Water Jacket or Hot Exhaust?

[ffjme]

Need advice on my current exhaust configuration. I have an universal m4-30 located in the bilge. A custom fabricated water jacketed exhaust pipe leaves the manifold and runs 2.5 feet in the bilge and 3 feet vertically behind the aft bulkhead. Raw water then leaves the jacket entering the top of the exhaust riser falls to the muffler and exits the boat. The water jacketed section has failed, leaking raw water in the manifold. No major damage this time but I hate this system. Question? Is 5.5 feet of wrapped hot exhaust to long/ to hot? Am I stuck rebuilding the current pipe? Thanks for any advice.

[RaymondR]

Both my engines have had exhaust water locks connected to the exhaust/water mixer directly behind the engine with about 5m of hose rising to just under the deck and then through the transom and I have never had a problem with water back into the engine. I have an anti siphon loop with rubber air bleed in valve between the raw water outlet on the water cooled manifold and the exhaust/water mixer.

[djmarchand]

5-6' of wrapped hot exhaust pipe is certainly unusual. But if you ventilate that compartment well and keep the pipe away from adjacent structures, you should be OK. Replacing that jacketed pipe in kind will be expensive, but better.

[RaymondR]

If you have room to install a mixer immediately following the exhaust manifold and a waterfall device immediately after the mixer you should not require the water jacketed intermediate pipe or lagging as the cool raw water cools the exhaust gasses when they mix in the mixer. Can you provide an overhead image of the back end of your engine?

[colemj]

We had the exact engine and exact mounting location and exact runs in our previous boat. While not ideal from a maintenance standpoint, it was never a problem.

I replaced it once in 15 years. The beauty of that engine is the exhaust flange is a simple female NPT fitting. So you can make your own exhaust riser and injector out of common inexpensive pipe and fittings. Black pipe is best. Galvanized pipe will release fumes - so run it hot with a lot of ventilation for the first few hours until they stop.

McMaster-Carr sells the fiberglass heat wrap for the standing part very cheaply (just bought some for a similar project).

Mark

[ffjme]

Mark how hot would that pipe get? Thanks

[Cheechako]

The other thing to consider is you should install a bellows or two to avoid vibration cracking on that dry exhaust link. i would definitely stay wet. Water jackets go bad occassionally, just get 'er replaced.

[rw58ph]

What you might want to move the whole exhaust closer/next to the engine which would eliminate the 2 ½ ft horizontal jacket, so would be only the 3 ft vertical. Where is the boats water line so the raw water injection might be lower? So you might be able to shorten the total length of the hot exhaust. Would still have to have to heat wrap or water jacket the exhaust but no as much.

[colemj]

The pipe before the water injection will get hot enough to burn things.

If your installation is like mine, you don't have many choices because the engine is mounted to close to a bulkhead to shorten the horizontal run and too low to shorten the vertical rise. And there is not enough room beneath it to make a close water injection with anti-syphon loop.

Mark