avoiding water backflow from exhaust

[robh]

Last year, soon after buying my 26' boat, the repair to my volvo MD1A (regrinding valves, etc) seemed to be caused by some long term corrosion , suspected as water getting back up the exhaust hose to the engine. Space in the bilge does not let me fit a separator anywhere in the exhasut hose that is sufficiently lower than the engine, and indeed the hose does not really get much lower anywhere out to the transom outlet, so I feel I'm in danger of a repeat of an expensive disaster. (On advice, I now shut off the cooling water 10 sec before stopping engine in the hope that exhaust blows out most of residual water in hose).
Can anyone advise whether it is possible to fit a valve in the exhaust hose between engine and the exhaust lowest point, which is either a "flap valve" flapping open for the exhaust gas, or, alternatively, a sealable valve to be turned off (thoguh this would be hard to reach) once the engine is stopped ?
It seems a fairly simple soution, but if so, why is it not found anywhere (that I know of) ? Any other ideas ?

Many thanks, Robert Hedges

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[ianabc]

Nigel Calder has a diagram of a "standpipe" exhaust system. (Boatowner's Mechanical and Electrical Manual a book that appears to me to be Boat Mechanical 101)


This system seperates the cooling water from the exhaust using a standpipe.

Checking with the diesel shop next to my boat yard, they tell mw that a large number of repairs are due to water ingression back through the exhausts of the boats they service.....


<hr width=100% size=1>Building a steel sailboat in Comox, B.C. Canada.

 

[billmacfarlane]

I had a Volvo valve in the exhaust system but it was useless and I had it changed to a Vetus which seems to work well. I've got a Volvo 2030 but I don't know if they do a valve for a smaller exhaust

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[sailorman]

i think u require an "Antisiphon" device.
cut the coolong raw water inlet hose ,after the water pump, insert a "Tee" & run a small bore pipe either overboard or onto the cockpit @ high level ( & allow it to drain through cockpit drains)
this will break any "vacuum" caused by the piston on a down stroke
look in the Vetus catolouge there is a schemeatic drg

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[schilde]

I might be wrong, but here's what I think.

Your exhaust and water have to go uphill to get out of some part of the exhaust. This is so that the water gets taken out and so that the water silences the exhaust.

When it stops the water in the uphill section runs back toward the engine.

Seems to me that all you need is to make sure the tank / sump / pipe / section it flows back into has enough volume to hold it, so that it doesn't rise high enough to spill over back into the engine.

So I think you need to think about the size of the different sections. You could make the uphill section shorter or make the bottom section fatter / wider or both.

If I am wrong will some expert let me know too?

s.childe@rya-online.net

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[claymore]

The vetus antisyphon device does work. Ours didn't for a day and gave us an exhaust pipe full of water which was seemingly on its way to the cylinders fairly soon. We turned off the inlet as you say to break the syphon - but its a real pain and reliant too much on memory to be a safe long-term option. The anti syphon device is a fairly simple lowish cost solution and being quite small will fit somewhere high up in a locker against a bulkhead.

<hr width=100% size=1><font color=purple>regards
Claymore<font color=purple>
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