Chainplate Crack Cause, Effect, Theory, RFP

Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
The backstay chainplate of the '84 Ericson 381 was cracked. Missed by surveyor, because when a boat's transom extends past the slip, it's impossible to see without a dinghy--and when hauled out, a ladder is required.

This hefty piece of stainless is 2" x 13" and almost half an inch thick. I believe the crack was caused by a very small deviation in the true pull of the backstay.

The ruler photo shows the relation of the crack to the lateral bend induced by subtle misalignment.

1-Chainplate ruler.JPG

This piece may have gone another 30 years without incident, but nobody in his right mind could sleep knowing it was there.

3-chainplate side.JPG4-chainplate bottom .JPG

The piece is easy to have fabricated--but what caused the crack?

My theory is that the factory installed it with no margin for error or to permit natural alignment.

The holes for the bolts were drilled such that the bolts cut threads in the fiberglass of the transom before entering a stainless backing plate, split washer and nut.

Removal required unscrewing. They couldn't be hammered out. They were threaded in.

This seems like extra strength but I think it's an error. Such tight holes fix in place a chainplate set by eye--how else would you do it?--that must align with a point 60 feet away of uncertain location (i.e., the top of the mast).

On reinstallation of the new chainplate, I plan to over-drill the transom holes slightly, so the plate can align naturally.

Counter arguments invited.
 
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supersailor

Sustaining Member
That appears to be flex cracking. Could the chain plate have been out of alignment or the top bolt loose? There is a lot of stress back there. Looks like work hardening. Good catch! A reminder for all of to look at our chain plates.

What I like on my E34 is it is a straight pull on the chain plate without the angle bend at the top.
 
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Kenneth K

Sustaining Member
Blogs Author
I think you may be right about the uneven axial forces contributing to the crack. The uneven wear/corrosion on the port side of the attachment point (under the big arrow) looks like this side may have taken the brunt of the force.

1-Chainplate ruler.JPG
And, as Supersailor points out, the bend in the plate would cause additional force in another axis on that same area of the plate. The crack is an equal distance from the bend compared to the attachment point from the bend. The bend would probably always concentrate the forces in this area the plate, but then, many, many boats have such a bend in the chainplate. Like you say, the uneven axial force is what probably pushed this one to failure. Scary....

1-Chainplate ruler 2.JPG
 

Spirit Moon

Member II
The chain plates on Ericsons of our vintages, and unfortunately with no new Ericsons being produced we all have vintage boats, are hitting that point in time that replacements are going to be much more common. The previous owner of my boat, a 1980, had a rigging and fabrication shop replace all the chain plates because they were prepping for some serious offshore time. The danger is those cracks that are not visible without a dye test on the metal. By the time a crack becomes visible it can be a big problem that can lead to a major rigging failure. One thing to keep in mind about stress on chainplates is that it is not always just the usual forces of sailing that can stress them. A rigger told me that it is common for motoring to set up resonant vibrations in the rigging that flex the chainplate hundreds of thousands of times, causing the metal to fatigue. Updating to current standard motor mounts that are designed to reduce vibration can be a big bonus for your rigging life span. The more rattle and roll the engine is doing the more stress on your chainplates. Keep up the good work on your boat.
Frank
 

supersailor

Sustaining Member
Perhaps, you might want to consider a little redesign. The pix is of the chain plate for an E-34. Perhaps it could be adapted to an E-38. It would appear to solve the problem of stress cracking. P1000801-001.JPG
 

u079721

Contributing Partner
Fascinating - and scary. I'm sort of amazed that the failure wasn't at the bend. When I used to look at that chainplate on our 38 it bothered me, because to my eye the bend was actually MORE than needed to make the pull fair, and it looked as if they made the bend a bit bigger than optimum just to give them room for the toggle.

I'd be a bit worried about making the holes too big, as wouldn't that allow the unit to move a bit under stress, which would lead to widening of the openings?

What I would like to try if I were doing this on my boat would be to maybe just attach the new chainplate with just the top bolt, and then lightly tension the backstay. That would cause the chainplate to swing into its natural alignment before drilling out the other holes. Of course that would only make sense if you filled the original holes in the hull with epoxy and used a new backing plate, right?
 

bolbmw

Member III
The holes for the bolts were drilled such that the bolts cut threads in the fiberglass of the transom before entering a stainless backing plate, split washer and nut.

