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Vibration from weed weed hatch


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On 16/07/2021 at 20:01, PeterF said:

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.

The base plate of the weedhatch cover seems to sit above the step in the boat body, should it fit snuggly to the body?

On 16/07/2021 at 22:11, dmr said:

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

After vacuum broken sound is still there, should the bottom plate of the weedhatch cover fit snugly to the body? Am sure it does not

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2 hours ago, umpire111 said:

The base plate of the weedhatch cover seems to sit above the step in the boat body, should it fit snuggly to the body?

After vacuum broken sound is still there, should the bottom plate of the weedhatch cover fit snugly to the body? Am sure it does not

There is a 1cm gap between the bottom plate and the “step” in the boat body, should the plate sit on that ledge?

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Just now, umpire111 said:

There is a 1cm gap between the bottom plate and the “step” in the boat body, should the plate sit on that ledge?

If it sits on the ledge you probably wouldn't be able to seal the top, it needs a certain amount of clearance

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4 minutes ago, ditchcrawler said:

If it sits on the ledge you probably wouldn't be able to seal the top, it needs a certain amount of clearance

Understand, so 1cm about right?

8 minutes ago, PeterF said:

The bottom of the base plate on the weedhatch should fit level with the bottom of the boats base plate (uxter plate) to give the smoothest flow and least turbulence.

 

See the linked sketch showing this, not perfectly level but close.

 

https://images.cm.archant.co.uk/service/social-media-image/3844940/6242846/1/6217232-2/5tech-2.jpg

Useful Tx, so are you saying the anti cavitation plate should fit level with the step around the weedhatch, accepting that it will be a smaller size than the hole so not forming a seal?

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On your boat is the hole in the uxter plate smaller than the weed hatch chute leaving a step. A sketch would clear up any chance of misunderstanding. I will have a look at my set up as I also have a recent Colecraft hull and no issues with noise.

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OK, my old boat was like Tony's and flush with the uxter plate, which is what I am used to. I have just had a look at my new boat based on a Colecraft shell and taken some pictures and measurement and the layout is different and i now understand your questions.

I

IMG_20210719_193123.jpg.3471344a3728b176b002544ed3e502b1.jpg

 

IMG_20210719_193129.jpg.282bbbef115bbd628c83fa54f32fb150.jpg

 

My weed hatch construction is identical to yours. The distance from the bottom of the gasket to the bottom of the anti cavitation plate is 215mm. The anti cavitation plate is 392mm x 190mm.

 

The distance from the upper face of the chute where the gasket sits to the bottom of the uxter plate is 240mm. This means that the bottom of the anti cavitation plate is 25mm above the uxter plate.

 

On my boat the hole in the uxter plate is smaller than the chute giving the step you mention. As the uxter plate is 10mm thick, there is 15mm gap between the bottom of the anti cavitation plate and the top of the step.

 

The inside of the chute measures 405mm x 200mm so there is a 5mm gap between the front and back of the plate and the chute with 6.5mm at the sides. My hatch does not generate a vacuum and is easy to remove. It does not vibrate and so far I have been on a range of depths and speeds.

 

I hope that this helps.

Here is a sketch.IMG_20210719_200450.jpg.38b7d0afb92a4bc6855bce84d152954e.jpg

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4 hours ago, umpire111 said:

The base plate of the weedhatch cover seems to sit above the step in the boat body, should it fit snuggly to the body?

After vacuum broken sound is still there, should the bottom plate of the weedhatch cover fit snugly to the body? Am sure it does not

 

Sounds like the vacuum is a red herring, this is sad as I was working on a really novel theory 😀

 

We don't know that the weedhatch is a factor, it could just be the "prop singing" that happens from time to time.

 

The weedhatch design is really interesting, why have Colecraft introduced this flange at the bottom of the weedhatch, my boat (2001) does not have one. Maybe the OP should phone Colecraft and ask? It appears to slightly limit access to the prop. It also means the bottom (anti cavitation?) plate can not sit flush with the Uxter plate (well I suppose it could be it doesn't). On my boat the bottom plate is about 15cm above the uxter plate anyway.

 

I have an idea, its an anti sinking thing because it limits the thrust of water coming up through the clearance gap so if you don't seal the weadhatch correctly you get less water in????

 

and one last thing, I note that you all have the sealing tap on the removable weedhatch whilst mine is on the top flange of the fixed chute. Any thoughts? I reckon your way is better.

 

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

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10 hours ago, dmr said:

 

and one last thing, I note that you all have the sealing tap on the removable weedhatch whilst mine is on the top flange of the fixed chute. Any thoughts? I reckon your way is better.

