Chainplate materials
 

I am replacing the chainplates (6 in number for capshrouds, forward and aft lowers) on my Rustler 36. Dimensions:10" x 1 9/16" x 3/8", material 316L. These yachts are still being built and still using 316L as chainplate material.
I have removed and tested the chainplates on my yacht (7 years old) and a sister ship (12 years old), both of whom have spent significant time in the tropics. Both suffered from surface and stress crevice corrosion (SCC) where the chainplates pass through the deck. My test for SCC is to (a) suspend the chainplate with a screwdriver through the bottom bolt-hole and tap in the middle with a hammer, then (b) to repeat the test suspending the chainplate from the hole for the rigging clevis pin. If SCC is present, then test (b) gives a dull "clank" as opposed to the clear ringing tone with test (a).
As I am about to leave for a 4 year sailing "odyssey" I want to replace the chainplates with a more corrosion resistant material, and after trying unsuccessfully to obtain Nitronic 50 at an affordable price, have decided on Nickel Aluminium Bronze (Ni 5%, Al 11%). This material seems to have at least the strength of the austenitic stainless steels and is very resistant to fatigue. It is slightly more expensive than 316L.
Although the decision is made and executed, I should be very interested to hear other views on my decision. I hope not to hear that I have taken a stupid decision, but all views (pro and contra) will be welcomed.
:)

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Hello,
And first, that's an elegant little test; why does it work? Is it the length of the unaffected sections?
As for the materials, nickel-aluminum bronze is good, with the only qualifier being that bolts made from it might be slightly stress-corrosion-vulnerable ( Warren ). Actually, a number of bronzes would be just about as good, as would grade 5 titanium, though if you bend the latter, some skill, care, and heat is required. At 3/8" thick, the chainplates are reassuring massive for this boat.
Fair leads,
Brion Toss

Elegant test ;-)
 

Hi Brion,
Thanks for the speedy reply, and of course the thumbs up! I intend to use the existing shafted bolts (A4) to fix the chainplates. Although I could obtain the appropriate rod and thread it, I decided to stay with the stainless because: 1) the underdeck location is dry 2) there will be a HDPE washer between stainless washer and chainplate 3) will use Tefgel or Lanolin to isolate shaft and chainplate hole and finally 4) I can monitor (see) and withdraw all the bolts easily.
The test was recommended to me by someone in the heavy lifting industry, and the key is the metal between point of suspension and the hammer stroke. If SCC is present, then the metal between the clevis pin hole and the point where the chainplate is struck doesn´t conduct the vibrations properly - I guess a bit like a guitar string when not cleanly fretted.
I have your excellent book "The complete riggers apprentice". An old seadog pointed out a small "improvement" which was new to me. "A knot and a tail" - means that all ropes with a stopper knot should have a tail long enough to grab hold of and pull when the worst happens and the stopper knot does its job. Quite logical when you think about it.
Yours aye,
Bill Attwood

Chainplate Material
 

The Nickle Aluminum Bronze , for your new chainplates, is a great choice from my research.

Have you found a source for Nickle Aluminum Bronze ( C95810X ) in 3/8" plate or flat stock ?

I would like to find a source other than casting ingots from Atlas !

I would second the recomendation for titanium, specifically Grade 5. While it is slightly more expensive than siliconized bronze, in normal marine service it doesn't suffer from corrosion at all. At tempratures below 180F the engineering allowables say you don't even need to provide a corrosion allowance for it.

Brion is correct that to cut and weld it takes a little more work than stainless, but the major differences are pretty easy to handle. Just TIG weld it under argon like you would aluminium, and hold the gass a little longer. Frankly titanium bends fine, you just have to remember how much stronger than stainless it is. With a yield strength roughly five times 316, you need to use a lot more force for the same size part.

We actually make a lot of titanium chainplates as replacement items, and are slowly moving into OEM parts as prices have come down. I would really recommend exploring titanium.

Titanium vs NiAlBr
 

I am delighted that this thread has generated some interest, and thanks for the replies. I haven´t looked at Titanium because of the difficulty I had trying to source Nitronic 50. I feel that in the US it is much easier to source "exotic" materials, whereas in Europe it is either difficult or expensive - or both.
I was surprised to find NiAlBr, forged not cast, at such a reasonable price. Replacement chainplates in 316L would have cost approx $40 each, and I shall be paying approx double for NiAlBr. The only quote I could find for Nitronic 50 was approx $300 each - although to be fair about 1/3 of the price was for machining the available bar stock down to size.
I think that overall strength in chainplates is not an issue. The breaking strength of chainplates is generally 3 to 4 times that of the wire rigging attached - on my boat about 19 tons/7 tons. But corrosion really is an issue, and I am surprised that it doesn´t receive more attention on forums. The real problem with stainless is that SCC results in catastrophic failure (although tell when a chainplate failure isn´t catastrophic). ;-)
But I shall certainly go back to my supplier and ask what Titanium would have cost. Too late to change this time, but useful experience.
Thanks again for the replies.
Yours aye,
Bill Attwood

NiAlBr
 

Hi Douglas.
My source for forged NiAlBr in 3/8" bar is:
Chrome Nickel Alloys Limited
The Barn, Manor Farm Barns
Baines Lane
Seaton, Rutland
LE15 9HP
www.cnalloys.co.uk
Andrew Clark is the MD and is very helpful. Mention my name when/if you contact him - I might get a discount next time!.
Yours aye,
Bill

Kinsa,

You are correct that strength in chainplates is rarely the biggest concern. They are typically the strongest part of the system when new. The problem is that there is no good way to attach them to a hull that doesn't result in corrosion problems. Either you go with external plates that leaves an entire side in an anaerobic situation, or thru-deck ones that almost guarantee a ring (where they pass thru the deck) where corrosion will occur. This requires relatively large corrosion allowances be used in order to have a long enough expected lifespan.

This is where titanium is really a great OEM material. The strength and size can be reduced because the corrosion allowance can be reduced to nothing, and they just need to be strong enough to handle the loads, not the lost material.

In fully engineered chainplates I have actually quoted some that were less expensive than the stainless original parts because we needed so much less volume.

Titanium chainplates
 

I have a quote from my supplier for chainplates in Grade 5 titanium alloy, about 5 times as expensive as NiAlBr. Not a fair comparison, as the quote is based on the same chainplate dimensions as for NiAlBr. If the dimensions were reduced to take advantage of the superior strength of titanium, I´m sure that the price comparison would be more favourable. However, for a crusing sailor (with a budget) I am happy that NiAlBr is a better solution than 316L for strength, corrosion resistance and price. Bur thank you for your comments, which have widened my knowledge!

Would you mind sharing the supplier and the cost? Five times sounds a bit high to me compard to nialbr.