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Old 01-03-2013, 08:58   #1
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Exhaust Refit

I'm in the process of replacing the old stainless steel muffler, hoses, etc on my perkins 4.108. When looking over the current setup, it seems to differ from standard exhaust systems. Seems that most systems inject the raw water prior to the muffler. My setup does not inject the raw water until just before all is exited from the boat.

I believe that injecting the water prior to the muffler has two benefits. One is that it helps the muffler reduce noise (this has not be an issue) and it cools the exhaust system.

So here is the question, should I leave the system as is, or should I retrofit to a more traditional setup.

Thanks for the input.

Bill
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Old 01-03-2013, 09:49   #2
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Re: Exhaust Refit

Hi Bill, I'm not experienced at boat exhaust systems but since I've been reading up on them a little, I'll chime in and hope someone with experience can address this too. According to Calder and Gerr, most sailboats are water cooled, with raw water injected into an elbow coming out of the exhaust manifold. This transitions to a hose that is rated for wet exhaust systems such as SAE J2006 or UL 1129. Amongst other standards, this means the hose is rated for at least two minutes of operation with the water supply removed (not indefinite operation). A vent loop is provided. The hose enters a water lift type muffler and carries the exhaust gas and the raw water with it. Pressure forces the gases and water out of the muffler output and out to the outtake through hull. Your existing systems seems different. Your exhaust would have to be a dry type exhaust with stainless exhaust pipes leading to a dry muffler as exhaust hose is not rated for dry exhaust. If it is a dry exhaust system, then why would it have water injection just before the through hull?
I recommend Boat Mechanical Systems by Dave Gerr for a good discussion of different ways to design and install boat exhaust systems
Boat Mechanical Systems Handbook: How to Design, Install, and Recognize Proper Systems in Boats: Dave Gerr: 9780071444569: Amazon.com: Books

There are some ways to significantly screw up when building or rebuilding a boat exhaust system so it's a cheap investment to buy a good book or get professional advice. For instance the size of the muffler compare to the volume of the outlet hose and the position of the water injection fitting to avoid flooding the engine with raw water. The hose diameter, length and number and type of bends is important for controlling back pressure.
If anyone who actually knows about this stuff is around, feel free to jump in!
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Old 01-03-2013, 09:51   #3
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Re: Exhaust Refit

Wow, Danger! I had that old system, it failed and flooded the engine. The pipe rotted and water came right into the dry pipe and then into the engine, Nasty! Switched to a water lift and am so happy. Luckily for me it happened in a slip at something like idle.
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Old 01-03-2013, 10:01   #4
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Re: Exhaust Refit

Wow, that is a strange system you have and I don't understand how it works. Exhaust from a normally aspirated engine can be 600+ deg F. This will melt rubber hose and fiberglass muffler components in addition to heating up the engine compartment unless all metal and fully insulated.

Put your hand on the exhaust hose going to the muffler. If it isn't metal and insulated and not very hot- less than 200 degrees, then water is cooling it somehow. Go back upstream to the engine and look for a water injection point.

David
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Old 01-03-2013, 10:23   #5
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Re: Exhaust Refit

ICEDOG - Thanks for the reference. I just ordered.

All others - I'm not at the boat and have never touched the rubber hose just aft of the exhaust exit from the engine. I did notice the other day that the hose leading to the muffler was cracked which started me down the path of further investigation and now learning. I did take a long look at the raw water path and from the raw water exits the heat exchanger and that hose goes directly to the exhaust elbow which is 6 inches from the exhaust output through the hull. I replaced every hose in the raw and fresh water system so I'm familiar with the path of both systems.

Looks like I have yet another refit and re-engineering task.

Thanks for the reply.
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Old 01-03-2013, 10:39   #6
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Re: Exhaust Refit

Good luck with it. It may turn out to be just a variation that is workable with some new parts. If you have rated exhaust hose as opposed to pipe, then it has to be cooled at that point or it would have burned up a long time ago. I'm looking at Calders wonderful boat manual right now and it shows some variations in system design with additional raw water components such as auxiliary heat exchangers etc. I bet you will find another raw water injection point before the muffler inlet hose.
My upcoming project apparently has an unusual but not unique variation with a keel cooler for the engine coupled with a wet exhaust (most have a dry exhaust I guess).
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Old 01-03-2013, 10:47   #7
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Re: Exhaust Refit

Change it to a more conventional system. What you have is a hot exhaust until the injection point. I assume it is wrapped in something?
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Old 01-03-2013, 13:31   #8
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Re: Exhaust Refit

It is not wrapped. Maybe originally it was and maybe the PO changed. I'll look again if there is another injection point but I have been around the engine room and engine and can not think of another place it could be injected. From the raw water pump there is a pipe/water hose combination that carriers the water to heat exchanger. Off the back of the heat exchanger is a outlet with hose that I mentioned previously that carries the raw water to the injection point near the exhaust outlet for the boat.

I'm thinking I was lucky this has not caused problems. Number one priority at this point.
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