Underwater exhaust exit.

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sdowney717

Guru
Joined
Jan 26, 2016
Messages
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Location
United States
Vessel Name
Old Glory
Vessel Make
1970 Egg Harbor 37 extended salon model
Instead of above water exhaust, why not exhaust completely underwater?
Advantages are less soot, less fumes, less engine exhaust smells, less CO, less air pollution for you to breathe in.
I was thinking a contoured or angled pipe type through hull, as long as you can not reverse siphon, so might have to have a small exhaust air hole above the water?
 
Sudbury did make an exhaust muffler that a person could place over the existing transom exhaust which would direct the exhaust down into the water. The muffler had several small relief holes above the water line in order to not direct the entire exhaust under water. I place one on a boat and it did work well.
 
i run a 58 hatteres with 16 92s DD that has a under water EX but it also has small ones above water to lessen pres.when running slow,never a problem
 
Old deckhand said:
Sudbury did make an exhaust muffler that a person could place over the existing transom exhaust which would direct the exhaust down into the water. The muffler had several small relief holes above the water line in order to not direct the entire exhaust under water. I place one on a boat and it did work well.
Click to expand...

Salisbury Hydro-Vac. I installed one a number of years back - it bolted right over the exhaust port flange. It made the aft deck somewhat more pleasant underway, but the exhaust gas still bubbles up and drafts back towards the boat. Since I already had a large Aqualift muffler it did almost nothing for sound underway - and burbling rather than splashing at idle.


Keith
 
Underwater exhaust is common on larger boats and yachts. There is always a slow speed bypass as described in previous posts because without the bypass relief backpressure would become excessive and could damage engine.

Outboard motors with thru-hub exhaust have bypasses too on the back of the leg below the cowling as an example.

I have seen on smaller boats (approx. 40 ft) an exhaust pipe extending just below the hull bottom and turning aft 90* as in pic 1 and 3 below

What this does and what larger boats do with a cowl (pic 2) is create low pressure at speed that sucks exhaust gasses out the bottom. This creates kind of an automatic switch from the low speed bypass to the underwater exhaust path as the vessel increases speed as exhaust gasses will travel the path of least resistance.

:socool:
 

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Lots of boats with them...early 2000's Sea Rays had them..

Can't say they were terrific.
 
I have a Salisbury HydroVac muffler on my lobsterboat. Its been installed for 30 seasons now and I would not be without it. My exhaust was originally offset on the transom but I moved it to the center. Now it is inline with the propeller race and works even better.
My HydroVac is made of heavy neoprene, streamlined, and fits right over the exhaust flange. It came with neoprene shims so it could be aligned with the flow when offset on a curved transom. It does have a small relief hole so that water is not drawn in as the exhaust cools after shutdown.
I am sure I have run over things over the years but the neoprene is flexible enough that it has never been knocked off.
 
I don't know if this is what you're looking for, but when we bought our boat I had an "exhaust water separator" installed on the gen exhaust. It takes the water component out of the exhaust and sends it down below the boat via a thru hull. What comes out the back of the boat (above the water) has the sound of just a quiet hum. If you're standing on the dock next to it you don't even notice it is running unless you look down and see the ripples on the water.
 
sdowney717 said:
Instead of above water exhaust, why not exhaust completely underwater?
Advantages are less soot, less fumes, less engine exhaust smells, less CO, less air pollution for you to breathe in.
I was thinking a contoured or angled pipe type through hull, as long as you can not reverse siphon, so might have to have a small exhaust air hole above the water?
Click to expand...

the problem is back-pressure on exhaust system kill you engine if you underway with specific exhaust underwater that ok but at low speed or no speed you need system with by-pass to go up of waterline.
problem is also noise and spray for exhaust air coming at the surface.

Hugues
 
Brooksie said:
I have a Salisbury HydroVac muffler on my lobsterboat. Its been installed for 30 seasons now and I would not be without it. My exhaust was originally offset on the transom but I moved it to the center. Now it is inline with the propeller race and works even better.
Click to expand...

