Yanmar 4JH4-TE Catastrophic failure

[DougR]

The conrod bolt that is in one piece loosened and fell out of the rod, and the engine continued to run at the moderate 1400 RPM with low load, and the rod cap was held in place by the second bolt. When the vessel surfed down the wave the RPM raced and overloaded the remaining bolt and it fractured, allowing the piston and rod to remain up in the cylinder until the next revolution of the crank. When the crank came around next time it jammed the rod up against the block, spinning it 90 degrees, bending it and fracturing the piston.

No evidence of any other damage.....no seized piston, no damaged bearing shells, no scored journals.

The bolt that is not broken shows no evidence of rubbing or wear between it and the rod cap, it just loosened and fell out. But the broken bolt shows lots of shiny wear evidence against the inside of the rod cap holes.....evidence of flexing and rubbing as it continued to run.

Its difficult to see the grain structure of the bolt fracture surfaces, so we cant see evidence of a progressive fatigue failure (beach marks or two different grain structures) on the threaded end which is still in the conrod. However it probably fatigued and then failed from an abrupt overload.

[kev_rm]

I am not buying hydrolock from exhaust while running. Banana in the tailpipe? Banana launches across the parking lot, 100+ psi in exhaust. 100+ psi = 200 foot wave.

[Jim Cate]

The OP reports a surf (or wave slide) from a following sea which as others have suggested can force exhaust water back towards the engine.

While the list of possible causes grows, I'm troubled by the above concept. When our boat is surfing, no water impacts the transom or exhaust outlet. There is a nice clean flow in the usual direction. Only if the boat is slowing and the wave breaking or nearly breaking does water impact the stern, and that is a rather brief action. I'd expect the Bene to be similar, but have never sailed one.

It is now clear that the rod did not break... it bent and a big end bolt broke, a different circumstance than I was envisioning before. Sure sounds like it was jammed, whether by hydrolocking or hanging a valve, who knows? One thing that comes to mind is that I suspect the valve springs on low speed diesels are pretty weak, so if the governor failed to control engine speed when the prop either came out or engaged some highly aerated water, perhaps valve float occurred to the extent that interference with the piston happened.

In light of the recent revelations, I'd have to retract the thought of a latent defect causing the rod to break... 'cause the damn thing didn't break!

Jim

[DougR]

Regarding the idea that a propeller cannot overdrive an engine, I beg to differ......
Back in the not so old days it wasn't uncommon to start a recalcitrant diesel engine in a commercial fishing boat by pulling the vessel with another, engaging the mechanical "clutch" (Cajun for marine transmission) and the large spinning prop would spin the engine over fast enough to start it.

There are many types of transmissions in use on today's sailboats that will overdrive the engine. Hydraulic transmissions, like the ZF45a that I have in my boat, will keep the clutch plates firmly engaged as long as the engine is running and hydraulic pressure is produced.

Most of the saildrives that are built by ZF use a series of steel balls on ramps to create pressure to hold the clutch plates together. The higher the RPM of the shaft, the higher the centrifugal force on the balls and the more firmly the plates are pressed together.

In my opinion its very possible for a Beneteau 50 surfing at 12kts to over speed the engine enough to overload an already compromised connecting rod bolt and cause it to fracture.

[kev_rm]

If 12 knots will drive an entire engine and transmission assembly to 4000 rpm (say, 20 horsepower of energy) - I am totally confused why 7 knots will drive a free wheeling shaft only at about 60rpm (say 1/10hp).

Originally Posted by DougR

Regarding the idea that a propeller cannot overdrive an engine, I beg to differ......
Back in the not so old days it wasn't uncommon to start a recalcitrant diesel engine in a commercial fishing boat by pulling the vessel with another, engaging the mechanical "clutch" (Cajun for marine transmission) and the large spinning prop would spin the engine over fast enough to start it.

There are many types of transmissions in use on today's sailboats that will overdrive the engine. Hydraulic transmissions, like the ZF45a that I have in my boat, will keep the clutch plates firmly engaged as long as the engine is running and hydraulic pressure is produced.

Most of the saildrives that are built by ZF use a series of steel balls on ramps to create pressure to hold the clutch plates together. The higher the RPM of the shaft, the higher the centrifugal force on the balls and the more firmly the plates are pressed together.

In my opinion its very possible for a Beneteau 50 surfing at 12kts to over speed the engine enough to overload an already compromised connecting rod bolt and cause it to fracture.

