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  #1  
Old 09-21-2011
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Replacing Exhaust again...need to get it right this time

The PO of my 1972 Northstar 38 re-powered it in 1992 with a new Universal M4-30. The exhaust setup consists of a riser where raw(salt) water cools the hot exhaust via a double-walled metal riser that's fastened to the exhaust manifold on one end and flanged to a DRY, wrapped, black-iron exhaust pipe. The water from the riser exits via fitting into a rubber hose which marries up with the dry exhaust pipe 12 fee away just before exiting at the stern.

The black-iron dry exhaust pipe failed in 2002, and after replacing it, is now failing again. The riser is just fine.

I'd like a longer-lasting solution because the dry pipe takes some tough turns and it's a very difficult, custom job to replace it. Access is very limited

I'm wondering if some of the newer Trident rubber hoses could take the place of the black iron. Or should I consider replacing the riser and black-iron pipe with a whole new setup, like a Vetus water-lock. I'm a bit scared of the water-muffler approach as I'd be inventing the new design and I'm worried about really botching it - as in, water back-feeding into the engine if I get it wrong.

Thoughts on either approach? or do you have an alternative?
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Old 09-21-2011
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We replaced our old riser and stainless muffler with one of the new VETUS waterlock muffler and it has had great results in two years. Boat is substantially quieter at the exhaust.

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Old 09-21-2011
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Thank you. Do you or does anyone know a good source to assist in a redesign?
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Old 09-21-2011
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Normally the water is injected just after the riser and it both cools the exhaust and makes the system quieter. Your 12' of dry exhaust is rare in a sailboat.

Rubber exhaust hose is designed for the water cooled section and will not work for the dry section of the exhaust.

Properly designed and with the desired (required I think) anti-siphon there is virtually no risk of flooding the engine.

Here is a Yanmar diagram showing their ideal layout, but it is not brand specific. The anti-siphon can be a small hose exiting in the cockpit well above a drain to act as a telltale so you know the cooling water is flowing.

Hope this helps.
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Old 09-22-2011
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Agree that hose is only good after the water injection. It will melt.

My previous boat had a Universal 5411 and the exhaust system was standard black iron fittings and nipples for the loop, hose after the water injection, and a Vetus waterlock muffler. The black iron loop lasted 15 years and was very inexpensive to replace since the pieces were standard plumbing parts.
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Old 09-24-2011
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Mititempo and JimsCal, thank you. but i spent some time talking to silicone hose distributors. I haven't gotten to the actual manufacturer, but the distribution network challenges your conclusion: meltdown. They say, some of it, anyway, will withstand 500 degrees F. that's enough headroom to do what I need (I think).

Engineering question: if i measure the surface temp of the existing metal exhaust at the hottest point, just after the water-injected riser, and the black-iron reads, say, 300, how hot is the gas inside?
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Old 09-24-2011
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Hotter inside I would guess but I am not sure what temp.

Is your concern siphoning into the engine or offshore seas flooding the engine? Siphoning is taken care of with a layout like the Yanmar one I posted above. As sea it makes no difference, if the stern is clobbered with seas the engine will get wet unless a seacock is installed at the outlet.
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Old 09-24-2011
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Hmmm. The exhaust gases in a diesel running under full load that has reached equilibrium temperature exit the cylinder at 1500 to 2000 degrees. By the time it gets out of the manifold, I would wag a guess of 1000 - 1200 at the manifold exit, although less under more normal conditions. I would not trust any formulation of rubber to stand up to those kinds of temps. I expect it would melt and char from the inside out.

Concern over overheating when you are in nasty conditions and really need the engine is bad enough. Personally, I would not want to worry also about whether running my engine flat out for too long would do in my exhaust. How do you feel about cleaning charred rubber bits out of your water muffler?

Exhaust has corrosive gases. Chemical reactions happen faster as temperature goes up. You say your exhaust pipe was replaced in 2002 and you want to get it right? If this pipe lasted 9 years I would say you got it right last time and should do the same thing again.
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Old 09-24-2011
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Exhaust gas temperatures can be well in excess of 500F after operating at load for a period of time. dacap is correct in that gases leaving the cylinder are well in excess of 1000F. A lot depends on the construction of the water cooled exhaust manifold, but temps of 600 to 800F at the exhaust flange would not be out of the question. There is a reason why the section before water injection is commonly wrapped in high temperature insulation.
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Old 09-24-2011
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Pub, photos would really help I don't know how well SS works in Saltwater Exhaust systems. I found this site. Schedule 40 fittings and pipe nipples there. http://www.buyfittingsonline.com/1-14nptschedule40.aspx
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