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The LowGun · 16-01-2017, 17:04

Does anyone have experience silencing a 2kw inverter/generator?
My thoughts are to build a small insulated box and run the exhaust into the water.

svninkasi · 16-01-2017, 17:26

If it's air cooled it will overheat quickly inside an enclosure. If it is not a Honda Quiet or similar, more than likely most of the noise is mechanical and not in the exhaust being that it's hash bushings vs bearings inside. This is a quick take from what I've read from your post. If it is water cooled, it's another animal. More details would help.

brianlara 3 · 16-01-2017, 17:28

I have one. It runs at least two hours per day.
It sits on the poop seat but is covered loosely with canvas which doesn't restrict the essential cooling air flow. I was also thinking about boxing mine but the radial fan on the crankshaft is designed to pump air from the immediate environment thru the engine, no more. Atmospheric pressure supplies the air to the fan. If you box the thing you will cause a depression area at the fan and overheating will likely result.
This ancient mechanic also thinks that sending the exhaust gases thru a long tube will cause excessive back pressure resulting in exhaust valve failure.
Additionally, sending the exhaust gases into a solid brick wall of water .....I could write for some time why it wont work. Someone else can elaborate if they wish.
Yes, I'd like to shut mine up to. It it could be done I'd have done it.
Goodluck.

Bill Seal · 16-01-2017, 17:45

And while you're at it, find a way to shut up those hundreds of damn KADAK-KADAK-KADAK-ing golf carts that destroy the serenity of Avalon, Catalina, starting at around 7am when the hotel maids start working.

leftbrainstuff · 16-01-2017, 17:46

If you want a quiet exhaust you cool the exhaust gas. The bark is proportional to the temp delta between exhaust gas and ambient air. A wet exhaust on a boat does a good (actually great) job of cooling the exhaust.

Much of the noise is structure borne. Isolation through decent engine mounts is key here. Most generator installs are mounted with about 1890 thinking. 1, 2 or 3 cyl motors that are severely unbalanced are problematic.

Once you've cooled the exhaust, sorted our your mount damping then the next noise is induction. The rush of air into the engine. This is the biggest noise source in your car.

A decent filter and induction insulation help here. Ensuring your intake plumbing is large in diameter will help. You'd be surprised how easy it is for air to transiently achieve supersonic speed.

Finally metallic clatter is the final noise source. In most generators this can be quite significant. Some of this is structure borne and some is airborne. A soundproof box will help here.

Note that you are dealing with both structure borne and airborne noise transmission. To make your generator quiet you need to tune both.

Structure borne noise transmission on a boat is a big issue. Your car has mostly engineered this out. This noise will travel down your ground out engine mounts and breakout on large flat or unsupported surfaces. The hull, walls, etc.

Our 1 cyl (very unbalanced) 8kW genny hard mounted to a stainless frame in our engine room is noisy down below. It's mostly structure borne. I'll start with active damping mounts on a floating platform which the enclosure will sit on. The mounts on a floating platform allow me to tune the mounts, in two places, to avoid resonance on and off load. Mechanical clatter will be quiesed by the enclosure.

The LowGun · 16-01-2017, 18:03

Quote:

Originally Posted by svninkasi

If it's air cooled it will overheat quickly inside an enclosure. If it is not a Honda Quiet or similar, more than likely most of the noise is mechanical and not in the exhaust being that it's hash bushings vs bearings inside. This is a quick take from what I've read from your post. If it is water cooled, it's another animal. More details would help.

I haven't yet purchased the gen set. I hadn't considered the cooling. It is an issue on one of the Honda/Yamaha types that I am considering. Thanks for making me think of it.

SecondBase · 16-01-2017, 18:26

Sounds like you don't have the deck space for solar? or maybe supplement with some solar and drop a size in generator?