You had the HydroVac in both settings? My exhaust is also offset and I would consider moving it. What percentage "better" after moving?


Keith
 
I actually have 2 of these underwater devices, similar to Salisbury HydroVac.
I dont know who manufactured these, but did find their patent number US3162171A.
these came off an 1966 wooden cruiser that was being crushed at the marina.
http://www.freepatentsonline.com/3162171.pdf
https://patents.google.com/patent/US3162171A/en

I traced them by their embossed patent number. They are heavy hard rubber.
My transom has a curve, and these came with a piece to allow for the curve.


Do these devices cause an exhaust restriction?
Reading the patent info, they do not.
I painted mine but they are not on the boat.
These wont snag, the bottom is inline with the hull.
Actually I am trying to convince myself to put them on the boat.









photo sharing websites
 
Last edited:
Nordic Tugs have under water exhaust systems with a three inch above water bypass.
 
We have a Volvo Penta D4-260 attached to a DPH drive. The exhaust is completely underwater at displacement speeds. It is at the cavitation plate level, so basically at water level when on plane.

We have had no issues with this system. It bubbles quite a bit while idling in neutral - there is a lot of air going through that engine. The engine is electronically controlled, so theres little/no smoke produced and I have never seen any.
 
I've been on lots of boats with underwater exhaust. They can work, but not a big fan. Unless designed right, backpressure can be pretty high or even over spec. I prefer a good muffler and tailpipe above waterline. Nice to be able to see, at a quick glance, that you have good water flow and a clean burn.

The IPS really needs a relief line. Constantly blowing bubbles under the boat is annoying.
 
Ski in NC said:
I've been on lots of boats with underwater exhaust. They can work, but not a big fan. Unless designed right, backpressure can be pretty high or even over spec. I prefer a good muffler and tailpipe above waterline. Nice to be able to see, at a quick glance, that you have good water flow and a clean burn.

The IPS really needs a relief line. Constantly blowing bubbles under the boat is annoying.
Click to expand...

Run a tell tale, a 1/4 inch polyethylene (ice maker) plastic tubing shunting some water flow from the end where it dumps before the mixer out the hull near the exhaust outlet, that way when you see water coming out the little tube, you know the raw water pump is pumping.


These ones I have, a hole exists above the waterline in them. Hole is about 1/2 inch. I think it modifies the idling sound, less gurgling.

I think if your burn is really bad, you would still notice even with underwater exhaust.
To me the entire purpose is reduce the air pollution you will be breathing from exhaust fumes. I know some boats suck exhaust fumes back into the boat. Or wind blows back into the boat exhaust fumes.
 
From the dock I can see that the stern is now 3 inches lower than before the modifications. Reaching back while laying on the swim platform I can feel that the exhaust hose that was previously just above the water line is now entirely below the water line. That is not what Beta Marine calls for in their installation manual Could this be the cause of overheating at idle and slow speed? I will contact Beta on Monday for advice. It would seem to me that the exhaust gases escaping under water would eliminate any back pressure for the cooling system water, but I am still concerned with this configuration about water backing up into the engine when the engine is not running. The patent drawings for an underwater exhaust device are interesting. Is that device available for sale?

The serpentine belt is not slipping, and I'm convinced that 50 hp is more than enough power to run the 120 amp alternator without overheating, even at high output.
 
"It would seem to me that the exhaust gases escaping under water would eliminate any back pressure for the cooling system water,"
This is exactly backward thinking. As has been stated previously several times, there are bypass or relief passages on all the well designed u/w exhaust systems for when the vessel is at displacement speed, even the Salisbury unit.
The reason is works for propulsion engines is that at displacement speeds and lower rpm the bypass size is adequate to not negatively impact backpressure.
As for hydrolock, if the exhaust is per specifications it should only happen under a specific set of circumstances that would be rarely encountered. It's not that the exhaust is underwater, it's what happens inside w the exhaust plumbing that matters.
I'm betting Beta tells you to move the exhaust above the waterline
 
I'm going to pull the boat out and resolve this problem.
 
