68 Renegade #147 chain plates & bolts

[Polecat]

When cleaning up the 3/8" chain plate bolts on a bronze wheel one of the heads just cracked almost off the bolt. It was black most of the way thru. I found 2 more that were stressed and had cracks at the head. I'm thinking - 41 year old stainless shouldn't be trusted much. also 2 of the six chain plates proved to have cracks near where they went thru the deck.
jim

[Hirilondë]

Crevice corrosion is not uncommon in stainless steel, especially 41 year old material. It is usually more common in the bolts than the chain plates, but as you can see, it happens. I would definitely remove, clean and inspect them all. Even if the plates themselves seem in good shape I would replace the bolts.

[Homer]

Old chainplates can certainly be a worry. Even worse, chain plates that cannot be inspected at all are a time bomb. My 41-year-old boat had them replaced at some unknown point with a strange and rather massive fiberglass-encapsulated, stainless steel structure. The whole lot will be carefully sawed out and ground away this spring. Yankee will get new foam-cored knees and Farrier F-Boat type carbon chainplates. No more leaking. No more worrying.

[Rachel]

I know, I know, I'm like a broken record on this topic. But you might want to consider bronze, which does not crevice corrode in the way that stainless does (sneakily).

Skene's has the forumulae to calculate the sizes with (page 188 in the 8th edition). The design is based on a factor of safety of 2.5 and a strain of 4/3 of the breaking strength of the appropriate wire. I don't know what size wire your renegade uses for stays/shrouds, but for example, based on 1/4" 1x19 wire with a 1/2" pin, the headstay and backstay tangs would be 5/16" thick, and 1-3/8" wide. Center of pin hole would be 13/16" from the end, and the end would be rounded with an 11/16' radius. The shrouds would be the same except 1/4" thick.

Whenever I look up this information, I take a moment to think of Robert Gainer*, who cheerfully sent it to me a few years ago (I did not have a copy of Skene's). Of course if you are going to use these figures it would be best to double check them, but it gives you an idea (I was surprised to find that -- at this size anyway -- bronze and stainless carried the same specifications (only aluminum was beefier).

Rachel

*Among other things, he crossed the Atlantic solo in a Sea Sprite 23 when he was 21. I only knew him from online forums, but he was always most gracious and helpful. He passed away in 2008, while planning a spring departure for the Maritimes in his Tartan 34c, which he lived aboard. Plastic Classic all the way :)

[Polecat]

Chain plates were around $45 for six. Will have to drill them myself. New bolts/washers/nuts were around $26 -- not near as much as I thought they would be. Wonder what you should torque a 18-8 X3/8 bolt & nut to - so as not to over stress them. I'm sure that's what causes the internal corrosion just under the head.
Thank you for your thoughts
jim

[Hirilondë]

Once in a great while the materials for a boat project cost is reasonable. Enjoy it, it will rarely happen again!!

I'm sure that's what causes the internal corrosion just under the head.

There may be an optimum torque, but I don't think it affects corrosion. It will happen again should any moisture including condensation get in. And because boats are in water it will happen. The only safe procedure will include periodic inspection.

[Polecat]

After 41 years all discoloring/corrosion I found cleaned off with a bronze brush with the exception of the black corrosion which went right through the bolt under the head - the corrosion had to get in there some way - IMO it was over torqued either on installation (unlikely) or during inspection.
The cracks in the chain plates also showed up as tiny black lines once cleaned with the bronze brush. Took them to a flying buddy and he did some trick with chemicals that caused the cracks to show up under a light.
jim

[Hirilondë]

I have found that more often than not over tightening a bolt will damage the threads than cause a crack or crevice. The consensus of most professionals I have worked with is that the crevices they find, sometimes when failure occurs, were there when the bolt or piece of stock was purchased rather than caused by installers or work afterward. But unless you have the means and can justify the expense to test all materials it is rather moot.

[Rachel]

I like to understand how things fail, and I had not heard of overtorquing causing corrosion on fasteners before. Wouldn't that be crevice corrosion you're seeing? It sounds like you may know this already, but stainless steel likes oxygen, and when it's deprived of oxygen, it can suffer crevice corrosion.

I can't see your chainplate, but it seems like the area under the bolt head might not get oxygen, since it could have been installed with caulk or the like.

Wouldn't overtorquing damage the threads, as Hirilonde mentioned, or perhaps (probably invisibly) "stretch" the fastener?

In any case, you're replacing them, so that's good. I'm still curious though.

Rachel

[Quetzalsailor]

I second the notion that the bolts failed in crevice corrosion over the notion that the bolts failed in excessive tension. I also do not favor the idea that tension promotes corrosion.

I recall, from a long-past engineering course (Professor David Selby, it you must know), that there are two types of bolted connections that are loaded in sheer. One is a bearing connection and the other is a friction connection. A bolt in a bearing connection is like a pin in a clevis at the upper end of most chain plates; an important detail there is that that pin is loaded in double sheer, that is the pin is resisting the force in two sheer planes. This is nice since the pin/bolt is loaded symmetrically and does not tip or load the holes at one or the other edge. A bolt in a friction connection is itself in tension and the load is borne by the friction between the faces of the joined items. This is dandy because the load on the bolt does not change with changing load on the joined items, thus the bolt is not subject to cyclical forces and thus not subject to fatigue failure.

I suppose most bolts that mount ordinary chainplates to the hull are loaded in single bearing and that they resist tipping and loading the edge of the boltholes in the 'glass simply because they're tight enough. They're not so tight as to make the connection a friction connection. Two reasons for my supposition are that the glass isn't that flat and the glass is susceptable to crushing. This supposition is supported by the fact that the bolts are not the high-strength types seen on building framing or engines (the 3 or 6 radial lines found on a high strength bolt's head tell you about the strength) and no one talks about torquing them.