chainplates

[George Dubose]

I wrote Uma to ask what they paid for all new chainplates, covers, bow
fitting, etc. Check "Sailing Uma - Episode 474".

Here is what they wrote:

All 8 plates, caps and backing plates fabricated here in Olbia cost us I think €2200. Including the bow fitting he did a great job. Buy in typical Italian fashion, a “one week” project took them over a month. So there is that.

We get a lot of doom and gloom comments very similar to that guy about this mysterious “crevice corrosion” everyone seems worried about. It’s like the boogie man!!

Removing, inspecting, and resealing every 5 years maybe. But not replacing. Unless there is visible damage. Crevice corrosion happens AFTER surface corrosion. And you can inspect for it with polishing and dies.

--
George DuBose
www.george-dubose.com

Wiedstrasse 16
D-50859 Cologne
Germany

[Kevin O’Brien]

Final comment from me on this: crevice corrosion is not mysterious. It occurs when an acid contacts stainless in a low-oxygen environment, and the inability for oxygen exchange to occur results in a differential aeration cell between the material and the surrounding area (e.g., the crevice, typically the deck penetration for chainplates).

Chloride ions migrate into the crevice, consuming the passivation layer (chromium oxide) on the steel, and without oxygen it cannot be replenished. As the chloride ions migrate into the steel itself, they combine with seawater to form hydrochloric acid, which further attacks the metal, accelerating the process until the chainplate fails structurally.

The chloride ion incursion can and does occur microscopically, beneath the existing visual shiny layer - which is precisely why you cannot simply visually inspect chainplates. Dye testing or ultrasonic inspection can find areas of corrosion, yes, but you’ll pay for it - and if you do find evidence of corrosion, which is incredibly likely in quarter century old metal - you’ll still have to replace them.

Kevin O'Brien
Email: kevin.mor...@gmail.com

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[Dan Pfeiffer]

For reference the 3" x 3/8" thick 316 SS bar stock to replace the primary 10M chainplates would cost on the order of $150.  Thy are unfortunately 13" long so a bit of extra waste for standard sizes. That's from onlinemetals.com.  Then there's machining.  Not a difficult job but maybe a few hundred?   That's just for the two primary chjainplats.  There are two for the forward lowers and one at the stern.  The bow unit is a more complex fabrication.  I expect most Pearsons under 38 feet would be similar. 

Kevin gave a very nice description of the corrosion issues.  I would recommend this book:
Collier, Everett 2001 A Boat Owners Guide to Corrosion, International Marine
ISBN 0-07-155019-4,   LOC # 00-033488,   Cat # VM951.C585 2000 

And here is an interesting video about corrosion in stainless steel for the curious...
https://www.youtube.com/watch?v=KHJmQdvnfPE

Dan Pfeiffer

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[Dave Cole]

Hi George,

Exactly. If you are concerned, you could have them x-rayed as well.  Welding companies often provide this service to verify weld integrity on critical welds.  

Best,
Dave Cole