 

If the tape is on the top of the chute it can get damaged dragging sharp objects out of the chute, having it on the lid keeps it out of the way of damage.

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2 hours ago, PeterF said:

If the tape is on the top of the chute it can get damaged dragging sharp objects out of the chute, having it on the lid keeps it out of the way of damage.

 

In my 12+ years of boating this has never been an issue....till a couple of weeks ago when I had a spectacular prop foul that took 3 hours to clear, and during which I did slightly damage the tape. So rather a co-incidence to now see pictures of the tape on the lid.

 

A tiny advantage of tape on the chute....my tape very slightly overlaps the size of the lid so its easy to see the lid indentation and get the lid back in exactly the same position.

 

........Dave

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37 minutes ago, Tracy D'arth said:

That weed hatch lid and anti cavitation plate will vibrate.

 

It is only stiff in one dimension. It will resonate like a tuning fork.

 

You could get a couple lengths of (say) 50mm x 50mm timber to fit between the lid and the anti-cavitation plate en bolt the tight through the web (vertically). That should stiffen it up and if it works get a couple of steel struts welded in place if the wood.

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8 hours ago, PeterF said:

If the tape is on the top of the chute it can get damaged dragging sharp objects out of the chute, having it on the lid keeps it out of the way of damage.

OTOH, tape damage is more easily seen if the tape is on the chute, rather than the hatch ... swings and roundabouts! :cheers:

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5 hours ago, Tracy D'arth said:

That weed hatch lid and anti cavitation plate will vibrate.

 

It is only stiff in one dimension. It will resonate like a tuning fork.

 

But its bog standard Colecraft design so there are 1000's in service with no problems.

I note that Colecraft do one or two things that are not "elegant" engineering but do work reliably.

 

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

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3 hours ago, dmr said:

 

But its bog standard Colecraft design so there are 1000's in service with no problems.

I note that Colecraft do one or two things that are not "elegant" engineering but do work reliably.

 

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

Mine was a bog standard GT hull

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On 19/07/2021 at 18:24, PeterF said:

On your boat is the hole in the uxter plate smaller than the weed hatch chute leaving a step. A sketch would clear up any chance of misunderstanding. I will have a look at my set up as I also have a recent Colecraft hull and no issues with noise.The boat builder is also at a loss…he’s coming out next month to have a listen

Been suggested that I change the 3 blade prop for a 4 blade, bit more expensive but may stop turbulence….any views please?

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9 minutes ago, umpire111 said:

Been suggested that I change the 3 blade prop for a 4 blade, bit more expensive but may stop turbulence….any views please?

 

No, that's a way over the top fix and will likely have disadvantages (dunno what but everything on a boat is a compromise and 3 blade props are popular for a reason).

 

Fix 1    Learn to live with it.

 

Fix 2  Wait, it may well fix itself as the prop gets a bit of wear.

 

Fix 3 At next drydocking get somebody who maybe knows a little bit about props to adjust the blade edges with a file. 

 

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

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Just now, dmr said:

 

No, that's a way over the top fix and will likely have disadvantages (dunno what but everything on a boat is a compromise and 3 blade props are popular for a reason).

 

Fix 1    Learn to live with it.

 

Fix 2  Wait, it may well fix itself as the prop gets a bit of wear.

 

Fix 3 At next drydocking get somebody who maybe knows a little bit about props to adjust the blade edges with a file. 

 

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

Think that maybe sound advice, value it, tx

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35 minutes ago, dmr said:

 

Fix 3 At next drydocking get somebody who maybe knows a little bit about props to adjust the blade edges with a file. 

 

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

Funny you should mention that, my builder did exactly that  to the prop fitted by Colecraft before the boat was launched.

 

I have noticed  some flow noise on mine when I increase power which reduces as the boat speeds up, I put this down to the prop demanding more water than is arriving based on the boats forward speed and with an increased water supply to the prop as the boat picks up speed, the noise reduces. It is more noticeable on shallow sections but it is so slight it has never bothered me.

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58 minutes ago, PeterF said:

Funny you should mention that, my builder did exactly that  to the prop fitted by Colecraft before the boat was launched.

 

I have noticed  some flow noise on mine when I increase power which reduces as the boat speeds up, I put this down to the prop demanding more water than is arriving based on the boats forward speed and with an increased water supply to the prop as the boat picks up speed, the noise reduces. It is more noticeable on shallow sections but it is so slight it has never bothered me.

What did he do, file the prop?

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