Runaway diesel

[lw395]

I wondered that too so I tried to answer it. A quick and dirty analysis of reports here found:

3 reports of "Own Boat" runaway
1 report of another boat
1 report of a non-boat runaway
4 hearsay reports

The problem is defining the size of population from which these reports have come.

32 different people have contributed to the thread which suggests a 1 in 10 chance of experiencing a runaway on your own boat. That's scarily high.

The forum front page reports that there have been 2465 views of the thread but I don't know if that's 2465 different individuals. If it were, then the probability of an own boat event is down to 1.3%, a lot more manageable.

I'm assuming that anyone who is aware of the thread and has experienced a runaway would let us know about it. I know I would.

I'm concluding that it's a more likely event than I had imagined and my own previous ill-considered contribution demonstrates that it's a bit late to work out how to deal with it after it's started.

Derek

I think the probability of it happening is very low.
I've heard of it happening once, to an acquaintance of a friend, who had a fairly clapped engine and mistakenly put in far too much sump oil.
Since my friend knows a lot of people from many years in the trade and has a good memory for such events, I think we're down in the 1 in the 1000s chances of it happening in anyones boating lifetime, and even then it's a really a cause-effect thing, you need a crap engine which uses a lot of oil, plus an excess of oil.

Our engine uses say 0.5 litre/hr of diesel just to tick over?
In normal use, it consumes about 0.25 litres of oil in 100 hours? Some of that is not getting burned, it's weeping from the odd seal and getting wiped off the dipstick.
So when it starts to use 200x as much oil, we might have a potential problem, if it's not too clapped out to even start.
But, it's a Yanmar we can pull a string and decompress it. Which is far more likely to be needed if a Morse cable jams or something.

I've never exhaustively studied the breather systems of boat engnes, but many engines have a pipe which takes fumes from the rocker box into the air inlet.
If you block the intake to the filter, will the engine not draw more oily air from the rocker box and possibly make the situation worse?

The proper shut-off devices go right next to the manifold AIUI.

 

[oldharry]

My impression is that the two common causes are failed turbo oil seal, and excessive blow by pushing oil up the breather tube. As few beyond the Mobo brigade use turbo diesels this is not a significant problem for us yotties. The second cause only (usually) comes from excessively worn engines, or overfilled oil sumps, bearing in mind that the sump can fill itself while running, with diesel or seawater. An engine that has not been turned over for some years can also push enough oil past the rings if they have stuck. So take care with that Easy Start! It can overcome low compression and start an engine with seized or damaged rings, leading to runaway. Usually older small marine diesels will refuse to start unless there is good compression anyway. So I think the chances of it happening to a boat engine are very low indeed, unless you have a fancy turbo diesel.

Turbos in boats are prone to corrosion in the salty atmosphere which can quite easily damage the oil seals in them, so if you have a turbo, extra care with anti-corrosion measures are needed.

As far as air being sucked through the breather tube as lw395 suggests, the sump itself should be airtight anyway so it wouldnt happen unless there are other external breathers such as the oil cap.

 

[scottie]

What is the likelihood of this happening on my boat engine, is it more or less than getting run down by a nuclear submarine?

You could ask the late Sir Arnold,s skipper

 

[Major_Clanger]

Years ago I was mate on a sailing barge based in St. Katherine Docks. She was fitted with an ancient Kelvin 66; a 3-cyl long stroke diesel of great beauty but totally worn out. The engine liked to run away occasionally and normally when we had punters on board. Anyway, one day we were at the end of a trip to the barrier and back with 12 punters aboard, strong flood under us and shaping up for the outer lock at St. Kats. All was fine until the revs started to rise and we knew the Kelvin had taken matters into its own hands. Engine speed quickly rose above the max RPM of circa 650 and we couldn't take her out of gear as once there was heavy load on the g'box the chain telegraph to it wouldn't work.

Our passengers had no idea there was a problem and sat in blissful ignorance sipping their bubbly. We'd made a big arc over to the south side of the river with the poor barge at hull speed pulling a bloody great quartering wave along! We could see the outer lock beginning to open so began our approach. I went below to the chuntering Kelvin hoping it wouldn't explode while I stood holding the great decompression lever at 12 o'clock. Very slowly it lost momentum and we were able to take the box out of gear - the fact that we were still doing 8 knots across the river had nothing to do with it! By the time I came back up on deck the engine was stopped and we were sliding through the outer lock. I threw a spring and, by pure luck, managed to lasso the bollard and so was able to slow us to a halt. The passengers and onlookers thought everything was quite normal and didn't question the hysterical laughter coming from the aft deck! The workboat gave us a pluck to our berth and all was well.

Sadly that was the end of the Kelvin; that winter it was junked in favour of a 6-cyl Ford. A more souless engine it would be hard to imagine.

 

[lw395]

....

As far as air being sucked through the breather tube as lw395 suggests, the sump itself should be airtight anyway so it wouldnt happen unless there are other external breathers such as the oil cap.

I can't work this out from the diagrams TBH.
I suspect the question might be whether the crankcase/rocker box/timing chest can withstand full vacuum without any gaskets or seals letting go. I'm not convinced I want to rely on that, particularly on a random engine that's knackered enough to have a problem in the first place. I wouldn't trust a sheet metal rocker cover not to distort and leak for one thing.