Crevice Corrosion?

While cleaning my engine room today I found the below sheered off bolt under my dripless seal on the Stb side. A little investigation led me to see that it came out of the shaft flange, sheered off clean.

All three other bolts in the flange appear to be fine. Our last trip was a long one, 120 miles non stop at fast cruise, the boat ran great, no vibration or anything. I haven't hit anything recently, at least to my knowledge. I was under the boat a lot in the last few weeks (we were in the Bahamas) and the props are fine.

Any idea what could make the bolt sheer off like this?

Crevice corrosion maybe?

I would take out at least one of the other bolts to verify they are fine, no marks, etc. If so, then I would assume a flawed bolt.

If that was a chiller end bell bolt, I'd say it was torqued too far. If you have torque data to show otherwise, I'd say you had a defective bolt (and bad luck). Doesn't look like crevice corrosion to me though.

If your access to the flange is relatively easy, I would replace all the bolts and torque to spec. Cheap and peace of mind.

For sure!

That's my plan, 4 new bolts. Thanks for the replies.

It looks like a clean break. There's no visible evidence of crevice corrosion on that bolt. Crevice corrosion makes it look like worms have been eating the metal.

Ken

OK thanks. Pretty odd. Must have just been a defective bolt.

Looks like it could be a fatigue failure. Bolts can get cyclic loading if they are not torqued sufficiently and flanges have relative motion. Check to see if other bolts are tight.

But after checking them, replace them. Use grade 8 fine thread.

That's probably the clue, the broken bolt is likely too poor a grade.

Photo of the actual bolt head top should settle which grade the bolt is.

Shaft flange bolts should be grade 5 minimum. Fine thread is best and shakeproof nuts or safety wire. Replace them all.

Replaced them all. Of the three remaining bolts 2 were pretty loose, one of them I could finger tighten. I think it was probably fatigue. I had my dripless seals changed when I bought the boat in late 2013, I think the tech maybe didn't get them tight, and it's not something I check in my maintenance/inspection schedule. I guess I should now. The bolts on the other engine were all tight.

"That's my plan, 4 new bolts. Thanks for the replies."

Add some of your favorite bolt glue like Locktite, the RED stuff.

Ski in NC said:

Looks like it could be a fatigue failure. Bolts can get cyclic loading if they are not torqued sufficiently and flanges have relative motion. Check to see if other bolts are tight.

But after checking them, replace them. Use grade 8 fine thread.

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Agree. Looks like pretty classic fatigue failure. Counterintuitively, insufficient preload will fatigue a fastener more than excessive preload. Thumb rule is try to achieve 60% of the materials yield strength. That's what most of the torque tables use to calculate.

Threadlocker is a good idea, but I would use blue, not red. Red generally needs heat to remove.

Crevice corrosion is a stainless steel problem and your bolts look like mild steel. You want grade 8 steel bolts if you can get them. I would recommend a very close look at all the engine mounts to see if something is moving and, you should very carefully check the alignment. I think you may have had misalignment ant it may have fatigued the bolts.

The alignment response could hold water. My motors are tough to align, ( I guess all of the 400 twins are) as they don't have a real straight shot at the stern tube. The tech who redid the shaft seals when I bought the boat made a point of telling me that. He felt I might wear through seals faster than normal. He worked to align the motors (in and out of the water) but said there would always be some slight alignment issues. I don't recall the exact details of how they are out.

My opinion is that the alignment would have to be pretty bad in order to fatigue fail the bolts. This is "gut feel" based on my engineering experience)
So I'm not buying the "slight" alignment issue.

Jay, in my many years running boatyards, I have aligned or been responsible for aligning hundreds of engines. I have seen bolts fatigue and fail. As recently as last fall I had a customer with a bad vibration, had 3 sheared coupling bolts. In all cases the engine was out of alignment. At 2000 rpm, the typical engine will endure about 4 million revolutions in an average year (round numbers) So in 3 seasons an out of alignment engine will pass 10 million cycles of loading and unloading. Coupling bolts do shear and alignment accounts for much of it.

If bolts were initially torqued properly, a slight misalignment will not cause relative motion between the two flanges, so the stress in the bolts remains absolutely constant.

If bolts were never tightened, then the alignment being off will cause bolt stress to vary. Also, diesel output through the shaft is not constant due to firing pulses so that will vary the stress cyclically. Sets up a classic fatigue situation. Alignment could be perfect, but if the bolts are loose, they will break.

tadhana said:

Jay, in my many years running boatyards, I have aligned or been responsible for aligning hundreds of engines. I have seen bolts fatigue and fail. As recently as last fall I had a customer with a bad vibration, had 3 sheared coupling bolts. In all cases the engine was out of alignment. At 2000 rpm, the typical engine will endure about 4 million revolutions in an average year (round numbers) So in 3 seasons an out of alignment engine will pass 10 million cycles of loading and unloading. Coupling bolts do shear and alignment accounts for much of it.

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I don't doubt that, but I can't believe an out of alignment condition of 1 or 2 thousandths would cause it. That is what I would call slight misalignment.

It's not difficult to align an engine, just time-consuming and to an old fart like me, exhausting. Up down, back front, repeat until perfect. Also, an alignment done anywhere but in the water is a pointless exercise, the hull is loaded differently on the hard vs in the water; the hull flexes.