Exhaust Back Pressure

[Ralph Johnstone]

Gordon,Recently I saw an article which claims that diesel engines on sailboats are looking at shortened lives due to the water lift exhaust systems they discharge into.On our 1999 H310 (Yanmar 2GM20F), I can certainly see how that last loop in the wet exhaust could generate as much as 24 in. of back pressure. The author's claim is that the backpressure will cause the valves to burn.Do you agree with this idea, and if so, would you please explain the mechanics of how the backpressure contributes to burned valves ? (Something else to lose sleep at night).Regards,s/v Island Hunter

 

[Randy]

Back pressure

Perhaps you read David Pascoe's article on sailboat exhaust systems? I can't remember exactly where it is, but it's on one of his three sites: docksidereports.com, marinesurvey.com, and yachtsurvey.com. His articles usually make for interesting reading. I don't know why excessive back pressure will cause burnt valves, but I've heard that said about car engines, too. High back pressure can also reduce power, raise engine temps, accelerate wear, reduce fuel economy, etc. I don't know anything about the exhaust system installed in your Hunter, so I couldn't really comment on that. I'm sure others will be able to contribute...Randy

 

[Pete]

back presure

the article was on sailboat engines by Don Pasco @marine survey. He talks about excessive back presure in a exhaust system that has MORE then 36" of height in the exhaust hose as a riser. Your 24" should not be a concern (as I read the article and know that I also checked mine). I thought it was a good article and I'm more concerned with the heat of the engine room and will most probely install a full time blower on mine boat. I find his web site to be very informative ! I would be very interested to hear what Gordon,Yanmar rep or someone from Hunter has to say! HOW about it? Ralph- not a issue to lose sleep over!

 

[Pete]

correction

article on sailboat diesel engines is a www.docksidereports.com . Go to section on marine deisels

 

[Gordon Torresen]

The other posts led me to read some of the David Pasco writings. About the only thing therin with which I totally agree is his statement that he doesn't know much about sailboats. His diesel engine writings are very opinionated and force certain beliefs which are not correct, especially as relates to sailboats. When it comes to exhaust back pressure, the height of the rise is irrelevant if the prescribed pressure is not exceeded. The length and diameter of the tubing is probably the greatest factor. As far as burning valves, too low a back pressure would be the cause, if it was to be blamed on back pressure. The specified back pressure for small diesels is usually around one PSIG or 28" of water with a range between .7 PSIG and 2.5 PSIG depending upon the engine. It is not easy to measure and it is doubtful that boat builders can give you the actual number with any accuracy. The pressure given as 28" of water does not relate to the height of the discharge of a water lift muffler in that the measurement is made with a solid column of water and the waterlift only makes spurts.

 

[Ralph Johnstone]

Too Funny ..........................

Just roared at you responce that your only point of agreement with David Pasco was that he did not know much about sailboats.Thanks for a well written explanation of how valves may be burned (or in this case not burned). As you said, a low backpressure MIGHT cause an excesssive pressure differential across the valve and burn but highly unlikely.Regards,s/v Island Hunter