Could this cause Vibration at higher rpms?

[barossa]

Hi all,

I am trying to solve an issue I am having with vibration during cruising.

Mounts seem ok.

Shaft seems in line,

gland is firm and parallel,

shaft is well lubed.

Oil in both engine and gearbox,

Prop seem secure and free of contact ,

 

What I was wondering was could this bottom plate on my weed hatch be the cause of vibration. ie; is it causing turbulance from cavitating? ( is that the right word? )

Anything over 1500rpm and it starts. Increasing with more revs

It can be felt and heard up at the bow when cruising at this rev.

This is the only photo I have from the build of the bottom plate

Cheers and thank you in advance once again all.

 

Chris / Barossa

[ChrisPy]

if it projects below the bottom plate like that I should think it is likely the cause, or is this photo not the final set-up?

[nb Innisfree]

Hi all,

I am trying to solve an issue I am having with vibration during cruising.

Mounts seem ok.

Shaft seems in line,

gland is firm and parallel,

shaft is well lubed.

Oil in both engine and gearbox,

Prop seem secure and free of contact ,

 

What I was wondering was could this bottom plate on my weed hatch be the cause of vibration. ie; is it causing turbulance from cavitating? ( is that the right word? )

Anything over 1500rpm and it starts. Increasing with more revs

It can be felt and heard up at the bow when cruising at this rev.

This is the only photo I have from the build of the bottom plate

Cheers and thank you in advance once again all.

 

Chris / Barossa

 

Do you know the clearance between propellor and weed hatch?

[barossa]

Not sure of the clearance but will have a look tomorrow and see if I can work it out. This photo was taken early on during the build I 'll see if I can find any images from the lift. cheers

[LEO]

Hi,

 

I have a similar arrangement for the weed hatch on my boat. It does vibrate if the 4 fixing screws are loose.

 

Mine also has two supports connecting the top plate to the bottom one, makes it very rigid, not sure how many you have but is your system rigid enough?.

 

Hope that helps.

 

Loe.

[barossa]

I have a better photo just uploading now should be up in 5!

[barossa]

I have a better photo just uploading now should be up in 5!

This photo was from the lift. it's hard to see but might give you an idea of clearance.

I only have 1 main bolt holding the weed hatch down which is very firm always.

The weed plate when I take it out is heavy and solid.

[LEO]

Hi,

 

Mmmmmm! not sure about the pic!. As I said mine has 4 bolts, tightened with an Allen key, provided they are tightened up there are no rattles.

 

The whole unit is very heavy and as it forms part of the floor of the counter makes it virtually impossible to flood the boat with the weed hatch loose.

 

I would say that your rattle is attributable to the arrnagement you have.

 

Leo.

 

This weekends job for me - to master the technique of posting pics!!!!.

[barossa]

Hi,

 

Mmmmmm! not sure about the pic!. As I said mine has 4 bolts, tightened with an Allen key, provided they are tightened up there are no rattles.

 

The whole unit is very heavy and as it forms part of the floor of the counter makes it virtually impossible to flood the boat with the weed hatch loose.

 

I would say that your rattle is attributable to the arrnagement you have.

 

Leo.

 

This weekends job for me - to master the technique of posting pics!!!!.

One more photo

[Smelly]

I'd agree with Chris, that looks like a good candidate for culprit... I've never seen a weedhatch protruding underneath the boat like that. Is it a new build? I'd be expecting a builder to resolve issues like that!

[LEO]

One more photo

Hi,

 

You've got a 4 blade prop!.

 

Sorry can't help with this pic. Have been looking though my pics, can only find one of the counter showing the hatch at the moment.

 

Remove your weed hatch system and take some pics and may be able to help more.

 

ATB

 

Leo.

[nb Innisfree]

Difficult to see but looks a bit close to plate to me, could you clamp a temporary plate to the top, minus a bottom plate, just to eliminate clearance theory?

[Keeping Up]

You could try cruising without a weedhatch for a short while and see if the vibration goes away (but don't go out into rough water, or anywhere that you might need to engage lots of reverse in a hurry). At a steady cruising speed most boats are fine without it as a test measure.

[Yoda]

With a four blade prop you would expect less pressure pulse vibration around the circumference area of the prop. As you can experience it at the bow, there is something very wrong.

 

First try Allen's idea as this will eliminate the hatch cover. If there is still a noticeable vibration at all revs, check that the prop blades are not bent as this is the prime candidate.

[steelaway]

Hi all,

I am trying to solve an issue I am having with vibration during cruising.

Mounts seem ok.

Shaft seems in line,

gland is firm and parallel,

shaft is well lubed.

Oil in both engine and gearbox,

Prop seem secure and free of contact ,

 

What I was wondering was could this bottom plate on my weed hatch be the cause of vibration. ie; is it causing turbulance from cavitating? ( is that the right word? )

Anything over 1500rpm and it starts. Increasing with more revs

It can be felt and heard up at the bow when cruising at this rev.

This is the only photo I have from the build of the bottom plate

Cheers and thank you in advance once again all.

 

Chris / Barossa

 

 

Hi

 

I had exactly the same problem last year. I checked the engine mountings, I thought the engine was bouncing on it's beds.

