Why didn't my mast break

[allene]

The day after the last race of the year I took Papoose to the boat yard. I discovered that the starboard upper (7/32 wire) had almost failed.

I should point out that the previous day this shroud was loaded up such that we were healing at least 30 degrees and likely more at times. We had some guest crew that ended up on the low side and needed considerable help getting up and over to the high side.

Cree at BMC said he had only seen this kind of failure once before and it turns out it was under the same circumstances, having to do with an antenna. I traced the cause of this and wanted to share in case anyone else might have a similar arrangement. The cause of the corrosion was a stray current of 20 mA. My UHF antenna is on the stern pulpit. That grounds the pulpit and that grounds the lifeline, which are uncoated stainless. The lifeline is tied to the upper shrounds for support there instead of a stanchion, something that is typical I am told for boats of my configuration. The shroud is attached to the bronze chainplate which is in the wet wood. There is the battery between the zinc on my grounded prop and the bronze chainplates and thus the current. Ions go through the water and electrons go through the rigging and lifelines. The moisture in the twine where the lifeline was tied to the rigging likely helped to make this worse and also hid the damage. I disconnected the radio antenna and the current stopped. Obviously I replaced the rigging and I have insulated the antenna and the pulpit as well as making sure there is no electrical connection between the rigging and the lifeline.

I feel very lucky. I also should have knows better. Is this another reason to make lifelines out of Amsteel?

My questions is, with so much of the shroud gone, why didn't it fail? There must be a large safety factor there and for that I am grateful.

Allen

[Brion Toss]

Hi there,
Beautiful picture. I'm glad your mast is still up, and that you and your crew are, too. As for why the wire didn't break, let's start with the wire. I magnified the picture, and it appears that the core was far less damaged than the cover; is this so? If it was, then we can count on at least some strength there, up to about a third of the wire strength.
Next, even at 30 degrees heel, you wouldn't have been approaching the ultimate load possible on the wire, so there would be some reserve. And then there is the safety factor, which was bound to help to some extent; how much you had would depend on the load.
Finally, tune might have been a component; if the intermediates were tuned even a little slack, less load would be borne by them than the design would indicate.
Altogether though, and even assuming the most favorable combination of factors, it seems likely that you were an erg or so of righting moment away from a dismasting.
Fair leads,
Brion Toss

[allene]

Originally Posted by Brion Toss

...
Finally, tune might have been a component; if the intermediates were tuned even a little slack, less load would be borne by them than the design would indicate...
Fair leads,
Brion Toss

The lowers were and are relatively lose but this was the upper. Just looking at the wire now, there are 10 strands completely broken and there is a core with the remaining 9. Your estimate of 1/3 the strength is probably good given that the remaining wires are not pristine. In thinking about it a bit more, I sail in SF Bay and I don't get a lot of pounding. I have talked with the tech people at Loos and they say it is the shock loads that break the wire so that was probably helpful. Maybe I wasn't as close as I thought to having the rig come down but I am still thankful that it didn't.

Brion, my friend Steve caught the lecture at the boat show where you demo'ed how to make my soft shackle. You talked to him after. Thank you for the mention. Sorry we could not get up your way on our trip. Next time I will time it for the boat show.

Allen
L-36.com

[allene]

I checked more carefully and there are 12 wires broken and a pretty solid 1/8 inch core. 1/3 of strength estimate looking good but even perhaps a little optimistic. It would be 35% if the core were at full strength, which I doubt.

So roughly, 4400 pound keel at 5 feet at 30 degrees is 2200x5 = 11000 ft pounds. If all of that was from the top of the mast 40 feet up but acting through only 13 degrees is 1,250 pounds. A bit close to the 2000 pound strength of new 1x19 1/8 wire which this was not. But then again, all the load was not on the upper as you point out.