PBO lessons learned.

S

silverdawn

Well-Known Member
Joined
30 Jan 2005
Messages
380
Location
mountnessing essex
Visit site
Hi, Just looking through Dec issue of PBO pages 76/ 77 reading Best Laid Plans, the summary lessons learned item 7
Don,t motor with your exhaust below the waterline! my exhaust outlet is about 2"above the waterline when motoring at about 3Knts, above this speed the stern squats down and progressively the exhaust outlet gets lower below the waterline, at 5Knts the outlet is just in,view under the water but its clear that exhaust water is being pumped out, I have motored for hours like this the engine has never overheated or shown signs of malfunction.

I have had the boat 4 years with no engine problems, the exhaust system is quite normal ,anti syphon on the bend 3" dia hose from the engine rises to a gooseneck then down to a water trap which discharges through the 2"Dia outlet.

Perhaps I have misunderstood the article.
 
It's one persons view in relation to one specific situation. The great thing about this forum is that EVERYTHING is peer-reviewed by a huge number of peers. A definitive conclusion may not be reached, but you will certainly hear every possible opinion :D
 
It is good to sail and have new experiences, and new learning. However, as you say, the 'lessons learned' perhaps has a bit more learning to go.
 
My previous boat had its exhaust outlet under the counter and so was subject to submersion in some conditions. However it angled down, rather than lining up with following seas so never had a problem with either overheating or water backing up. The boat had also done 40 years and a transatlantic circuit ( not by me) unmodified, so probably no problems.
Haven't read the article but maybe the installation was crucially different
 
VicMallows said:
It's one persons view in relation to one specific situation. The great thing about this forum is that EVERYTHING is peer-reviewed by a huge number of peers. A definitive conclusion may not be reached, but you will certainly hear every possible opinion :D
Click to expand...

+1 :D
 
Spuddy said:
Haven't read the article but maybe the installation was crucially different
Click to expand...

I only skimmed it, but if I remember rightly the problem was on the inlet side - an air leak, worn pump, something of that nature. The author had assumed (almost certainly erroneously) that dipping the exhaust underwater was somehow related.

Pete
 
prv said:
I only skimmed it, but if I remember rightly the problem was on the inlet side - an air leak, worn pump, something of that nature. The author had assumed (almost certainly erroneously) that dipping the exhaust underwater was somehow related.

Pete
Click to expand...

And then proceeded to drain the wet box on the assumption it was creating excess back-pressure (losing the drain plug into the bilge in the process).
 
I seem to remember the recommendation to turn off the exhaust thru hull when sailing.. And of course remember to turn it back open at the end of the sea voyage..this was before Mr Clever Vetus and his water trap..

I do remember the bubble bible noise on wooden boats each time the low freeboard exhaust outlet went under water, on each passing wave quite often!
Presumably the increase in back pressure would lower the already non-stunning ' auxiliary petrol engine' horsepower..?
An old engineer btw told me each extra bend in an exhaust was equivalent to an extra 10feet of length and weakened output correspondingly...
Not uncommon to have a waterjacketed bit and basically a straight through 'dry' exhaust, with lots of asbestos lagging and toasty feets in the cockpit at night...
 
There have been countless threads on this site describing a failure of the seawater pump to prime. Many reasons including wear of the pump body, face plate, failed shaft seals and others. This is a far more likely cause of the problem as described.

The exhaust of an engine can be likened to a positive displacement compressor, which in theory will continue to generate pressure indefinitely until something breaks. This doesn't apply directly to an engine, which will suffocate on its own gases if the exhaust is closed off, but mechanically it is true. The little bit of pressure resulting from a few inches of water head is negligible compared with the pressure being generated by the engine. When motorsailing on port tack my exhaust is immersed, something we have done for very many miles.
 
I was amazed that there was no editorial comment on this article! The owner starts by stating that he keeps his boat in excellent order, and I don't doubt that he does to the best of his ability and experience. Unfortunately, setting out toward a tidal gate starting two hours late in a good breeze with brand new rigging and a failed VHF doesn't suggest that his experience is up to the task. Whilst making little progress against the tide in choppy conditions, he sailed past the alternative havens all along his route after losing his genoa - still trying to fix the engine and hampering the efforts of the helmsman to sail off a lee shore. It seems the lessons he didn't learn are more important than those he thinks he learnt!

And yes, most engines have to run with a submerged exhaust for extended periods on one tack. Why no comment from one of the PBO "experts"?

Rob.
 
vyv_cox said:
The exhaust of an engine can be likened to a positive displacement compressor, which in theory will continue to generate pressure indefinitely until something breaks. This doesn't apply directly to an engine, which will suffocate on its own gases if the exhaust is closed off, but mechanically it is true. The little bit of pressure resulting from a few inches of water head is negligible compared with the pressure being generated by the engine. When motorsailing on port tack my exhaust is immersed, something we have done for very many miles.
Click to expand...

My exhaust is on the port side, near the back (no transom). If the engine ticking over and the boat heeling enough to put the exhaust under water, the engine note becomes quite erratic; it slows down a lot then suddenly races a bit, as, I presume, the water pressure is overcome and the submerged section of the exhaust is emptied. This does not happen if the engine is doing useful work. Considering the big loop the exhaust has to get through to escape in the first place I am mildly surprised that a few inches under water does anything.
 
JumbleDuck said:
My exhaust is on the port side, near the back (no transom). If the engine ticking over and the boat heeling enough to put the exhaust under water, the engine note becomes quite erratic; it slows down a lot then suddenly races a bit, as, I presume, the water pressure is overcome and the submerged section of the exhaust is emptied.
Click to expand...

Is that actually the engine changing speed, though, or just the exhaust note changing?

Pete
 
JumbleDuck said:
My exhaust is on the port side, near the back (no transom). If the engine ticking over and the boat heeling enough to put the exhaust under water, the engine note becomes quite erratic; it slows down a lot then suddenly races a bit, as, I presume, the water pressure is overcome and the submerged section of the exhaust is emptied. This does not happen if the engine is doing useful work. Considering the big loop the exhaust has to get through to escape in the first place I am mildly surprised that a few inches under water does anything.
Click to expand...

That's just the governor not reacting instantly or giving 100% response to changes in back pressure. They all do that sir.
 
The exhaust on the Moody 31 is only just above the waterline when in flat water. When motoring at hull-speed, the "squat" effect pulls it several inches below the water, with no discernible affect on performance. Of course, at sea wave action puts it underwater on a regular basis. There is a water trap, of course.
 
You must log in or register to reply here.

Share this page

Top