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umpire111

Vibration from weed weed hatch

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Boat has started to develop an annoying  grinding/ cavitation noise from the weed hatch. If I rev up then decrease revs it goes. I assume it’s air in the weed hatch? When I lifted the hatch there was a great vacuum in there and I had to prise the hatch up slightly and air gushed in for a out 10 seconds then I was able to lift it.

 

Any ideas how to address this?

 

Beta 43 with PRM 150 gearbox,  Colecraft hull

Edited by umpire111

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Has your boat just been blacked?? It sounds like the gap between the uxter plate and the cavitation plate has become almost sealed....and a vacuum is being generated by the prop water passing under it.

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Just now, matty40s said:

Has your boat just been blacked?? It sounds like the gap between the uxter plate and the cavitation plate has become almost sealed....and a vacuum is being generated by the prop water passing under it.

 

My thoughts exactly, but you put it more succinctly than I was going to!

 

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Just now, matty40s said:

Has your boat just been blacked?? It sounds like the gap between the uxter plate and the cavitation plate has become almost sealed....and a vacuum is being generated by the prop water passing under it.

Its a new boat, just developed it after 40 hrs, it’s annoying more than anything. Don’t know what a cavitation or uxter plates are

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Odd. I guess you've no leak in through the stern gland else you'd have mentioned it.  The vacuum might be a distraction - are all the propshaft mountings  / bearing secure and running sweet? Is the prop securing nut tight? 

 

Edit - possibly still worth checking but matty knows more than I do for sure. 

Edited by twbm

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Isn't this the same boat on which we had the stern gland question recently, and are the two problems connected (other than in the obvious way)?

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3 minutes ago, twbm said:

Odd. I guess you've no leak in through the stern gland else you'd have mentioned it.  The vacuum might be a distraction - are all the propshaft mountings  / bearing secure and running sweet? Is the prop securing nut tight? 

 

Edit - possibly still worth checking but matty knows more than I do for sure. 

Checked all nuts etc and all seems ok. Did have a leaky stern gland but a small turn of the two bolts sorted that

Just now, Machpoint005 said:

Isn't this the same boat on which we had the stern gland question recently, and are the two problems connected (other than in the obvious way)?

Same boat, don’t think they’re connected?

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Cavitation plate too large so drumming against the weed hatch trunk?

 

Has the cavitation plate been fitted with a rubber seal for some reason?

 

Uxter plate also sometimes called the swim plate is the large plate with the V shaped cut out for he hull sides that surround the engine.

 

Cavitation plate, the horizontal plate that often hangs down from the underside of the weed hatch cover.

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On the other hand perhaps it's not complete nonsense - thanks Tony!

It occurred to me though after posting that if there is a partial vaccuum under the hatch, it is probably screwed down!

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2 minutes ago, Machpoint005 said:

On the other hand perhaps it's not complete nonsense - thanks Tony!

It occurred to me though after posting that if there is a partial vaccuum under the hatch, it is probably screwed down!

Bit confused Re this, sorry

5 minutes ago, Tony Brooks said:

If so that might be the leak in the other topic.

As I said, I had to prise the weed hatch open to ease the vacuum, def no water coming in thru there

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4 minutes ago, Machpoint005 said:

On the other hand perhaps it's not complete nonsense - thanks Tony!

It occurred to me though after posting that if there is a partial vaccuum under the hatch, it is probably screwed down!

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.

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1 minute 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.

Could hear the air gushing in but seal was strong

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Can't see it being a vacuum.

Is the resistance just the weight of water in a full hatch on the cavitation plate? I think that is more likely. Sounds like the prop is forcing water up into the hatch, unusual as most boats will run forward with the hatch open without sinking, until you stop suddenly!

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3 minutes ago, Tracy D'arth said:

Can't see it being a vacuum.

Is the resistance just the weight of water in a full hatch on the cavitation plate? I think that is more likely. Sounds like the prop is forcing water up into the hatch, unusual as most boats will run forward with the hatch open without sinking, until you stop suddenly!

 

That's a good thought, especially if the cavitation plate is a tighter than normal fit in the trunk. The sucking could be air going in as the water runs out.

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1 hour ago, Tracy D'arth said:

Can't see it being a vacuum.

I can Tracy - mine does it. 

Mine doesn't have a cavitation plate either, but then Simon Piper trads don't - not just cos Piper's don't think it needs one, but also cos there's not enough clearance between the uxter plate and the counter above it to lift one out!  I think that, when cruising, the hatch trunk is full (or nearly) of water, so it has to draw air in to drop to normal level if you open the hatch. Perhaps it's a lack of cavitation plate thing?

 

I think we may barking up the wrong tree here if we blame this vacuum for the issue as I don't have such a problem.

Edited by Sea Dog
Changed unter back to uxter!

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2 minutes ago, Sea Dog said:

I can Tracy - mine does it. 

Mine doesn't have a cavitation plate either, but then Simon Piper trads don't - not just cos Piper's don't think it needs one, but also cos there's not enough clearance between the unter plate and the counter above it to lift one out!  I think that, when cruising, the hatch trunk is full (or nearly) of water, so it has to draw air in to drop to normal level if you open the hatch. Perhaps it's a lack of cavitation plate thing?

 

I think we may barking up the wrong tree here if we blame this vacuum for the issue as I don't have such a problem.

But that is exactly what I said, its full of water not a vacuum! It sucks as the water drops out.

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54 minutes ago, umpire111 said:

Could hear the air gushing in but seal was strong

Mine often builds up a vacuum if I open it as soon as I stop

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47 minutes ago, Tracy D'arth said:

But that is exactly what I said, its full of water not a vacuum! It sucks as the water drops out.

Ah, then we almost exactly agree! The water trying to find its level once stopped either causes a vacuum above it or just draws air in behind it when the hatch is cracked, or both. Whether there's actually a vacuum "bubble" will depend on the surface area and head of water I'd have thought, but the effect is the same here, right?

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1 minute ago, Sea Dog said:

Ah, then we almost exactly agree! The water trying to find its level once stopped either causes a vacuum above it or just draws air in behind it when the hatch is cracked, or both. Whether there's actually a vacuum "bubble" will depend on the surface area and head of water I'd have thought, but the effect is the same here, right?

👍

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2 hours ago, Tony Brooks said:

Cavitation plate too large so drumming against the weed hatch trunk?

 

Has the cavitation plate been fitted with a rubber seal for some reason?

 

Uxter plate also sometimes called the swim plate is the large plate with the V shaped cut out for he hull sides that surround the engine.

 

Cavitation plate, the horizontal plate that often hangs down from the underside of the weed hatch cover.

When our boat was new we had noise sometimes from down there. But it always coincided with a trip down the weedhatch. If I pushed it to the stern, no noise. If I pulled it forward, noise.

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I have been on this boat (a very short distance) and heard this sound. Its in both forward and reverse (I think) at low cruising revs which is odd. It sounds like a prop thing, almost a cavitation/turbulence/sucking air sort of thing rather than a mechanical problem. Its a very quiet boat which makes the sound more obvious, in fact its the dominant sound, but I would say it is louder than any normal prop noise.   Colecraft have made a lot of boats so are unlikely to have done anything wrong or non standard.

 

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

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Go down the weedhatch and have a really good feel of the prop. Check that its really tight...push in and out and try to rock it as hard as you can. Check extensively for a very small amount of prop fouling. Turn the engine off and take the key out before you go down the weedhatch 😀, and the water is pretty warm at the moment. You are going down through Mnchester so you need to get good with the weedhatch 😀. A very small bit of prop fouling could just explain this noise.

 

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

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