304 vs. 316

[pgillihan]

I have a 36 foot corsair catamaran. Upon a return trip from Bermuda I discovered severe breakage of the capshrouds just as they entered the swage on the mast t terminal. Both cap shrouds had breakage and I noticed at least one of the wire strands had broken several feet away from the swage fitting on one of the shrouds. The rigging is 3/8 and the boat displaces 11 to 12 thousand all up. The cat routinley hits 9 and 10 knots and you can feel it load up . We made it to Bermuda in just over three days with steady 18 knots at 60 degrees apparent. Sometimes with a reef in the main, sometimes not, just at the break point for a reef. The boat carries a lot of sail especially the main , and is fractionally rigged. The caps and forestay terminate several feet below the masthead.

There seemed to be no problem with allignment of the shrouds and the t terminal, but i did change the top fitting to a noresman. My rigger convinced me 316 was the way to go for corrosion resitance, and that strength or resitance to shock load will be ok with 316. ( he thought the wire was poor quality and /or the swage was incorrectly done) Now I think i should have gone to 304 for the increased strength. Upsizing the wire is just too expensive. I still have the forestay to replace ( i dont know if its broken i cant get enough slack in the furler system to inspect the swage) and if i have to take the furler off i might as well replace it, although this wire was apparently replaced at a different time with prehaps different wire than the cap shrouds. None of the wire is over 5 years old.

Should i have gone with 304 in the caps and should i go to 304 in the forestay. I guess its a matter of corrosion resitance vs. strength , but how bad is the corrosion in a tropical climate. I do a lot of single handed sailing and i have lost one rig in my days which is enough.

[Brion Toss]

Hi there,
We have a couple of variables to deal with. First, we do not know the original quality of the wire, nor that of the swage. And second, we do not know the state of tune. We do know the multihulls, especially hard-sailed multihulls, tend to impose shock loads on rigging by their nature, and that therefore any compromises in quality or tune will bring about wire failure more quickly than with most monohulls.
The Corsairs are great designs, but fabrication and installation vary in quality. I think it's likely that 3/8" is appropriate for your boat, and that the above variables are the likely culprits, in combination.
Some type 316 wires can be 15% or more weaker than those made from 302, and this might possibly be a contributing factor to future failures. Does the rigger have any data on average break strengths for the wire they used? It should be findable, if it is good quality. Find out that strength, and then check with the designer on shroud loads to discover what your actual safety factor is; you might find that, once you are up on quality and tune, that even a weaker 316 is plenty strong.
The tropics are extremely hard on wire, which is why we bother with 316 in the first place. So if you find that your 316 is notably weak, and that it might compromise your safety factor, I'd go with a better grade of 316 rather than 304.
As for the furler, what brand is it? It would be extremely unusual that there be no mechanism whereby you could see the wire. In any event, replace that jibstay, pronto; it was under the same loads as those failing shrouds.
Finally, do you have gear to take up slack on the leeward shroud? This is a critical piece of multihull tuning gear, as flexion of the hulls will always put a lot of slack, and thus schock loads, into the shrouds. A stout bungy cord can make an amazing difference, but you can also use block-and-tackle.
Fair leads,
Brion Toss