Thank You for your replies and affirming my suspicions regarding diameter.
I have removed the sheave box and inspected the assembly. Chafe is not a factor.
The NA's design load for the reacher tack is 8600 lbs, so by way of the 2:1 purchase, the tackline shouldn't see much more than 4300 lbs, however the tack is tensioned by a hydraulic ram that can generate up to 20,000 lbs!
The reacher halyard that parted looks like it was stretched thin and broke like taffy.
Given your responses, I hope to proceed with a line that fits within the sheave specs and perhaps a pressure relief valve on the hydraulics.
Regards,
Martin
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