Underwater exhaust pros and cons

dasabalon

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I have dual underwater exhaust on my Wesmac. It's an amazing system. Gotta be 75% quieter when running compared to other boats. We also have a relief tube out the back when at idle or slow trolling water and exhaust comes out the back. I agree these fish feed right off the back of draggers way louder then my boat. I never smell fumes and never have soot on my transom. Genny is the same way. I hear the motor run but not the exhaust noise. It's very easy on the ears after a two day trip, all you hear in the cockpit is wind and water. No drown from the engine. I would highly recommend it on any NEW build. My muffler in the engine is made by sound down and is about 4ft wide and 3.5ft tall feeding dual under water exhaust. My wife loves to fish with me and has never once complained about the fumes since ownership.
I’m not sure I’m asking this correctly but with your underwater exhaust where on the bottom of the hull does it exit? Is it a dual exhaust because of the horsepower? Also do you have any pictures of where it exits.
Thanks in advance. We are thinking of doing this on the Wesmac I’m having built.
 

Soundownsam

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I’m not sure I’m asking this correctly but with your underwater exhaust where on the bottom of the hull does it exit? Is it a dual exhaust because of the horsepower? Also do you have any pictures of where it exits.
Thanks in advance. We are thinking of doing this on the Wesmac I’m having built.

The exhaust runs out the bottom of the boat through venturi scoops that suck the exhaust out as the boat moves forward. Because these scoops extend under the hull using two of them (one each side) will keep the running surface symmetrical.

The location of the scoops should be based on a number of factors including submergence and relative location to the prop. If you go too deep you will have back pressure or need a larger scoop, to shallow and the exhaust will break the surface right behind the boat and you will hear it. You also want to make sure that the exhaust gas does not travel into the prop where it will cause issues of all types. Scoop design is mostly a science, but, also a little bit of an art.

sam
 

dasabalon

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The exhaust runs out the bottom of the boat through venturi scoops that suck the exhaust out as the boat moves forward. Because these scoops extend under the hull using two of them (one each side) will keep the running surface symmetrical.

The location of the scoops should be based on a number of factors including submergence and relative location to the prop. If you go too deep you will have back pressure or need a larger scoop, to shallow and the exhaust will break the surface right behind the boat and you will hear it. You also want to make sure that the exhaust gas does not travel into the prop where it will cause issues of all types. Scoop design is mostly a science, but, also a little bit of an art.

sam
Thank you for the reply. How do you figure out the location of the scoop? Is there any kind of math? Also if your using an 8”exhaust out the stern, 6” off the turbo increased to 8. If you go with the dual underwater exhaust tubes can they remain 6” or can they be reduced since you are using two exhaust outlets? Do you use a muffler and then split the exhaust?
I read seaboards article on underwater exhaust for a QSM 670 they use a muffler with one underwater exhaust outlet and a relief tube. is this dated material? Is there more articles on the topic I could research?
 

Soundownsam

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Thank you for the reply. How do you figure out the location of the scoop? Is there any kind of math? Also if your using an 8”exhaust out the stern, 6” off the turbo increased to 8. If you go with the dual underwater exhaust tubes can they remain 6” or can they be reduced since you are using two exhaust outlets? Do you use a muffler and then split the exhaust?
I read seaboards article on underwater exhaust for a QSM 670 they use a muffler with one underwater exhaust outlet and a relief tube. is this dated material? Is there more articles on the topic I could research?
A muffler is always recommended for underwater exhaust as the gas will ride along the bottom of the hull and you will will drive the hull at low frequency. This is a concern in DE boats, but, not as much as larger cruising vessels with aft cabins. Unsilenced underwater exhaust can make those cabins uninhabitable in many cases.

The relief hole or low speed bypass is needed as the scoops do not create any suction at rest or very low speeds and the exhaust will bubble from the underwater outlet.

The sizing of the exhaust tubes and bypass will be pretty system dependent. In general the piping after the water injection will need to maintain the same cross sectional area so if you can run in a single 8" then 2 x 6" should be fine.

As for articles I am sure that there are some available, but, I would be careful about trying to design something based on what you read. There is a fair amount of experience that goes into laying out one of these systems. For example the scoops need to be high enough on the hull that the static head pressure does not cause back pressure issues. At the same time they need to be deep enough that they do not bubble out the chine or immediately aft of the transom. There a host of different factors on top of the fact that different hull forms have different flow characteristics under the hull.

If this is something that you are considering for your current project I can send you contact info of our guys who deal with exhaust. I am pretty good with a lot of layouts, but, I'm not the pro when it comes to going underwater.

There are also other options for very quiet systems that you may wish to consider.

Sam
 
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