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Old 02-23-2019, 08:23 PM   #1
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New Install, Northern Lights 5Kw Genset

I bought a NL 5kw Genset, with a sound shield. I'm replaceing a well used 1979 Onan 3kw MDJA. It will go in a different spot in the lazertte because of the larger footprint of the soundshield.

Question. What is the most effective way to mitigate structural noise transmission? The lazertte has a "floor" made up of 3/8" Plywood painted sheets placed on top of a 2"x2" stringer structure. The fiberglass hull is heavily laid up 1981 era fiberglass.

The generator will be placed on a 3/4" plywood base that will be screwed to the plywood floor, which intern will be screwed to the stringers.

Should any thing be placed between the plywood and stringers or plywood basse and plywood floor to reduce noise transferance?
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Old 02-23-2019, 08:41 PM   #2
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First, the 3/8 plywood is not strong enough to make a base for the genset. I would laminate the 3/4 plywood to it and see if you need more support in between the stringers. Then maybe check with a company like Soundown for flexible mounts to mount the genset to the strengthened deck.
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Old 02-23-2019, 09:15 PM   #3
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I have this genset, which by the way is a good choice. Mine is mounted on a 2" thick plywood shelf between the inboard stringers, aft of the engines. The shelf is bolted to the stringers with no padding. The genset is attached to the shelf on shock mounts which I believe are NL issue. With this setup, I have no vibration or resonance transferred to the hull. I also don't use a sound shield and things are quieter than the Onan I removed that had a sound shield.
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Old 02-24-2019, 04:35 AM   #4
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I have an 8 KW Northern Light. Although it has a sound shield I leave the front cover off so I can see things. It is still very quite. Having to do over again I wouldn’t bother with a sound shield. My 2 cents.
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Old 02-24-2019, 05:47 AM   #5
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"What is the most effective way to mitigate structural noise transmission?"

Mount the noisemaker to a heavy structural board or steel frame on its floating mounts.Noise cover too.

Mount the board or steel frame to the boat on a second set of floating mounts,sized for the combined weight..

Expensive to do with a propulsion engine as a thrust bearing and special shafting is required, BUT not a big job for just a noisemaker.

All the items like fuel lines , exhaust , electric wiring and controls must be extra flexible .
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Old 02-24-2019, 08:26 AM   #6
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I have a 5kw Northern Lights generator, it’s mounted to 3/4” marine plywood. The generator plus sound shield weight is 400 lbs +- with a small foot print for mounting. The installation was 9 years ago and the mount you see in the photo doesn’t transfer noise. The generator is shock mounted to the stainless steel drain pan for vibration noise transfer elimination. This install has proved to be very quiet.
Note, The black electric cord in the photo is to our 450 watt Extreme bilge heater.
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Old 02-24-2019, 08:48 AM   #7
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You can get from NL or fabricate yourself a steel mounting plate which also serves as a drip pan. Mount the genset to the "pan" with genset motor mounts. This is how our Westerbeke is setup too. Paint the "pan" white.
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Old 02-24-2019, 09:46 AM   #8
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I have the 5k NL in my engine room just outboard of the main engine. Drip pan hard mounted to a shelf attached to a stringer and chine board. No soundbox at all. Vibes and noise are very tolerable. In the salon and in the cabins, it is just a pleasant purr. Great for sleeping.

A 3cyl has a minor first order vibe (really a "waddle") that tends not to be offensive. 1, 2 and 4cyl are horrible. On those it is good to mount them on a structure that is itself floating on resilient mounts. I find no such need on the little 3cyl.

You may even find the soundbox to be unnecessary, but you already own it.
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Old 02-24-2019, 09:58 AM   #9
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Quote:
Originally Posted by Ski in NC View Post
I have the 5k NL in my engine room just outboard of the main engine. Drip pan hard mounted to a shelf attached to a stringer and chine board. No soundbox at all. Vibes and noise are very tolerable. In the salon and in the cabins, it is just a pleasant purr. Great for sleeping.

A 3cyl has a minor first order vibe (really a "waddle") that tends not to be offensive. 1, 2 and 4cyl are horrible. On those it is good to mount them on a structure that is itself floating on resilient mounts. I find no such need on the little 3cyl.

You may even find the soundbox to be unnecessary, but you already own it.
Ski
Excellent point on the 3 vs 4 cylinder vibes. Some years ago we had a 3 cylinder and it was soft mounted in a pan, I thought that was SOP. Other than that all our units have been the rambunctious soft mounted 4 cylinder units that jump around a bit during startup and shutdown.
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Old 02-24-2019, 10:59 AM   #10
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All diesels with low cylinder count will jump around on startup and shutdown, especially on soft mounts like most gennies. Normal.

The three cyl both ends of the engine wants to rotate a bit, with the front doing it 180deg out from the back. This vibe is 1:1 with engine rpm But add the mass of the rigidly mounted gen end and there really is not much vibe left. With factory soft mounts very little vibe gets into boat structure, and at the low frequency it does not tend to bother anyone.

The four cyl is a different story. It has a second order vibe (often called a buzz) the shakes the engine straight up and down at twice the frequency of crank rotation. It turns out that at 1800rpm this buzz is 60hz, twice that and much more intense than the 3cyl vibe. This is why lots of higher end 4cyl engines have dual balance shafts with weights in oil pan spinning 2x crank speed. I don't think any smallish 4cyl gennies have such.

I can walk onto a boat with gennie running and tell right away if it has 3cyl or 4cyl.

On a four, it is good to mount gennie on a heavy board with a secondary set of isolators between the board and boat structure.

Won't even talk about 1 and 2cyl. The are infernal beasts. Six cyl smooth as silk.
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Old 02-24-2019, 11:11 AM   #11
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The new NL machines come with a stainless pan and additional plates of stainless welded onto the bottom of pan at the ideal mounting location. I used pulltruded 4 fiberglass tubes to lift the machine of the deck to allow better access since I had the room. The nl rubber isolators between the pan and the generator rails are really all you need. These machines are very quiet. Here are some photos of a 6kw I put in last june.Click image for larger version

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