Another Stainless steel failure

[delmarrey]

This winter I decided to change out the 25-year/ 7000 hour motor mounts. With all the shaking of a diesel after all this time I figured it was time to change out the old motor mounts while I had the rudder, prop shaft, shaft seal and a bunch of other stuff out.

While un-screwing this 5" SS lag bolt out, it twisted off just as the broken area reached the surface. Boy, was I lucky, I thought. Will explain later.

Anyway, you can see the electrolysis had eaten away at the inside. But this has never been in any water other then the wetness of the wood. At first I thought the wood may have been wet, but the moisture meter says it's normal for wood. So, I guess with just that moisture mixed with aluminum and steel, the stainless was the anode.

In the mean time, trying to get the motor re-aligned I discovered that the motor never was in-line (.035" on a 4" OD coupler). With the exact same brand and model of motor mounts, the original holes would not allow the couplers to line up. Beside some of the bolts were in at an angle and had bent the bolt heads cockeyed. So, I ended up making some sub-plates with multiple threaded holes (straight up/down) and bolted them down to the old glass covered beams using twice as many shorter galvanized lag screws. The motor came right in............................._/)

[CaptainK]

OUCH!!!!

Lucky Man.

[phorvati]

My motor mounts are adjustable. So you can always adjust for allignment. Left-right, fore-aft, and up/down. But I need new ones as well, since my old ones never had any rubber. Probably why I had so much vibration.

[delmarrey]

Yeah! These are adjustable too. But with the old lag bolt holes into the wood beams. The forward ones wouldn't move another 1/2" sideways to bring the coupler inline.

No rubber! Hummmmm????? It's not a diesel, is it?.................._/)

[Brent Swain]

Pulp mills , despite their extemely corrosive atmosphere ,never use stainless anywhere vibration is involved, It stretches , then crumbles.
Brent

[Alan Wheeler]

I just had to rebuild my SST exhaust riser. I had a small leak under the heat insulation and thought it was just a cooling entry fault. I opened up the heat insulation to find an utter mess. The pipe had cracked and fallen apart right were the cold water meets the hot exhaust. The water blast into the pipe and hits one wall only leaving a hot side and a cold side opposing one another. So I made a defuser to allow the water to enter the exhaust more evenly and rebuilt the pipe. Looks great, now I just have to re-lag the hot side of the riser.
SST exhaust systems commonly have cracking problems due to exhaust pulsing. If you have a SST exhaust, it pays to regularly check it.

[hellosailor]

I don't know if this is part of the problem, but woodworkers have told me never to use "iron" screws in cabinet work, because the iron reacts to acids in the wood causing a black stain in it. The solution is supposedly to still use brass for top-quality work because it will not have the reaction.

I wonder if the underlying basis for the stainless failure is partly the same thing, that some woods simply react badly to any iron fitting? Or perhaps, someone simply whacked that bolt during the initial installation, and the rest was just fatigue failure building after 2o years.

[Alan Wheeler]

SST will not remain stainless at all if it is in timber and the timber is wet. The SST will corrode away to nothing more than a rusty core in the timber. It just gets eatin away.
When I fist discovered it personaly, I at first thought that some twit had fitted the timber window surrounds using steel screws and that the steel had rusted away. Every now and then I would come across screw trhat were SST and scratched my head as to why the idiot had used a mixture. Then I discovered that all the screws were infact SST and some had been wet enough to simply disolve away in the acid solution the timber produces.

This application is a very bad one for stainless. It's not the vibration that is the issue, but rather the fact that stainless steels require exposure to oxygen to stay "stainless". Buried in damp wood, they have no exposure to oxygen and have no protection.

This is an application for bronze (NOT brass) bolts only. As strong as stainless, but not dependent on oxygen for corrosion resistance. They will be hard to find, but worth it!

[hellosailor]

I'm not sure that brass is the wrong material--bear in mind, that recommendation is from cabinetmakers for *furniture* not for marine structural mountings. Different context.<G>

Another good reason to overdrill mounting holes & bed things in epoxy though! Might be quicker, cheaper, and simpler than trying to find large bronze bolts, and then trying to figure out "what is bronze?". We played that game some years ago looking at through-hulls, every maker defines their alloy differently and they get really upset if you ask "but doesn't that make it brass?"

[Richhh]

Ill bet most of the farm that the failure in your lag bolts was simple fatigue failure. Stainless steels have a very low 'endurance limit' versus fatigue strength and if you design at much over the ~30Kpsi fatigue limit ( although 90Kpsi ultimate tensile strength) you only get about 1 million load cycles .... then 'snap'. If replacing with stainless steel fasteners, significantly 'beef-up' the cross section of the fasteners.

Wood is quite permeable to gaseous oxygen transmission, so I dont think that depletion of the chrome rich surface is the 'problem', although its quite permeable to water vapor. If the piece experiences 30kpsi loads, fatigue cracking may begin to form on a 'new' unit immediately, and the microscopic cracks are then very subject to crevice corrosion which makes the fatigue failure acclerate which make the crevice corrosion accelerate, etc. etc. etc. etc. until you get catastrophic failure. Fatigue is 'additive'.

[Brent Swain]

A mixture of seawater and hot diesel exhuasts will disolve stainless readily.
Stainless gets along well with dry exhuasts .Go dry exhuast and eliminate a lot of problems.
Lloyds won't approve any wooden boat unless it has stainless keel bolts , which corrode rapidly in soggy floor timbers . So much for Lloyds credibility.Is that why so many round the world racers are losing their keels ? Bad advice from a not so bright source?Possibly.
Brent

[hellosailor]

Brent/Louis-
"Is that why so many round the world racers are losing their keels ? Bad advice from a not so bright source?"

In case you hadn't heard, most round the world racers are not using wooden floor timbers these days. They don't make their engineering decisions based on Lloyds, either. They sit around and say "Do you think this will break? Can we get away with making it any lighter?" And when the race is over, they say "If nothing broke, you built the damn boat too heavy and that cost us the race!"

[Alan Wheeler]

I have had experiance with SST piping cracking due to pulsing. Trust me, it does happen. Corrosion is one thing, but in this case, it just cracks as if it was egg shell.

[CSY Man]

Darn, now I have another thing to worry about..:

My 27 year old engine-mount bolts, and the mounts themself.

Wonder if I can pull just the bolts, one at a time, without upsetting the apple-cart?

Probably should lift the engine and do the mounts as well. That means taking the whole galley part...Lots of work...Lazy owner....