Hi, 

Who has an idea of the working load of the main halyard shackle?
What is the expected rating? 

Cheers.

 Is this for a 2:1 halyard?
 
If it is a 1:1 halyard we just loop the halyard through and put the end through it.

2:1.
Cheers.

Standard size D shackle is fine for the bit that goes through the sail.
 
A wide d shackle with a 5mm pin is what I used to use on the top of the mast.
 
Neither are anything fancy, and probably overspecified - you might have been looking to reduce the size/weight? I'd stick with what works :)

Hey Chris

Are you currently using a halyard lock or a 2:1?

Which of the two do you prefer?

I noticed a definite difference in he 2:1 over the straight through on a chipstow mast. I later fitted a halyard lock and did not notice a difference in performance, though less rope in the boat was a bonus. Whilst I only had trouble with a jamming lock once or twice in about 5 years I think that the proneness to being temperamental leads me to say that I'd stick with the 2:1 and live with the extra long tail.
 
Its fairly academic at the moment as nobody makes a lock that fits a skinny mast! 

True no lock options for the new rigs