Wednesday, 9 December 2015

The Stainless Thing That Went Bang In The NIght

Successive Stresses - Click To Enlarge The Image
Some weeks back I reported on a failure of our stainless steel boom bail, when a rain squall brought about a fairly gentle but inadvertent jibe - you can see that story here.

We're still working on the boom repairs, and on installing webbing slings as main sheet and preventer attachment points for the boom.

However the failure of that part is an intriguing lesson in the finite mortality of stainless steel on board the vessels we call home.

This image shows the failed part, and after consulting with experts I can see that this was not an instant total failure - in fact it clearly started to fail a long time back, and subsequent stresses have continued to weaken the bar until less than 50% was holding the thing together. Then came our latest jibe, and away it went.

Naval architect Evan Gatehouse, sailing on the catamaran Ceilydh, showed me how the "beach sand" tide marks on the right hand side are the record of successive stress events, and the final failure is visible with its granular structure at bottom left.  You can't trust old stainless steel it seems.


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