On Tue, 14 Mar 2006 12:02:58 -0800, "Capt. JG"
wrote:

I believe the shackle was
defective, so I'm not sure if that was the reason or if it would have
happened anyway.

Probably not defective strictly speaking, just a spring too weak for
the flogging forces generated. My issue was on spinnaker halyards if
memory is correct. When the wind was blowing hard the spinnaker would
get about halfway up, and poof, the shackle would magically open. I
originally assumed that the shackle had not been properly closed or it
was some kind of weird one-time event but after several more fiascos
it became clear that the shackle was actually opening under its own
volition. The cure was a stronger spring which fortunately my rigger
had in stock.