I checked the prop - I checked everthing

For some reason I loosened the weed hatch while going along and ......Silence.

The bloody weed hatch was vibrating in the hull. I ground a bit more clearance around the bottom plate but that made it worse.

I then packed it higher with 2 thickness's of sponge sealer..... Perfect.

I hope yours is that simple

 

Alex

[mykaskin]

One more photo

 

There seems very little clearance between prop and hull, and this can cause a low frequency rumble/rattle. You may want to try a smaller prop (with a higher blade area), but you could try moving the bottom plate of the weedhatch upwards to see if it improves.

 

You could also try moving the prop backwards as clearance to the swim also is important - this may require a new longer shaft. Don't go too far though as you'll hit the rudder!

 

BTW - There should be a minimum of two inches around the prop.

 

We did both things on our boat, and it's much improved.

 

Cheers,

 

Mike

Edited March 31, 2009 by mykaskin

[DHutch]

You may also be able to refit the weed hatch upside down, cavitation plate end in the air, and test like that without risk of taking on water.

- First picture does show it a little low, but with 6-8mm of foam/rubber seal in there it must be close to flush i would have thought, seams so in the later images.

 

 

Daniel

[alan_fincher]

You may also be able to refit the weed hatch upside down, cavitation plate end in the air, and test like that without risk of taking on water.

Now that's an excellent thought.

 

Although if it's the type that is secured by a bat and big T screw, it may need to be temporarily held another way, (G cramps ?)

 

Or just use something else to cover the hole temporarily - it doesn't need to be sheet steel, a suitable bit of plywood would stop the water coming in whilst you did some tests.

 

Many cheaper boats don't have that lower plate on the weed hatch anyway - there is just an open hole up to the cover plate at the top of the hatch. Before anyone says so, it may not be good practice, but it's a fact!

[Theo]

Now that's an excellent thought.

 

Although if it's the type that is secured by a bat and big T screw, it may need to be temporarily held another way, (G cramps ?)

 

Or just use something else to cover the hole temporarily - it doesn't need to be sheet steel, a suitable bit of plywood would stop the water coming in whilst you did some tests.

 

Many cheaper boats don't have that lower plate on the weed hatch anyway - there is just an open hole up to the cover plate at the top of the hatch. Before anyone says so, it may not be good practice, but it's a fact!

 

Certainly is. Theodora has merely the top plate. I wonder if the big hole above the prop reduces or improves efficiency.

 

N

[onthecut]

Hi all,

I am trying to solve an issue I am having with vibration during cruising.

Mounts seem ok.

Shaft seems in line,

gland is firm and parallel,

shaft is well lubed.

Oil in both engine and gearbox,

Prop seem secure and free of contact ,

 

What I was wondering was could this bottom plate on my weed hatch be the cause of vibration. ie; is it causing turbulance from cavitating? ( is that the right word? )

Anything over 1500rpm and it starts. Increasing with more revs

It can be felt and heard up at the bow when cruising at this rev.

This is the only photo I have from the build of the bottom plate

Cheers and thank you in advance once again all.

 

Chris / Barossa

 

Just a quick and simple thought ----- have you wound the engine up to the relevant rev band out of gear, just to confirm the issue isn't further forward than the prop ?

 

Mike.

[John Orentas]

Had similar problems on my first boat many years ago, not so much a vibration as a very loud rattle.. The cavitation plate was allowed sufficient movement to allow it to contact the sides of the rectangular aperture.. I solved the problem by welding 4 extra separators between plate and hatch cover 1" x 1" box section if I remember, it made for a very rigid assembly.

 

Not sure I would advise running a boat with the weed hatch removed, water can come up there at a fair rate of knots.

Edited April 23, 2009 by John Orentas

[Keeping Up]

Not sure I would advise running a boat with the weed hatch removed, water can come up there at a fair rate of knots.

Have you tried it? On mine, as long as I remain below 3mph and don't ever engage reverse gear, not a drop comes up.

[John Orentas]

Have you tried it? On mine, as long as I remain below 3mph and don't ever engage reverse gear, not a drop comes up.

 

 

No I am happy to say I have never tried it, but I do know that people forgetting to refit the weed hatch is probably the most common method of sinking a boat.. Because you are fortunate enough to get away with it on one particular boat is not a good reason to recommend it for others.

[tanunda]

Just a quick and simple thought ----- have you wound the engine up to the relevant rev band out of gear, just to confirm the issue isn't further forward than the prop ?

 

Mike.

Yeah tried all that and it seems fine.

Went for a spin last week and took the weed hatch out and used a piece of ply to seal off the top and the vibrations were still there but not quite as loud as when hatch is in placce.

[ditchcrawler]

Had similar problems on my first boat many years ago, not so much a vibration as a very loud rattle.. The cavitation plate was allowed sufficient movement to allow it to contact the sides of the rectangular aperture.. I solved the problem by welding 4 extra separators between plate and hatch cover 1" x 1" box section if I remember, it made for a very rigid assembly.

 

Not sure I would advise running a boat with the weed hatch removed, water can come up there at a fair rate of knots.

I had exactly the same thing with a GT hull, just one bar between the cover and bottom plate. Welded two pieces of flay there side stopped it completely.