Vibration problem - ideas needed

[peter]

Team
I have a vibration at the rear of my boat when I hit 3000revs. The vibration stays until I move past this range. The vibration is coming from the plate in front of the motor. The vibration disappears when I place a bit of pressure on the plate by pushing it.

Is there a way to stop it vibrating without having to weld a brace onto the plate. I've attached a photo with an X marked on the plate in question.

The noise of the vibration is annoying.

Any help appreciated.

Peter

[lunar_c]

Just a suggestion:
Maybe a couple of pieces of high density foam or rubber strategically glued to the inside of the panel.

Ben.

[henryk]

you are resonating between boat and motor at these revs and will need to do something physical to stop it. the risk is that you may only end up changing the revs that your bluefin will vibrate at. welding a brace will probably help. cant offer much else advice at this stage.

[toymod]

Is there rubber inbetween your motor and the hull? This may reduce the amount of vibration, what does the other side look like? Is there any thing loose?

Cheers

David

[familyman]

Peter if you can find a solid spot to base a car jack you could stress the panel slightly toward the motor then remove the jack and pop the panel back in like the base of an aluminium frying pan.Any other cure will require heat or penetrating the panel.Go slowly :o
cheers jon

[fishingrod]

It might be a bit primitave, but will a few pieces strips of nylon or a chopping board etc either siliconed or bolted on the other side might be enough change the properties of the section and stop the vibration ??? It might look ugly on the other side ....unless u have a rear lounge to hide it behind.

It could be an excuse to install some extra accessories ..... something like a battery switch or bait tank or anything bolted on might "weigh down" that panel and stop the vibration. But if it still oscillates silently it might shake the bugger out of the battery switch and cause a failure.

Or as FamilyMan said, try stressing or bending it very slightly and it might help.

It would be better to stop the vibration totally than just stop the noise. If you spend too long at 3000rpm you might cause damage to the boat. Just because you cant hear it doesnt mean its not there.

[Eagle]

Hi Peter,
What you are experiencing is what's called the "natural frequency". This is the the rate or speed an article will vibrate at when struck by a hammer. The best example of this is a tuning fork. Tap it on a solid surface and it will vibrate at its natural frequency. For your boat, this comes in at 3000 rpm. It is self dampening as the revs go higher or lower. The first place I would look in stopping it is to check the prop. It needs only a very small chip in a blade to start a natural frequency vibration. My guess is that you have a Stainless Steel prop. A slight bend in a blade can also cause this vibration. The vibration is dampened by the other parts of the gearbox, engine etc. being out of sync at revs above and below 3000 rpm. Did you ever see the film of the Tacoma Bridge in the USA that fell down as it vibrated at its natural frequency? It's a famous piece of film and it's usually shown to all engineering students at uni.
If you continue to run at 3000 rpm you may cause severe damage to your boat and or motor, but especialy the boat. You must find the cause and correct it rather than trying to stop the plate from shaking (vibrating). Such shaking (high frequency vibration) can quickly cause a failure due to fatigue. Aluminium is relatively soft and flexible for a long time (measured in number of vibrations or flexures over time) but it will certainly fail from fatigue.
This should give you something to look into. Let me know what you find out with the prop.
Joe (Eagle)

[Angla]

Has it always done this or has it just recently started.
You could put a pillow in the space between the two plates to dampen the vibration but as Joe(eagle) says. You must look for the cause.

Angla

[toymod]

Eagle you are right about the "natural frequency", but however it is not to the boat but rather that 1 peice of metal, at worst you will see weld damage nothing else.

I am against things like "glueing" another product on the back as it will not last as the metal will still want to vibrate and the glue will fail. This said the glue will only fail if it is caused as eagle said with an external constant source such as the motor.

I would be trying to find the source of the issue not a bandaid soloution. Find another boat owner with the same hull, are they getting this? As I said before, is there rubber between the motor and hull, are the welds sound (check inside and out? Have you brought this up with you mechanic on a service? Are you sure its that metal or when you push on it, you stop the vibration going through to the source such as your bait board etc that is atached to the hull.

Food for thought

Keep us updated

[bond_007]

Has this just occured, if so then I wouldnt alter your boat in any way, it is related to a rotational component the motor.

maybe 3000 rpm is the 1st critical, you need to understand the forcing frequency, which is the motor rpm.

If it is a chip or general unbalance of the prop, resonance will occur on the boat. By holding the boat in the point you describe you are stiffening the boat which alters the natural frequency up or down from 3000rpm.

Look for the root cause, and if poss do some simple checks, swap the prop, check all parts are secure and stiff, no looseness around this area is evident,

Another thing to think about, have you made any changes to the boat recently

[peter]

Thanks team,

I've taken all your ideas onboard, and agree with first trying to find out why it is vibrating. means I'll have to go back to the dealer where I brought the boat. (not looking forward to that as they have been really unhelpful since they got there hands on my $32,000.)

But will see what they say.

Thanks again, great help.

peter

[brentasauros]

mate..that..sux.
you..spend..32.grand...
they..should..be..making..damn..sure..your..a..sat isfied..customer.
Hope..you.sort..it..out.

[borisdog]

Eagle has it sussed. I have a Cat with twin 60 Mercs and destroyed 3 gearboxes in 5 years on the port motor. I ended up seeing a small casting imperfection in the prop ( I couldn't feel any vibration)
I took it to get it's balance checked out of curiousity - short version was the guy couldn't believe how bad the prop was with such a small blemish and was not surprised when I told him the gearbox horror story. Been good now for 3 years. Might be worth checking the prop just to be sure.

[mark221263]

Hi, resonance or natural frequencies will develop the vibration your talking about. Like some of the other guys suggest you could weld or glue a brace to help stiffen things up. Both will work but I don’t like welding a brace onto such a nice paint job burning things up.

Try putting the vibrating sheet under compression which will raise it’s natural harmonics and stop your problem. Use 1 or 2 pieces of 6 mm diameter 316 stainless rod that are long enough to run a cross the rear of the panel with an extra 40mm in total length. Then drill a 6.35mm (1/4”) hole either side of the out board-well piece about 20mm behind the vibrating panel edges. Have the rod long enough to go through the 2 outboard well sides and into the hull cavity. Get each end of the rods threaded, then put in place with a nut on either end and tighten up just enough to put the vibrating panel under compression and stop the resonance. No welding or glue to become unstuck.
cheers Mark


cheers Mark

[PG]

Shouldn't you just run the motor at 3000 revs and not in gear to see if it's the prop or not? Rev it up slowly in neutral and listen carefully...before you go to all this trouble. If it doesn't stop, them ya know it's not the prop. If it does stop, still could be a bunch of things I s'pose.

[baitchuka]

Also known as as harmonic vibration, some times the problem can be overcome by simply removing the prop turning 180 degrees and refitting.A slight imbalance in the prop combind with a slight imbalance in flywheel and or other moving parts in the engine can somtimes exagerate the problem. rotating the prop can sometimes negate the problem...Hope ya sort it out.
CHEERS DAVO......

[juicyfruit]

:Pvibration

Peter, did you get on to your dealer.

Curious to know what your dealer had to say.

Juicy