The LowGun · 16-01-2017, 19:15

No. I don't have the deck space for solar. But I am planning on putting a davit on the stern and may be able to put something there. However, I have one panel (a flexible one on the bimini) but I can only measure about 2 amps coming in. It must take a hellovalota panels to run a fridge. With two nights out on the water I was out of power. Thus the genny.

brianlara 3 · 16-01-2017, 19:33

Quote:

Originally Posted by SecondBase

Sounds like you don't have the deck space for solar? or maybe supplement with some solar and drop a size in generator?

1KV & 2KV, virtually the same audio levels.

valhalla360 · 16-01-2017, 22:59

If you are just running the fridge and some lights, solar is the way to go. Even on a modest size cruising boat, you should have enough space over the davits to install enough.

If you are trying to run the air/con or other high output load, then consider a generator but we found the noise from the air/con fan was louder than the generator when inside.

If you are looking at a Honda/Yamaha style generator, a box with a decent air space to avoid overheating is about the best you are going to do (if you can set it up with hinges so 1 or 2 sides lift completely it would be best). They are already pretty darn quiet and they have highly paid engineers who have spent a lot of time making them quiet.

**Dockhead** · 17-01-2017, 01:24

Quote:

Originally Posted by leftbrainstuff

If you want a quiet exhaust you cool the exhaust gas. The bark is proportional to the temp delta between exhaust gas and ambient air. A wet exhaust on a boat does a good (actually great) job of cooling the exhaust.. . ..

Say what? Could you provide a link to something which supports this idea? It contradicts everything I know about exhaust noise, so I'd like to get studying, if everything I learned is wrong. How does the Delta T between exhaust gas as it enters the atmosphere, generate sound?

As far as I know, exhaust noise has nothing to do with this Delta T. It is caused by pressure waves generated in the exhaust tract when the exhaust valves open, something very different.

Here is how it works, according to what I always heard:

"The muffler quiets the noise of the exhaust by "muffling" the sound waves created by the opening and closing of the exhaust valves. When an exhaust valve opens, it discharges the burned gases at high pressures into the exhaust pipe, which is at low pressure. This type of action creates sound waves that travel through the flowing gas, moving much faster than the gas itself (up to 1400 mph = 625.8m/s), that the resonator and muffler must silence. It generally does this by converting the sound wave energy into heat by passing the exhaust gas and its accompanying wave pattern, through perforated tubes and tuning chambers. Passing into perforations and reflectors within the chamber forces the sound waves to dissipate their energy."

Exhaust Flow | Exhaust Tuning | Performance Muffler

So according to this, a muffled exhaust gas stream would be actually hotter, than an unmuffled exhaust gas stream, since mufflers convert sound energy into heat.

Injecting water into the exhaust tract might reduce exhaust noise, but the mechanisms involved do not seem to include reducing Delta T of the emerging gas stream:

https://www.acoustics.asn.au/confere...papers/p34.pdf

Rapanui · 17-01-2017, 02:28

Having spent 7 years researching reducing exhaust noise of vehicles, the main effect of cooling the exhaust is to drastically reduce the speed of sound. As a consequence this reduces the wavelength of a particular frequency proportionally. The silencing effect of exhaust elements such as waterlock mufflers increases as their size to wavelength ratio increases, so cooler gas is silenced better at low frequencies.
The other factor is that cooling the exhaust takes energy out of the exhaust and sound energy in the exhaust is one component of this.
If you turn the water injection off for a few seconds, the exhaust noise will increase significantly. That is why, boats can get away without an effective exhaust silencer whereas the same engine in a land application will need a sizeable silencer to be effective.
If you really want to silence the exhaust of say a generator, water injection followed by a conventional silencer is the most effective way.
The main thing to remember with reducing noise that sound levels add and subtract logarithmically, so you have to attack the highest noise sources first to make a significant difference. Identifying these and their pathways can be complex. In general you have to reduce exhaust, mechanical noise from engine, intake noise and if fitted cooling fan noise both airborne and structureborne in a balanced way to have a significant benefit.