Still Looking to Reduce Exhaust Noise

[Pete]

A few months back I wrote on this subject and received many interesting responses, most noting how they wish they could hear their Detroits in the middle of winter, but did not receive any suggested solutions. So I am going to try again with more detail. I own a 48 Long Range Cruiser. It is powered by 453N, at a shaft horsepower of 112 each. The key is the "N" rating that does not have a turbo. Several responses last try at this, noted the sweet sound of their exhaust. I am going to speculate that they have turbos which break up the sound pulses as the are driven through the blades. I agree, a turbo Detroit does have a sweet sound. However the non turbo Detroits have a much harsher sound, much more like a poorly muffled truck. I am looking for a way to take the harsh sound away.

Any ideas would be most appreciated.

I have two on which I would appreciate comments. First is to inject more water flow into the muffler from an additional pump to give the sound waves something more to bounce against. Second is to install a fiberglas "U" fitting after the muffler ( like a mild plumbing drain trap) to force the sound waves to interact with the water, as I believe the water just flows along the bottom of the exhaust and the sound waves travel above the water. I realize I have to be careful not to create additional back pressure, but suspect that if I can get the water and sound pulses interacting more it may help.

Thanks in advance for your ideas and comments.

 

[Maynard Rupp]

The biggest problem with Detroit's on the noise subject has to do with the fact that they are 2 cycle engines. You have a blower whose total job is to clear the exhaust gasses from your cylinders in the milisecond that is alloted for that purpose. A 4 cycle engine has an entire cycle devoted to eliminating exhaust. ANY restrictions that you put on the free flow of exhaust from your cylinders is bad. I'll bet your engines would run much better with 6" non-mufflered exhausts. If you try to force the stuff through a water trap or any other additional restrictor, you are making your blower's job very difficult. Buy ear plugs.

 

[SKYCHENEY]

I think you need to stay away from water and try to work with a muffler that has more baffling to break up the sound waves. It may have to be longer than the original mufflers or maybe you could add an additional muffler somewhere along the line.

 

[mike]

As a follow on with the previous responses, another point to consider is the closeness of the muffler to your exhaust manifold, the closer the more effective the reduction. You also may want to consider as already pointed out an additional down stream unit, there are systems made of a rubber compound that help to absorb and attenuate the sound wave. Given that low frequency sound is the hardest to reduce, another consideration is to decouple the engine via resilient motor mounts and flexible silicone exhaust couplings to help decrease the transmitted vibration and associated noise. Also make certain all air gaps are filled between the Eng room and the other areas. We found the cable runs up to the pilot house, the ER access door seals and exhaust pipes bulkhead seals as they exit the ER, all to be noise sources that are easly sealed with a little work. Soundown has some interesting mat-type absorption products for the floors and sides etc. that may help your situation. The next biggest problem after the exhaust is the blower intake noise; there is no easy solution for this one that I am aware of. Good luck.

 

[bobk]

Have you consulted with muffler manufacturers such as Aqualift? They must have dealt with the issue before.

Bob

 

[REBrueckner]

I assume its external exhaust noise at your stern that is of concern rather than interior noise emanating from your exhaust system. If that's the case, your best bet is as suggested to get some advice from muffler companies. Is it possible your existing mufflers are not functioning properly. Many just have baffles in particular shapes inside and overheating,for example, can damage the internal components letting exhaust thru without much muffling effect.

Check with Hatteras to see what the design backpressure has been measured..and how much leeway you have for your LRC. My understanding is that all production boats actually test their exhaust back pressure to insuire they meet engine manufacturer specifications. A muffler company should be able to tell you if the installation of a new (replacement) muffler or an additional muffler will keep you within engine manufacturer specs.

An alternative could be to add an additional exhaust "dump" thru a muffler either side of your hull but that involves are lot of work and ties up additional interior space.

 

[Fanfare]

When I went from our old 8-71N engines to the 6-92TAs my consulting engineer recomended that I go to waterlift mufflers which would have fit in each engineroom. The waterlift on our generator sounds great, and I understand that there are even better ones now which separate the exhaust gas from the water, eliminating the whoosh-whoosh sound. But I had had several experiences with water backflowing into the generator engine in heavy rolling seas after the gen stopped (probably because we rolled so far the oil pickup sucked air and the automatic stop system cut in) so I decided to stick with the straight-through exhausts with new, larger mufflers under our bunks (they were bigger--had to add a drawer under each bunk). This was a lot better than the old no-muffler system for sound, but you still hear it. There is no back pressure--probably quieter if there was some, but I like going faster.

I don't think the turbos have any noticeable effect on the exhaust tone. Sure can hear the whine though--I don't think we have any high frequency sound insulation built in. I added Soundown carpet padding beneath the salon carpet.

Give the muffler guys a call and look into both the regular and waterlift mufflers. They should know all about the DD engines and exhaust backpressure.