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Vibration from weed weed hatch


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4 hours ago, Tony Brooks said:

I agree but wonder if the so called partial vacuum was the hatch clinging to the rubber seal. Mine often needed a good tug to lift it and then it was Heavey to pull up.

My thoughts too. It's normal for mine to need a screwdriver (sorry for the tool abuse) under one edge to break the seal.

  • Greenie 1
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Our last boat was a GT hull athe the bottom and top plate were connected by a piece of tube welded between them, It just so happened that it resonated and the bottom plate started to wear a grove in the weed hatch, a second piece of pipe welted in with the first stopped it completely by changing the renaissance frequency  

 

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11 hours ago, ditchcrawler said:

Our last boat was a GT hull athe the bottom and top plate were connected by a piece of tube welded between them, It just so happened that it resonated and the bottom plate started to wear a grove in the weed hatch, a second piece of pipe welted in with the first stopped it completely by changing the renaissance frequency  

Sounds like a likely contender :)

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  • 8 months later...
30 minutes ago, umpire111 said:

I am the author of this post, boat been left for 6 months due to Covid...back to it, been out for 2 weeks and the noise has gone! A complete mystery..any ideas please

You have covid and have gone deaf.

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  • 2 months later...

Would welcome any more views…update. I’ve been out on it now for 4 weeks, down the GU the noise was on and off but when I got on the Thames…no noise. I am firmly of the opinion that it’s a turbulence, cavitation noise exacerbated with shallow water. There have been some suggestion that it’s to do with plates etc in the weedhatch causing problems, I agree with this. There is still a very strong vacuum when I try and remove weedhatch..any connection? Anything I could try . Tx to all again?

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48 minutes ago, umpire111 said:

Would welcome any more views…update. I’ve been out on it now for 4 weeks, down the GU the noise was on and off but when I got on the Thames…no noise. I am firmly of the opinion that it’s a turbulence, cavitation noise exacerbated with shallow water. There have been some suggestion that it’s to do with plates etc in the weedhatch causing problems, I agree with this. There is still a very strong vacuum when I try and remove weedhatch..any connection? Anything I could try . Tx to all again?

I would be tempted to just make a tiny nick in the weedhatch seal to break the vacuum. Not likely to sink the boat unless you do something very odd. Might make some difference but I think you are right, turbulence sounds like the reason. I suppose bolting a rectangle of 3/4" ply to the bottom plate might damp the resonance or frequency of the vibrations. The fact that it stopped after you had left the boat sounds like weed or mussels had attached to the plate.  You could try sticking some mussels on it in varying patterns to get the optimum effect.

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Posted (edited)
26 minutes ago, Bee said:

I would be tempted to just make a tiny nick in the weedhatch seal to break the vacuum. Not likely to sink the boat unless you do something very odd. Might make some difference but I think you are right, turbulence sounds like the reason. I suppose bolting a rectangle of 3/4" ply to the bottom plate might damp the resonance or frequency of the vibrations. The fact that it stopped after you had left the boat sounds like weed or mussels had attached to the plate.  You could try sticking some mussels on it in varying patterns to get the optimum effect.

Confused Re mussels? If it helps, when I’ve been on the Thames been at time 1400 revs, there is a noise which is acceptable and sounds much like the problem noise but much quieter. This is one of the reasons I think there’s a turbulence issue

7 minutes ago, umpire111 said:

Confused Re mussels? If it helps, when I’ve been on the Thames been at time 1400 revs, there is a noise which is acceptable and sounds much like the problem noise but much quieter. This is one of the reasons I think there’s a turbulence issue

Any dangers of trying the nick idea?

Edited by umpire111
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Do you have a down stand plate on the weed hatch lid? If so, depending on exactly where you position the lid when you put it back, it might or might not vibrate against the weed hatch trunk.

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2 minutes ago, David Mack said:

Do you have a down stand plate on the weed hatch lid? If so, depending on exactly where you position the lid when you put it back, it might or might not vibrate against the weed hatch trunk.

On ours I actually bound the bottom plate with cord to prove where it was coming from. When I mentioned it to some one they said why didn't you just invert the lid to try it.

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12 hours ago, David Mack said:

Do you have a down stand plate on the weed hatch lid? If so, depending on exactly where you position the lid when you put it back, it might or might not vibrate against the weed hatch trunk.

