Stopping Outboard/ Cockpit Vibration

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PotatoNavigator

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My boat is due to come out of the water soon and one of my jobs is to attempt to reduce the vibration noise caused by the outboard. It is unbearable after more than about 10 minutes, to the point where it ruins a trip if the wind dies and we have to motor sail.

I have a Tohatsu sailpro 6hp which sits in a well. The problem doesn't lie with the outboard itself (I put it on a dinghy once and it was near silent), but with the cockpit sole and seats. The construction means that the entire cockpit is essentially floating, only connecting to anything else at the companionway and the gunwales.

The mount for the engine is part of the same construction as the well, so the aft facing portion does connect to the hull, but only over a width of about a foot, since either side of the well are two lockers which connect to each other under the cockpit, in front of the well.

You might get a better idea of the design here:

http://sailboatdata.com/viewrecord.asp?class_id=3978

My current idea is to wedge rods with rubber feet on the ends between the hull and cockpit floor to dampen the vibration. Before I start a series of time wasting exercises does anyone have experience with anything similar, or any better ideas?

// Mounting the engine on a bracket off the stern is not an option, since the position of the lockers would make it impossible to control. Also fitting an inboard is out, partly due to cost and partly due to there not being enough room. It was an option specified when the boats were originally built, but the space only allowed for a lawnmower engine type set up, which will hardly improve my noise situation.
 
PotatoNavigator said:
My boat is due to come out of the water soon and one of my jobs is to attempt to reduce the vibration noise caused by the outboard. It is unbearable after more than about 10 minutes, to the point where it ruins a trip if the wind dies and we have to motor sail.

I have a Tohatsu sailpro 6hp which sits in a well. The problem doesn't lie with the outboard itself (I put it on a dinghy once and it was near silent), but with the cockpit sole and seats. The construction means that the entire cockpit is essentially floating, only connecting to anything else at the companionway and the gunwales.

The mount for the engine is part of the same construction as the well, so the aft facing portion does connect to the hull, but only over a width of about a foot, since either side of the well are two lockers which connect to each other under the cockpit, in front of the well.

You might get a better idea of the design here:

http://sailboatdata.com/viewrecord.asp?class_id=3978

My current idea is to wedge rods with rubber feet on the ends between the hull and cockpit floor to dampen the vibration. Before I start a series of time wasting exercises does anyone have experience with anything similar, or any better ideas?

// Mounting the engine on a bracket off the stern is not an option, since the position of the lockers would make it impossible to control. Also fitting an inboard is out, partly due to cost and partly due to there not being enough room. It was an option specified when the boats were originally built, but the space only allowed for a lawnmower engine type set up, which will hardly improve my noise situation.
Click to expand...

I have the exact same setup, and engine, and the same noise problem - looking forward with interest to what people have to say... in my case I was looking at noise dampening pads where the outboard clips to the boat - I managed to find some scrap offcuts of TekDek or the like - not quite as hard/rigid as wood, but no significant improvement in noise levels...
 
Fantasie 19 said:
I have the exact same setup, and engine, and the same noise problem - looking forward with interest to what people have to say... in my case I was looking at noise dampening pads where the outboard clips to the boat - I managed to find some scrap offcuts of TekDek or the like - not quite as hard/rigid as wood, but no significant improvement in noise levels...
Click to expand...

Good to know I'm not on my own. Having trawled the internet and not found anything, I imagine there are quite a few people that would like a solution.

I also tried dampening pads, started with sorbothane sheet over the whole mount, I didn't see a reduction in noise either. Then I took it further and welded up a ss cradle to sit in the well that was completely isolated from the boat by 25mm thick sorbothane feet, this almost completely solved the noise problem, but allowed for two much movement of the outboard (it would hit the hull in forward and the rudder in reverse). So I swapped the sorbothane feet for rubber ones, brought the deflection to within tolerance but the noise came back!
 
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I would suggest that you get some of those scissor jacks as used on small cars. Fit a big rubber pad on top and try putting these between the hull and the floor. Or cockpit side and hull. Then expanding the jack(s) to provide pressure while checking the results. Wedges might do the same job but more fiddly. good luck olewill
 
That would be my worry - boats flex naturally - hit a bit of chop and ... bang??

How about that expanding foam builders use, could you get any of it into the lip the outboard is clipped to??
 
This has been occupying my mind a bit too and interesting that you have tried a cushin mounted subframe, as that was one idea that I was thinking of. If the fore and aft movement was too much, would it have been possible to clamp a bracket to the leg, just above where it exits the hull and fit some buffer pads? The forward one needs to be the soft one as noise in reverse in not too important as long as it limits the movement.
You don't detail the subframe, but my thought was of one with the soft mountings right at the top front corners of the well and the lower ones as far down as possible, also in the corners and in shear.