Removal required unscrewing. They couldn't be hammered out. They were threaded in.
I discovered this today as well while removing a winch for serving, those bolts too were threaded into the fiberglass before the backing plates. Was quite the PITA to get those bolts out.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
At the point of reply #5, I sent this thread link over to a friend and professional rigger.
He said it was ok to share his comments:

" Interesting. Virtually no chainplate has a true lead to the opposite end terminal, that's why it's so important to have some type of toggle at the turnbuckle, and the proper fitting above. Not to mention the proper size clevis pin in the chainplate, something I see quite often.The tight fit of the fasteners through the transom is not good, surprised the fasteners didn't show signs of corrosion, they should be snug, but not to the point where they need to be hammered out or cutting threads into the laminate. Ideally the fasteners should have a shoulder where they pass through the laminate, and be threaded only on the outside of the skins.

It looks like a fatigue fracture to me from the close-up photo, possible that not all of the fasteners were sharing the load properly and the location of the crack had a slightly higher percentage. Could also be there was a small flaw in the casting at that point which lead to the failure.

Regarding the vibrations of motoring transferring to work hardening of the material, possible, but the boat absorbs a lot of that before it gets to the rigging. As I tell people during my seminars, your rig takes millions of vibrational loads sitting at the dock, from wind and wakes and the snubbing of the boats motion from dock lines. This all does lead to work hardening of the stainless and is a leading cause of rig failures. In any case, any boat over 30 years old should have all the chainplates removed and inspected, especially those that pass through the deck.

Just my $.25 worth, feel free to share "
 

Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
Maybe watching the Superbowl primed me for surprises, but in any case I looked again at the chainplate photos. And then at the fitting itself.

To my surprise, despite close inspection, I had missed the fact that the crack is spreading on the other side of the hole.

An experienced rigger wouldn't have.

close chainplate draw .jpg
 

sailorman37

Member II
Is that the side to the hull that shows the additional crack? It is quite possible that stress along with salt is causing stress corrosion/crevice corrosion. Very small cracks (microgap behing plate) allowed stagnant salt water to continue the process till failure. Due to the quality of steel and age, a number of plates cracked on my boat. The steel looked bright and shiny, but one tap resulted in the crack in this photo.
 

Attachments

Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
Yes. The side you can't see.

A further reflection on maintenance of our standing rigging is that there is no designed redundancy. This we share with general aviation aircraft, where weight is the factor.

The critical flying wires on hang gliders, for example, are designed only 50 percent stronger than maximum strain.

They are to be changed annually.

These wires--the equivalent of our stays--always look perfect, new, shiny stainless. You throw them away every year. Stainless looks beautiful just before it fails.
 

toddster

Curator of Broken Parts
Blogs Author
Maybe time to start thinking about carbon fiber chainplates and high-tech rope-rigging?
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Titanium

Maybe time to start thinking about carbon fiber chainplates and high-tech rope-rigging?
My rigger friend checked this thread again, and mentioned that he's always a little surprised that people do not change to titanium or bronze for these chain plates.
 

Joliba

Contributing Member
Carbon fiber isn't perfect either

Maybe time to start thinking about carbon fiber chainplates and high-tech rope-rigging?
Interesting idea. However, metal fasteners in contact with the carbon fiber chainplate will corrode much more rapidly than usual. This is especially a problem with aluminum contacting carbon fiber, but is true for stainless steel as well. There is probably a way to get around it, though. I once thought that carbon fiber was inert. Actually, it acts as a strong cathode relative to the metals we use on boats. (The high tech rope rigging could work well with our existing chainplates with the proper chafe proof attachments.)
Mike Jacker
 

Joliba

Contributing Member
Have you seen carbon fiber chain plates laminated as a retrofit? I've only seen it in new construction... But we don't have too many high tech boats around here.
 

toddster

Curator of Broken Parts
Blogs Author
Only on the internet. It wouldn't be easy, but replacing embedded chainplates with anything at all won't be easy.
 

ignacio

Member III
Blogs Author
+1 Titanium

I replaced all my chain plates with Grade 5 Titanium bar stock a few years ago. Yes, it's more expensive than SS, but not that much more. Half the weight, and 3x the strength. Does not corrode in the absence of oxygen (like where the chain plates pass through the deck). Corroded stainless chain plates appear to be among the top reasons for dismastings. The place I ordered from in So Cal sells grade 5 titanium rod, bar, sheet, plate, block, and other products. I'm not sure about it's welding properties, but the metal shop at Svendsens in Alameda was able to drill, bend, and cut it to match my original 304 stainless chain plates without issue. Two of my original chain plates had cracks. Half of them had visible pitting.

At least on the 35-II, 5 of the 6 chain plates are bolt-on. I had to grind out the one glassed in the stern, and replaced it with one that I through-bolted to the transom.
 
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