Generator exhaust outlet- below waterline

[Labrador Coaster]

Hey folks, planning on installing a new generator in the spring, have heard about the noise reducing benefits of installing the outlet slightly below the waterline.

Other than the obvious things to do to ensure that you don’t sink your boat, or flood your Genset, are most marine generators able to handle the slight “ back pressure” created by exhausting under water? would appreciate your thoughts, thx.

 

[AdmiralS]

You also need a gas/water separator. The exhaust gas is discharged above the waterline and the water through a separate below water through hull.

If you discharge both below the water line you will have a very noisy bubbling generator....

https://www.fisheriessupply.com/centek-gen-sep-water-exhaust-separator

 

[Labrador Coaster]

Ok, was thinking it might work like the exhaust of an outboard motor, the water would insulate the exhaust somewhat… THX,

 

[cks]

I have this separator on our GB. It’s excellent!!

 

[Lepke]

I've run several navy patrol boats with exhaust below the water. Boats were heavily insulated for sound, so at idle, no sound. Exhaust was in the stern with a fitting releasing the exhaust even with the boat bottom. Water stopped most of the noise, even at full throttle. As I remember, planing only drew 9". Engines were Detroit Diesels.

A small generator shouldn't experience more back pressure than a water muffler, but maybe both would be too much. Also depends on the electrical load.

 

[Keysdisease]

Plumb it just like Admiral S says, do NOT ruin a new generator by discharging the gas below the waterline.
Get a lift muffler, mount a gas/water separator (gensep) above it, discharge the water below the waterline and the gas at the boot just above the waterline.

What you do want to do with a new installation is use secondary isolation. that is install 4 isolators under the entire genset in addition to the factory isolators (mounts.) This is standard on larger generators, but smaller units benefit more because they vibrate more and you can't get 100ft away from them like you can on a megayacht. Best bang for the buck in noise reduction, should cost less that $200 for 4 isolators, makes big difference

https://centekmarine.com/solutions/generator-wet-exhaust/

 

[Comodave]

For below water exhaust absolutely use a water/gas separator.

 

[Ski in NC]

It will not cause a backpressure problem. But it will make an annoying bubbling sound that will resonate through the boat, and that means all day and all night!! Also makes it hard to verify the sea water pump is working, hard to see that water is coming out of the exhaust. Don't mount the discharge below the water line.

There are two strategies that work on keeping the exhaust quiet: First, as mentioned above. combine a water lift muffler with a separate exhaust/water separator. This will require two discharge through hull fittings, one for the relatively "dry" exhaust above, and a second for water, that one below. Adds more expense.

Second way is to oversize the volume of the water lift muffler and use that alone, discharge above WL. The larger the can, the more it muffles. Depends on the room in the generator area. For like an 8kW, the can like 16" dia x 16" high with 2" ports does a very good job. With that most of the noise comes from the gen engine itself and is handled separately.

 

[rslifkin]

For an above waterline discharge, another trick is to add an inline muffler between the waterlift and the thru hull. That'll quiet things down a little more, but it also acts as a surge chamber to smooth out water flow, meaning the water will come closer to pouring out smoothly rather than splashing out.

 

[Labrador Coaster]

Great advice all. I have a 4 new rubber engine mounts that came from a small Yanmar diesel. Thinking I can mount the generator on that. And also tap my generator exhaust into my 40'' x 12'' S\S wet exhaust muffler from my main motor. Or might that create issues that I am not thinking of? Maybe best to set up a separate muffler...

 

[cks]

I’ve not ever seen an installation where two exhausts were piped into on muffler.
It’s not clear to me what the ramifications might be and can’t imagine both main motor and generator running simultaneously is a good idea.
Set up a separate muffler sized to the engine.

 

[FF]

Vetus has a good collection of water lock mufflers and diagrams to explain what you need to do.


https://www.vetus.com › en › exhaust-systems › waterlock-muffler.html

 

[Labrador Coaster]

Thank you, will check it out.