Not sure what you mean by down plate, photos attached

1FCF78DD-EC2E-486C-ACDA-8FE888230419.jpeg

12 hours ago, ditchcrawler said:

On ours I actually bound the bottom plate with cord to prove where it was coming from. When I mentioned it to some one they said why didn't you just invert the lid to try it.

So simply wrapping cord around bottom plate…..how does that prove where it’s coming from? Tx

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7 minutes ago, umpire111 said:

Not sure what you mean by down plate, photos attached

1FCF78DD-EC2E-486C-ACDA-8FE888230419.jpeg

So simply wrapping cord around bottom plate…..how does that prove where it’s coming from? Tx

I'm not surprised if that vibrates. With that one vertical leg it is only braced in one direction. Are there any bright rub marks on the edge of the bottom plat where it has been touching the trunk?

There also seems to be no positive location for the top plate. Can it be moved about in the trunk before it is secured by the over bar?

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15 minutes ago, umpire111 said:

Not sure what you mean by down plate, photos attached

So simply wrapping cord around bottom plate…..how does that prove where it’s coming from? Tx

 

Because if the noise stops with the cord wrapping, you know it's the anticavitration plate that is causing the problem. Confirm by unwrapping the plate and see if the noise comes back.

  • Greenie 1
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13 hours ago, umpire111 said:

Not sure what you mean by down plate, photos attached

 

So simply wrapping cord around bottom plate…..how does that prove where it’s coming from? Tx

Are any of these edges shiny, mine where where they were vibrating on the shoot. wrapping the cord round isolated them so the noise changed

image.png.17545fef2ffe292278d93ce039ab01c2.png

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10 hours ago, ditchcrawler said:

Are any of these edges shiny, mine where where they were vibrating on the shoot. wrapping the cord round isolated them so the noise changed

image.png.17545fef2ffe292278d93ce039ab01c2.png

Just checked and no shiny edges

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Mussels. Sorry, just being silly. Where our boat is we have mussels that attach themselves to the boat and propeller. Our propeller has a tendency to 'sing' at certain revs but after being left standing for a few weeks the noise or resonance goes away until the mussels give up and fall off - then the resonance starts again.

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Back on the Oxford Canal, immediately the noise returned. I am convinced it is turbulence , cavitation noise. As suggested I wrapped rope around the bottom plate as shown but that made no difference. Also have released the vacuum by nicking the seal but to little effect. There is no noise at 1000 revs but put it higher and back it comes. Go into deeper water and it greatly reduces.  I’ve been asked for some measurements, 

 

top plate to bottom plate is 30 cm

tip of prop is 18 cm below water line

Prop is 8cm from bottom plate

 

any more suggestions please

96403919-A5FD-4D35-A7FA-8AD21C72518A.jpeg

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11 minutes ago, ditchcrawler said:

If it is osculating it will be going the other way, the rope was to cushion it against the front and back edge of the weed hatch, so wound at 90° to  what you have

Can’t wind it that way as their a down plate going that way

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1 minute ago, umpire111 said:

Can’t wind it that way as their a down plate going that way

Wind it at each end, clear of the down plate. The aim is to prevent metal-to-metal contact, which a rope at either end will do.

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2 minutes ago, David Mack said:

Wind it at each end, clear of the down plate. The aim is to prevent metal-to-metal contact, which a rope at either end will do.

Telling me something I hadn’t grasped. So the rope is to ensure the weedhatch cover doesn’t vibrate against any of the other metal of the boat? So why would it not vibrate in deeper water?

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The water flow will vary in direction, velocity and turbulence with changing water depth and if you have a flow induced resonance then it occurs at a very specific frequency which requires a specific water velocity or turbulence. Therefore, the amount of resonance changes with water depth. The fact that it changes with water depth suggests that it is not a mechanically driven resonance from the engine rpm.

 

Have you checked that the anti cavitation plate sits level with the baseplate. If it sits too high or too low then you may get unexpected turbulence in that area.

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When the sound came back did you open the weedhatch and did you still have the vaccuum?

 

What is the clearance round the bottom plate? with the rope on was it a snug fit or did you still have clearance?

 

Whats than stainless steel bracket thingy?

 

............Dave

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