WET EXHAUST SYSTEMS - IMPORTANT READ

[bob mitchell]

WET EXHAUST SYSTEMS

This such a critical topic that I have to say I am only studying it, and am in no way an authority, nor to I present this material in any way other than to make people aware of the potential danger of an improperly functioning or poorly designed wet exhaust system. Do not assume I know what I am talking about.

         My initial interest in wet exhaust systems started when I was replacing the floors and apron of my Newporter 40.

A sophisticated wet exhaust system, with all the bells and whistles from Vetus, looks like this.

       My 1967 Newporter had all the basic components, but  in other forms.  The "waterlock" dimensions demanded that a floor be cut out so that the waterlock could be lower - so since my waterlock intake was a little deformed from overheated exhaust gases, I was considering making my own that would fit between the floors instead of having to notch a floor out to oblivion .

         The waterlock appears to have two functions in the exhaust system. First, it is a silencer or muffler that I view as converting a lot of potential sound energy (heat from fuel combustion) into mechanical energy. The  system does this by taking the RAW WATER (that runs into the hull, through the raw water strainer, through the transmission oil cooler, through the raw water pump, and up to the heat exchanger where it remains separate from but cools the ENGINE WATER, and then into the exhaust manifold (itself a heat exchanger), still separate from the exhaust gases) -- and after leaving the manifold  the raw water is injected into the exhaust gases. One important heat energy sink is that the energetic (high temp - and probably fair velocity as well)  exhaust gases vaporize some of the water - probably a lot? It takes 540 Calories/ gram to turn liquid water into steam - so a lot of heat energy is used up changing water into steam. You can see the red/blue area on the above diagram where this happens.   The distance from that raw water injection to the waterlock is critical because it requires a certain distance for the gases to cool down enough for the waterlock not to melt. More energy is consumed when the exhaust gases have to push the liquid/gas mix out the exhaust.

           So since I wanted to design a new waterlock I had to know what was happening inside the waterlock - and of course - no manufacturer provided an internal diagram - so I took the waterlock from my ancient Onan generator and cut a view hole out of the bottom.

So there you have it. the water/exhaust mixture comes in from the side and the bottom fills up with water, I presume, until the exhaust gas pressure is great enough in the remaining chamber space  above to blow out a blurb of water. And with that in mind, it is a good time to mention "back pressure".

           If you did the Eddie Murphy thing, and stuck a banana in the exhaust pipe, there might be so much exhaust back pressure your engine would not start.  All though this is probably a gross simplification, anything that introduces exhaust back pressure (in this case exhaust back pressure exerted against the flow of gases through the engine - I am kind of viewing it as negative compression)  into the system uses up engine energy. But life is a compromise, so we introduce some back pressure with our exhaust system to cool and silence the exhaust gases.  Sharp bends, elbows, and additional silencers all add more back pressure.  Boat Design Forum had an excellent blurb, pointing out that while it is easy to calculate the pressure in a column of pure liquid (pressure = density of liquid x gravity x height of column),  it is very hard to calculate the pressure in a column of a chaotic mix of gas and liquid that is changing all the time with different rpms. So what a designers or installers might do is hook up a small tube full of water to the wet exhaust where they wanted to measure the pressure, and see how high the column of water in the tube was raised (a simple manometer) - and that would give them the pressure under the given circumstances.              "

back pressure on the exhaust is generally checked in a small yard with a simple small clear plastic hose connected with a temporary fitting after the mixing elbow, you nail this hose (diameter 0.6mm) fill up with water, on a small plank, install vertically, with a loop under the point 0 and do measurement how high the water is going up when the engine is running at different speed, most of the time 0.3 to 0.5 meter is the maximum admitted by engines constructor. (equivalent at 0.05 atmosphere) "

       

The second function of the waterlock is to act as a reservoir for the water that remains in the exhaust system when the engine is shut off, so that the water does not flood the engine. That is why the vertical height of the waterlock below the engine is critical. Basically you have a certain volume of water that the waterlock can hold before water starts flooding the engine. Under normal circumstances the amount of water depends mostly on the length of the exhaust pipe from the waterlock to the high point of the exhaust run (on the other side of the high point the water should run out the back).  However, if your gooseneck fitting (see first diagram) was not high enough above the waterline a following sea could push water up the exhaust pipe and flood the engine. It seems to make sense to me that the closer the high point of the exhaust run was to the waterlock, the less the  volume of excess water would be backfilling the waterlock once the engine is shut off.

Now one other critical idea is about SYPHONING

. Most of us understand that if we have a hose full of a liquid, and insert one end in a tank, and drop the other end lower than the tank, liquid will flow out until the tank is empty - ie the syphoning effect! Well consider the raw water pathway a tube that the raw water pump fills up. What if the location where the raw water is injected into the exhaust was below the waterline, and you had lost a vane of your flexible raw water pump impeller (so it leaked a little), so you shut off your engine and go home - holy poo poo  - you are filling up your boat with a syphon - first the waterlock, then the engine, then the bilge - how long before you come back after docking and shutting off your engine? So the raw water line has to have a SYPHON BREAK in it. The syphon break in my Newporter appears to be on top of the heat exchanger. There are warnings in the literature of the syphon vent stopping up from time to time, so it might be worthwhile to locate and clean yours!

In summary, there are at least two basic cases to understand, and better than I have explained them. 1) The filling up of the waterlock , and subsequent flooding of the engine, from the main exhaust hose going out the back. 2) The syphoning of the raw water line, which fills the waterlock and subsequently floods the engine, and may sink the boat.

The best article I have found is at

www.goodoldboat.com and titled Cool and Quiet and Trouble Free. This is an excellent, must read article - and available on line. As always, Nigel Calder's Boatowner's Mechanical and Electric Manual also has an excellent presentation with diagrams.

update: I was curious about my Onan generator waterlock being PVC. It turns out that PVC is rated as a combustion retardent, ie I think self extinguishing, which I found surprising. However, it can get soft and deform at 90 degrees C. But I think that is around 200 degrees F, which might be close to the exhaust temp at that point. At any rate, I scored a piece of schedule 40 8" PVC, and the caps cost $18 a piece, so I am going to experiment a little.