Mounting genset between stringers

I've yanked my gen-set from its hell-hole beneath the galley and carved-out a new hatch for it in the aft-deck (veranda) just ahead of the lazzerette hatch. I recon'd the gen-set and gently placed it into the hatch, check all the clearances, leveled the unit, and now I'm ready to make the mounts to rest the entire unit (including the drain pan) between the stringers.

The Northern Lights 4.5 gen-set rests on four new motor mounts on the SS drain pan. The idea is to double insulate the generator vibration by also making the stringer mounts rest on rubber. The pan is 17" wide and there is 19" between the stringers. The hull shape will require me to space it 1/2" aft and 2 1/2" in the front to make the unit level.

Am I going to far trying to insulate the unit further by using more rubber mounts? The unit is 380 lbs. or so. The original mounting was a simple plywood over stringers design, with the drain pan flat dab on the plywood with a few SS retainer clips over the pan.

Thanks in advance for the feedback.

The photo you posted appears to show a generator resting on the hull.

I'm assuming that either the generator is not permenantly installed, or that the mounts are hidden from view, or that I just missed them.

I would have no problem in theory having the generator itself being mounted on a rubber isolated mount, and having that rubber isolated mount further isolated.

Is it necessary? I dont know. I have a NL 9 kw generator and its really smooth with just the factory rubber isolated mounts, and the whole unit bolted to a plywood between the stringers type base plate.

From the picture, it looks like it high enough already without additional mounts under it. Based in that, if it were mine, I would be interested in having room over it for insulation or maybe a "sound box" that drops over it and making sure I had the best, shear type mounts in use.

The photo is of the gen-set simply sitting on 1/2" plywood at the rear motor mount and 2.5" of plywood at the front mount (only to level the unit), everything resting on the hull. Clearance is 3" from bottom of hatch to top of pressure cap. I couldn't have asked for a more perfect fit. The planned mounts would go underneath the drain pan and over the stringers on each side. I have some lead filled sheet for insulating the hatch and aft bulkhead further.

Larry,

I'd recommend going to vibrationmounts.com and downloading and reading the technical section and perusing some of their catalogs.

Generally speaking you want your "rubber" to have a natural frequency of approximately 1/3 of the frequency of what you are trying to attenuate, which is likely running speed or maybe bar passing frequency of the generator. The natural frequency is dependent upon stiffness, which in turn is effected by the load on the rubber. Too high or too low of a load would result in minimal gains. Worse case scenario (but not likely) is that you could make it worse by adding resonance.

Northern Spy said:

Larry,
Worse case scenario (but not likely) is that you could make it worse by adding resonance.

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Yeah, that's what I'm afraid of. I've been tossing back and forth ideas of how to and where to put the extra rubber. Ultimately, a simple rubber lining between the plate cradling the unit between the stringers and the drain pan would allow me to play with different compounds. I know nothing about the frequency absorbing abilities of different compounds, but I suspect that this is truly not a case of the "softer is better" concept. Thanks for the reference.

The US Fisheries boats and others that want a really quiet engine will create a box motor mount frame that is raised on rubber mounts and then has the engine bolted to more rubber mounts on the frame.

This works excellently , but does not cover the entire speed range.

Since a noisemaker is a single speed item , it might be worth some heavy extra angle iron and a second set of isolation mounts.

With a sound absorbing box cover , the noisemaker might be easier to live with.

FF said:

With a sound absorbing box cover , the noisemaker might be easier to live with.

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FF is right with this train of thought.

It appears that you are focusing on the mounts, which are something the nice folks at Northern Lights have already figured out for you.

Most of the rest of our boats simply bold down the generator, and vibration is minimal.

You will get alot more sound attenuation making a much more usable installation if you install a sound shield around that generator.

ksanders said:

It appears that you are focusing on the mounts, which are something the nice folks at Northern Lights have already figured out for you.
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Yes NL and Westerbeke have on site tech guys and engineers (like Spy) that get paid to come up with the best elastomers. Pay the extra dollars and order from them.

I think Larry is on the right track. He just needs to do the math to get the right durometer material. Unisorb is another company that has a lot of material on line that may be of help.

You may also consider adding some mass to the pan and on the plywood bulkhead seen in the photo.

I know I will be mocked for this. But I did three coats of Silent Running on the the hull and bulkhead of my engine room, before I put on the well fastened and taped sound down insulation. You can be standing on the dock, give feet away from my running engine and not hear it. Mass dampening works.

My next sound deadening project is to make an inertia block for my reversing autopilot pump.

Northern Spy said:

I think Larry is on the right track. He just needs to do the math to get the right durometer material. Unisorb is another company that has a lot of material on line that may be of help.

You may also consider adding some mass to the pan and on the plywood bulkhead seen in the photo.

I know I will be mocked for this. But I did three coats of Silent Running on the the hull and bulkhead of my engine room, before I put on the well fastened and taped sound down insulation. You can be standing on the dock, give feet away from my running engine and not hear it. Mass dampening works.

My next sound deadening project is to make an inertia block for my reversing autopilot pump.

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I would miss that annoying, squeaky grrrr sound letting me know MR Auto Pilot is hard at work

psneeld said:

I would miss that annoying, squeaky grrrr

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It's pretty common in "Off the Deep End." There are a couple of thousand squeaks and grrrs about one thing or other ... mostly one thing, sort of like a whiny pump - but doesn't achieve much useful like a whiny pump does.

Thanks everyone. I think I get it. True, the vibs were not all that critical when it was under the galley, but the sound was something I'd like to soften. Somehow, I can't bring myself to enclosing the unit in a sound box.... little or no air curculation. I'll try it with various levels of insulation on the firewall and hatch.