As post #13 mentions nordic tugs come stock with them. No back pressure issues. Easy to tell if you have good flow. Can look for sheen in the water to know everything is ok. An excellent way to handle this.
I’m also interested in learning about hybrid systems. Where it’s a mix of dry stack and wet exhaust. Wonder why that’s not more popular? Especially for those who boat in northern climates or put big hours on their engines.
 
Hippocampus said:
As post #13 mentions nordic tugs come stock with them. No back pressure issues. Easy to tell if you have good flow. Can look for sheen in the water to know everything is ok. An excellent way to handle this.
I’m also interested in learning about hybrid systems. Where it’s a mix of dry stack and wet exhaust. Wonder why that’s not more popular? Especially for those who boat in northern climates or put big hours on their engines.
Click to expand...
The confusion is probably mine, but even though my boat is "hybrid" in the sense of exhaust discharge both above and under water, the entire system is "wet". I have never heard of a dry system that discharges under water. Seems pointless.
 
MYTraveler said:
The confusion is probably mine, but even though my boat is "hybrid" in the sense of exhaust discharge both above and under water, the entire system is "wet". I have never heard of a dry system that discharges under water. Seems pointless.
Click to expand...
For the "hybrid" system I'm imagining something like a generator exhaust separator. So it's a wet exhaust where the water gets discharged near or below the waterline, but the gasses get separated out and run up a stack or to some other location.
I've done plenty of brainstorming on how exhausts could be routed, relocated, etc. with the idea of reducing station wagon effect issues. In my case, I'm a bit picky about how an underwater setup would be done as I hate dealing with an obnoxious gurgling/bubbling at idle. Plus I'd want any new setup to be at least as quiet as my current setup which is practically "the neighbors don't hear us backing into the slip until we're half way in" kind of quiet.
 
rslifkin said:
For the "hybrid" system I'm imagining something like a generator exhaust separator. So it's a wet exhaust where the water gets discharged near or below the waterline, but the gasses get separated out and run up a stack or to some other location.
I've done plenty of brainstorming on how exhausts could be routed, relocated, etc. with the idea of reducing station wagon effect issues. In my case, I'm a bit picky about how an underwater setup would be done as I hate dealing with an obnoxious gurgling/bubbling at idle. Plus I'd want any new setup to be at least as quiet as my current setup which is practically "the neighbors don't hear us backing into the slip until we're half way in" kind of quiet.
Click to expand...
Isn't the station wagon effect completely resolved by a conventional dry stack? The typical hybrid system, like mine in which low rpm exhaust is above water (but at the stern), while higher rpm is underwater, does a great job at low rpm, at least. At higher rpm, the engines (and how much they smoke), and the design of the boat (how much the configuration creates the draft of a station wagon) becomes important. However, in my case, the engines are exceptionally clean (QSM11 -- although I come from DD892's, so maybe I have a low threshold, but honestly, you can't see any exhaust smoke at any speed), and the house is perhaps 18 feet from the stern (and I suspect that the flybridge causes a lot of clean air to be sucked down into the cockpit), so there is rarely any station wagon effect (in fairness, I do have a vague recollection of exhaust fumes in the cockpit when backing down hard on a fish, but at that time I wasn't really thinking of the boat's exhaust performance, and may well be recalling a totally different boat.
 
Old deckhand said:
Sudbury did make an exhaust muffler that a person could place over the existing transom exhaust which would direct the exhaust down into the water. The muffler had several small relief holes above the water line in order to not direct the entire exhaust under water. I place one on a boat and it did work well.
Click to expand...
Sounds like the mufflers on the transoms of WWII PT-boats. They would open up the top ends to the atmosphere when they wanted to get best full power to escape after idling in close to the enemy with the engine exhaust sent underwater to silence them.
 
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