Check your bolts! Swan 40
Having noticed that some of the bolts sandwiching the chainplates to the hanging knees above the pilot berths of our Swan 40 were a bit short, I decided to replace them with longer bolts. The photos below show what happened today when I gently applied a spanner to unscrew the nuts!

The bolts are A2 stainless. I have replaced them with A4 stainless. The knees are tastefully hidden by a nice teak ply box, which the surveyer 6 months ago (when we bought Can Can) didn't bother to look inside. Good job I did, if not for my benefit, as a back injury means she is about to go on the market again, at least a good job for the next owner.

Anyhow, the point of this post is, if you haven't looked inside your chainplate boxes, do have a look, and if you see any signs of rusting, stick a spanner on the nuts.

(We're not giving up sailing - just sailing a smaller boat!)

Good sailing

John Trythall

Dear John,
mine is only an opinion and the professor will relate you much better.

- I do not see a reason for a 304 s/s being weaker than a 316 s/s
- Being your bolts of different length, I presume that some of them were not originally assembled. - You applied gently a spanner to unscrew them, but no one knows if they were screwed too much when installed. It is to presume that too much a torque was applied when installing, probably in the case of the shorter bolts.
- It appears that the bolts have been broken by torque, looking at the points of breakage, that is close to the heads and on the threads area.
- these bolts are intact in the central area of shear. In this case the plates and the bolts have to bear by project only the stress of shear. You might use a pin instead, as it is the assemblage of the troggle to the shrouds.

Thank you,
Matteo, Grampus 47/16 -

Thanks for that, Matteo

My interpretation is that the failure is due to crevice corrosion, cause by saltwater leaking through the chainplate seal, then sitting in the narrow gap between the bolt head or nut and the chainplate. This would be a stagnant and oxygen starved environment ideal for SS corrosion. I don't know whether A2 is any more prone to this than A4.

Regards

John

Dear John and Matteo
Right, this is a result of leakage and crevice corrosion.
There is a difference in pitting/crevice corrosion resistance between A2 and A4, the Pitting Resistance Equivalent Number PREN is approx 18 and 25 for these materials, the higher the better.
It would be of interest to hear from other owners, particularly Swan 40's, what markings there are on their chainplate bolt heads, A2 or A4? A4 has been specified.
Kind regards
Lars

Dear Lars et al.,

Storm Svale is getting prepped for another crossing. I will be on her the 8th of April and after seeing these messages this is the first thing on my to do list!!!

Back when I have explored the situation.

Fair Winds

Mike

Certainly the professor is right that 316 is better in pitting and also crevice corrosion than 304 - it is a big part of why 316 came along. Having said that, even 316 over time can suffer this fate - and the worst of that is this corrosion and type of failure is very insidious, sometimes not showing so much from a visual inspection. If you are to replace the bolts, consider super austenitic or duplex stainless steels which are almost immune to this type of failure - such as Avesta 254 SMo, or Ferralium - it might be very expensive to find or even get bolts made, but compared to a rig failure, not much at all, and the peace of mind well worth even a couple of thousand dollars. I will be checking mine, you can bet.

All the best,

Geoff, Corazon, 411 #41

Hi Guys,

The bolts on Storm Svale are in good shape, no corrosion. Another thing off my list of things to do.

Fair Winds

Mike

You had me really concerned, but it appears Cygnus is OK. Attached are two pictures of the bolt from the port side chain-plate, just inboard of the welded tang. There are no markings on the head of the bolt. It is non magnetic.