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Sunday, May 13, 2012

Engine Exhaust Beginnings

This is a test fit of some engine wet exhaust bits. Turns out the space planned for these, and the aft bilge pump anti-siphon loops (previously installed) was a bit too cramped. So, the pump anti-siphon loops had to be relocated. Turns out the new space for these has better access for maintenance anyway, so its all good.

I suspect most center-cockpit boats of this vintage had a similar limitation: where/how to run mechanical bits (exhaust, steering, etc) aft without interfering with the aft living spaces? I have seen some Westsail 42's do a port side-exit amidships for engine exhaust, which can be a simple and easy install giving you the shortest hose run possible (ideal). But I can't do that, given how I designed the engine room, without a big hose looming over your head. I would rather have exhaust discharge aft anyway.
Vetus Gooseneck

So I came up with a stern discharge solution that involves a Vetus exhaust "gooseneck". With wet exhaust you want the hose to "loop" up to a high point within the boat (the higher the better). This will prevent any following waves from "backfilling" into the exhaust and the engine. Saltwater backing into a diesel engine can kill it quickly unless you remedy it immediately (usually involves removing the engine heads before corrosion has a chance to work). If you have the space, simply looping the exhaust hose up-then-down from a high point is sufficient. But exhaust hose cannot bend enough for that to work in the small space we have.
Test fit of gooseneck in port-aft quarter.
So, this gooseneck provides the anti-backfill loop in a very compact space. I am somewhat loathe to use one of these. We tried one on a re-power of our small boat years ago. We removed it due to the excessive back pressure it created (exceeded the engine maker spec's for warranty). But, after a lot of thought, this is really the only sensible solution for this project. To help with the back pressure, the size of the gooseneck and tailing hose is 2 3/8 inch inner diameter. The engines in our target horsepower range (60-80hp) have 2 inch exhaust, so a reducer (expander?) will be inline near the top of the loop. Expanding the hose to 2 3/8 inch at the top to the "downhill" side will help the system to "breathe out" more of the exhaust pressure.
Discharge from gooseneck, down and toward the stern, underneath the aft-berth.

That is the idea at least.
More later...




4 comments:

Anonymous said...

I just read a blog 'S/V Earendil' They traveled from Nassau to Fla with following seas. Last year they had their exhaust modified to factory recommendations (Yanmar)...Still the engine is full of salt water! I read about this a lot. Fact is no matter how high you make that column if the boat is low and a towering wave is raising the stern ..problem. Why do I never see flaps on the outlet to stop reversed flow into the system. Will that stall the engine?
Colin

Robert Sutton said...

Agreed Colin. On the small boat we put a flapper on the discharge port. Easy enough to fit. Cheap insurance.

The Incredible Hull said...
This comment has been removed by the author.
The Incredible Hull said...

Second video is of my flapper flapping.

http://theincrediblehull.blogspot.com/2010/08/engine-running.html