, and even possible surging from the fuel delivery system, air intake system etc. for dirty filters and the like.
Thanks Bruce. It is generally constant. In this instance for over an hour journey back home. Both in Solent open water, Southampton water and on the last part of the journey on the river (if I went above the speed limit ;-) )
So where I was on the water did not seem to make any difference. Only difference did seem to be with or against tide.
I will try to get down this weekend and test again with the tide if possible, and see if I can see anything with the engine mounts.
 

BruceK

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I thought that different loading or pressure on the duoprop assembly may affect how it vibrates against the hull.

That was my initial thought too. Get a lot of that when in the Menai and estuaries where the water is churning with upwellings, back eddies and whirlpools
 
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