Stainless Hardware - Failure

[FLexpat]

This was the original 1/2" stainless stern lifting eye - 1976. From the outside it looked perfect. It took less than 5 ft-lbs to twist off both sides.


When I got it out, it was clear that it had spread about 5 degrees so it had already failed under lifting load. The thin little stainless backing plate was stretched out - it had caught the threads and was the only thing that kept it from coming out the transom. As far as I know, it was never overloaded and the fractures looked like they were from corrosion rather than simple stress. The other side 'looks' fine, even after I removed it - no apparent deep corrosion and only a little surface rust on the threads. I don't have access to xyglo or magnaflux inspection stuff any more so I have to rely on visual. I don't trust any of the lifting hardware now. I remember why I like slings too.

So what is the right replacement here? I'm thinking 316L for better corrosion resistance? The bow eye and lifting hardware - especially that threaded rod -gets added to the list too.

[McGillicuddy]

Classic crevice corrosion.

Chromium in stainless forms an oxide barrier on the alloy, once that barrier fails, corrosion however microscopic, will begin. Calder presents it as anytime there is an oxygen deficiency at the surface of the metal it leads to the oxide breaking down. In the case of the through bolts they simply got exposed by the years of movement, expansion and contraction of the wet wood led the sealant to separate from the steel, and exposed the steel to perpetual moisture trapped in the hole causing the metal became active and corrosion ensued. if your in altitude in UT there is even less oxygen than at the beach, so it stands to reason it might fail sooner, but that's splitting hairs. I'm sure the engineers on the site could give you a better explanation, but that my feeble minded interpretation.

I would guess the originals were 316 s.s., but yeah stick with your plan use the most austenitic steel you can find.

It did look like there is some moisture (rot) in the transom wood in you picture.

[FLexpat]

It has spent it's entire life in Florida and is currently in Palatka on the St. Johns River. I will get a better idea about how bad the transom is in 2 weeks when I start pulling it, but that side is the one I thought was still sorta good. I commute from UT for conjugal visits with the project - I can really strech it out that way.

[Bushwacker]

Gillie's right - crevice corrosion is a very common failure mechanism for those transom eyes. Anytime you see rust streaks from them, that's what's going on. Stainless has to be exposed to oxygen to maintain the protective oxide layer to remain "stainless". When it's buried in a thick transom away from the air but not sealed well enough to keep moisture away from it, that's the perfect environment for crevice corrosion. Silicon bronze doesn't have crevice corrosion problems and is often used for marine fasteners and sea cocks for that reason, but I haven't seen any u-bolts made out of it, maybe because they wouldn't have the strength of SS. Monel is a very corrosion resistant (and expensive!) nickle alloy sometimes used for prop shafts. A friend of mine that bought a Moesly 21 new in 1965 replaced the threaded rod attached to the bow lifting eye with a Monel rod on his boat because he kept it on davits and was worried about corrosion.

Best bet if you redo the transom is to seal the towing eyes with 5200 or a good polysulfide caulk such as Life Caulk that remains flexible. Drilling the holes oversize, filling with thickened epoxy, and then redrilling to size would also help by keeping moisture-retaining wood away from the stainless material.