Wet Diesel exhaust... Why?

Manureva · Sep 17, 2013

Jon Bastien said:
Hello everyone,

This is just a gee-whiz question, but I'm curious... Why do sailboats almost always have a 'wet' exhaust system?

It seems to have a few technical problems. I see lots of posts about clogged mixing elbows; there are also LOTS of Hunter-made sailboats with their exhaust outlets on the underside of the sugar-scoop transom (so it's impossible to tell whether there's good water flow out the exhaust pipe).

Seems to me, if we took the output from the cooling system and pumped it straight over the side rather than into the exhaust stream, we could solve both of those issues.

Is it to cool the exhaust? Quiet the exhaust (They make other mufflers for that...)? Reduce the number of holes poked in the sides? Is there some other function I'm missing?

It is for the purpose of cooling the exhaust gases, preventing fire . An uncooled diesel exhaust can be a well over 700 *F . Fibre glass and resin would be ablaze at those temps. It is also less dangerous then a hot pipe from the engine to outside the
hull

Don S/V ILLusion · Sep 17, 2013

The exhaust location low in the hull is by design to lessen the head against which the exhaust flows. Our old 40.5 Hunter was under the transom scoop but it was obvious when it was/not flowing.

Charles Erwin · Sep 17, 2013

Well -- I was wondering the same thing but cannot figure out how to place a dry stack on a sailboat.

Where does one put a dry stack assembly? It will surely be in the way of the things one needs to do on deck. The through-the-deck fitting has to be pretty waterproof and that means maintaining these fittings. It will be hot - and so I will probably sustain burns grabbing it someday on deck. Adding lagging (insulation) to reduce temperatures is troublesome and maintaining lagging is a nuisance. The motor room -- maybe the boat interior too -- is going to get way warm - lagging or not. There is no dry stack muffler that works well and unless the termination of the exhaust assembly is way tall above the deck the noise is intolerable.

In short dry stacks are too many parts, too much maintenance, too hot, and too noisy.

Wet exhaust systems avoid all these nuisances. They are reliable, cheap, and you get muffler-quieting for free.

Charles

jibes138 · Sep 17, 2013

However, many of the "problems" are related to combining the function of cooling the exhaust and cooling the engine. If a standard car radiator with fan was used for engine cooling then the wet exhaust would be much simpler with elimination of the heat exchanger. This would require dumping the hot air from the radiator (air-liquid heat exchanger) overboard adding another kind of complexity but would provide a great cabin heating system for cold weather.

Steve Dion · Sep 17, 2013

Charles Erwin said:
Well -- I was wondering the same thing but cannot figure out how to place a dry stack on a sailboat.

Charles

Charles, why wouldn't we use the mast for that?

Charles Erwin · Sep 17, 2013

Steve

In my case the motor is well aft of the mast. Hence - is a fair distance from the exhaust manifold outlet to the mast. Thus a tube passing through the living area is required. The tube gets hot, there will be joints that leak, etc.

Once we get to outside of the deck we still have to make parts to weather proof the tube to deck -- no matter that the tube path is inside or outside the the mast. Also, the exhaust tube termination up the mast will probably have ill effects on the main at main raising time - or if motoring of with main up.

For me anyway -- this is much complicated. Too many parts, too many joints, and too little one gains over a wet system.

Charles

Ross · Sep 17, 2013

The workboats on the Chesapeake often use a dry exhaust and bilge coolers for the engine cooling Just a pair of copper pipes that parallel the bilge outside of the hull and circulate the engine cooling water. Of course the engines are in their own box in an open cockpit and the exhaust is jacketed with perforated steel shields.

Johnb · Sep 18, 2013

English canal boats which are made of steel, are used in water that has large amounts of stuff that would block water intakes and all that I have been on have "air cooled" engines, like a car radiator. The exhaust goes through a muffler and then out above the water line. On rare hot days the system is inadequate and you have to back off for a bit.

Those engines also defy common logic and belief by giving decades of service while being run at low power for most of their lives - making wakes on the canals brings severe censure.

One nice side effect of our typical set up is how cool the exhaust manifold stays. On my Yanmar 3QM30 you can put your hand on it while the engine is running.

Given that so many boats have run for so long I suspect that convergent evolution has given us a fairly optimized system. Reading the above I had a mind flatus and pictured decoking the inside of my mast.

CalebD · Sep 18, 2013

And those Chesapeake work boats with dry exhausts are not at all quiet. You hear them before you see them.