[skipperpete]

DougR is right about engines that can be started via the propeller, in my first boat it was possible to start the engine by using the decompression levers and adjusting the Hundested propeller pitch while the clutch was engaged. This was a 36”propeller turning a 5LW gardner and it actually started by itself on one occasion.
I hadn’t considered the possibility that the undamaged conrod bolt had earlier just fallen out, well spotted.
More has been going on in this engine than meets the eye, in the 3rd photo (yellow paint showing) there appears to be substantial corrosion on the crank and rust tracks/streaks on the main bearing cap next to it which is unusual and implies water coming down from above the piston, past the rings and onto the crankshaft at an earlier time ....but seemingly no evidence of corrosion on the piston. Is there anything in the history of this engine that might explain the corrosion?

[RaymondR]

The conrod could have been bent a while back from water ingestion and chose to fail from the surging induced by the wave action.

[donradcliffe]

For the following sea crowd, don't forget there was a turbo involved.

[mvweebles]

First, I'll amend my no-hydrolock guidance and say that exhaust elbows are common sources of intrusion as they are neglected and often fail from inside-out thus invisible or are improperly designed with an injection point that is not sufficently past a 'hump' on the elbow, or has deteriorated and allowed water the reverse direction. This should have shown as increased steam at start-up, though not necessarily underway as the vapor would be driven off on a hot engine. Water scooping from the transom? Bottom of the list - by far. Still, getting that much water into the exhaust side of an engine that has been running for several hours - even at modest RPM - is difficult to imagine. The exhaust, by definition, is a positive-pressure system (especially with a turbo as someone else noted, though that may not kick-in until a higher RPM.....but still.....)

Otherwise, this is some flavor of lower-end bearing failure (I assume the wrist pin bearings are intact). The root cause of this would be low oil, severe/sustained over-rev, severe over-load (think: dragster that blows-up 100-yards from start, not a prop in water), improper oil lubricity (e.g. water in the oil), simple metal fatigue (unlikely with 1200 engine hours), or defect in the metal (cap bolts are commonly upgraded for high performance engines). Lubricity issues could have started a long time ago with a mistake at oil change or maintenance. Routine engine oil analysis would have very likely detected this as a trend over time of increased presence of metals (bearings) or sodium (raw water).

[Hunterdog]

Originally Posted by donradcliffe

For the following sea crowd, don't forget there was a turbo involved.

Maybe not: it was corroded stiff:
Mechanic says valves too corroded too.

But why the breakage right then? Is it just chance?
And I have receipts of full service just 8 months ago...
Should this be an inspection item?

[rslifkin]

Seems like it had been getting water through the exhaust in small amounts over time, causing damage over time in the form of corrosion and possibly an almost-hydrolock occurrence at some point. Then when you surfed, the engine experienced a sudden load reduction (not necessarily a big RPM change) and it's possible that was just enough for one of the damaged parts to go bang. The loading on rod bearings does change rather significantly when an engine gets suddenly unloaded.

[Hunterdog]

So...how would one detect this, so I don’t get stranded again and destroy another engine?

[rslifkin]

Make sure the replacement engine has an exhaust system with a water lift muffler. Make sure the highest point in the exhaust system before the muffler is higher than the highest point after the muffler output. Make sure the water injection point in the exhaust is after the high point in the downhill run to the muffler. And then make sure there's a decent drop from the muffler exit gooseneck to the exhaust outlet.

That setup should ensure that the muffler can't over-fill and feed water back to the engine and even if water gets pushed into the exhaust from outside, it'll be hard for it to make it up the hill into the muffler (and even harder to make it to the engine).

Detecting any water intrusion would involve periodically pulling the elbow to inspect it and the turbo, etc. to check for any signs of significant corrosion or water accumulating anywhere.

[mvweebles]

Originally Posted by Hunterdog

So...how would one detect this, so I don’t get stranded again and destroy another engine?

Most likely source is an issue with your exhaust design or the elbow is corroded and trashed (or both). This is often more complex than it looks - have to take into consideration height of engine relative to waterline. Best practice is for there to be a decent hump in the exhaust off the manifold

[mvweebles]

Originally Posted by mvweebles

Most likely source is an issue with your exhaust design or the elbow is corroded and trashed (or both). This is often more complex than it looks - have to take into consideration height of engine relative to waterline. Best practice is for there to be a decent hump in the exhaust off the manifold

Also, ongoing regime of engine oil analysis would likely have caught this before catastrophic failure.