Dyneema fail at masthead sheave!
 

We are scrutinizing our reacher halyard after it parted at the mast sheave on our last passage.
Our furling reacher hoists on a 2:1 purchase to an Antal 90mm tulip sheave with a maximum recommended line diameter of 14mm. The functional diameter of the sheave is 66mm.
Does the recommended max line diameter indeed include cover and core? Our broken halyard actually measures 15mm, but the cover is somewhat loose over a 10mm Dynex Dux core.
Would it be an overall stronger system to use a 12mm overall halyard so the strain caused by bend around the sheave is reduced? Other possible combinations include using the core only or core with chafe cover only.
How much is strength compromised when the minimum recommended bending radius is disregarded?
If we extended the bury of our eyesplice to continue past the mast sheave, will the increased diameter and stiffness reduce our functional strength or possibly increase strength?
Thanks in advance
Martin

The Max Diameter does indeed include cover and core--the final covered and chafe-protected diameter should not exceed the max sheave diameter--it will do violence to your line to stuff it into a space too small, especially if you have power winches and can't feel how much force it's taking to do so.
I recommend examining the sheave box for possible chafe spots and making sure you have a fair lead before you put a new halyard through there. If using a covered DUX (or Marlow MAX or whatever) halyard, and you want Dyneema chafe sleeve but it's making the line too fat, you can strip the poly/technora cover off of the portion to be dyneema sleeved before putting the dyneema on, and your line's diameter will remain constant.
If you extend the bury of your splice so it goes through all the sheaves, this will not make anything stronger--you'll just be putting a fatter line through what will then be a smaller bend radius for that fatness of line.
I suggest finding out from an engineer what strength of line you need, size your halyard accordingly, and upgrade to a bigger sheave if necessary.
Ben

Variables
 

Hi,
Dyneema eyes can get down to a 1:1 ratio of rope to pin before break strength drops -- in the lab. But a loose cover, a tight fit in the sheave, and what is likely some component of strong lateral load all are variables here, and that is without taking into account relative load, grit, splice efficiency, or sharp spots.
The break strength on that core should be around 13 tons, and you also have a 2:1 purchase, so the load on the rope should be utterly trivial here; sounds like you could specify a much, much smaller rope.
Fair leads,
Brion Toss

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