I can't remember any well installation that wasn't noisy to some degree. Mine is a 2T aircooled one and fairly bad, friend with a 4T in a similar situation also finds it intrusive.
DW
 
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It might just be unfortunate that your cockpit area resonates with the frequency/order of your outboard.
If so you must change the natural frequency of either.
You can not fundamentally change the natural frequency of the outboard , so try changing the natural frequency of the cockpit area.
To do this the best way is to use a mass damper , ie. change the mass of the surrounding area.
I would suggest trying a sandbag (or 2 or 3) on the cockpit floor or elsewhere and see if it damps the vibration. If so investigate other ways of adding the equivalent mass.

( sort of ex NVH engineer at Land Rover)
 
Fantasie 19 said:
That would be my worry - boats flex naturally - hit a bit of chop and ... bang??

How about that expanding foam builders use, could you get any of it into the lip the outboard is clipped to??
Click to expand...

Yes exactly! I think I would be OK if I used the scissor/ bottle jacks to test the concept, but then make a permanent feature by bonding the supports between deck and hull, so it's not under any pressure. The lip the outboard is currently connected to is wood with fg over the top. I thought about filling all of the void and seat backs with expanding foam, but it's not very elastic, so if it's compressed by the vibration it will stay smaller and stop damping the vibration. I guess it would absorb a lot of the sound reverberating in the void, but then it would be impossible to access the hull for repairs or anything else.

DownWest said:
This has been occupying my mind a bit too and interesting that you have tried a cushin mounted subframe, as that was one idea that I was thinking of. If the fore and aft movement was too much, would it have been possible to clamp a bracket to the leg, just above where it exits the hull and fit some buffer pads? The forward one needs to be the soft one as noise in reverse in not too important as long as it limits the movement.
You don't detail the subframe, but my thought was of one with the soft mountings right at the top front corners of the well and the lower ones as far down as possible, also in the corners and in shear.

I can't remember any well installation that wasn't noisy to some degree. Mine is a 2T aircooled one and fairly bad, friend with a 4T in a similar situation also finds it intrusive.
DW
Click to expand...

Yes that was pretty much the set up, I think you're right, having a buffer down at the leg would stop the forward movement, but I wasn't keen on the engine transferring its load through to the hull via it's casing, I could ask Tohatsu, but they're certain to say don't do it regardless.

jakeroyd said:
It might just be unfortunate that your cockpit area resonates with the frequency/order of your outboard.
If so you must change the natural frequency of either.
You can not fundamentally change the natural frequency of the outboard , so try changing the natural frequency of the cockpit area.
To do this the best way is to use a mass damper , ie. change the mass of the surrounding area.
I would suggest trying a sandbag (or 2 or 3) on the cockpit floor or elsewhere and see if it damps the vibration. If so investigate other ways of adding the equivalent mass.

( sort of ex NVH engineer at Land Rover)
Click to expand...

You're right, and my intention is to line the interior of the lockers either side of the well with self adhesive cork tiles, which should stop those vibrating as much. The problem with doing this with the cockpit is that it is already quite heavy, so to change the natural frequency of it away from the frequency of the engine would require too much extra mass, for example, there is no real reduction in noise when four people are sat in the cockpit.

I think the issue is the amplitude at which the sole is allowed to oscillate, rather than it being at resonant frequency.
 
*Update*

We might have had a breakthrough, Fantasie, I tried William's idea last night. I put a bottle jack under the cockpit floor propped against some sorbothane sheet, not under a lot of pressure, just pinched it up. The result was a marked drop noise, it was still there but you could hear it had dropped in frequency, and that was just with the one prop. I think it will work even better with two rows or four, also a lot of the noise comes from the lockers either side of the well, which are quite flexible, I think if I stiffen those as well it will bring the noise down to acceptable levels. I'll do another update after I've finished, but the boat is due to come out of the water for a couple months so might be a while.

The challenge now is creating some sort of bottle screw type device to go between the two rubber stops, ideas welcome, thinking I might have to buy a reverse tap and die set.
 
Dave_Plymouth said:
Please could you update us on your progress in reducing noise and vibration. - Dave
Click to expand...

So after the success of the bottle jack experiment I created something more substantial that spread the load over a larger area. I ran two lengths of wood under the cockpit sole, and two underneath along the hull, with threaded rod between them, loaded using nuts and washers.

The new arrangement almost eliminated vibration noise at idle, it was still louder than with the engine on a dinghy, but perfectly acceptable. Unfortunately underway the attenuation was not nearly as much, an improvement, but not enough to have bothered with.

I've since sold the boat to focus my attention on my restoration project, so I won't be able to do any further experimentation. For the benefit of anyone in a similar situation, my next step would have been to put some sorbothane sheeting between the wood and the fibreglass. The bracing method worked by increasing the effective mass of the cockpit, but it allowed the higher frequencies to resonate still, I think the addition of a damping effect might have been the final solution.
 
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