The mainsheet halyard has a 25cm wire leader which has a loop on both ends each held by a brass swage. The swage on the pulled end is designed to catch in the V-cleat on the mast so that the strain of the sail is taken at the top of the mast and not transferred directly to the cleat at the bottom. This has a number of issues:
The swage can come undone where it is clamped onto the wire causing the sail to fall down – especially in strong winds or if you pull the Cunningham on hard.
The swage can catch on the side of the V-cleat so that you think it is fixed in position but it then comes loose as soon as you start sailing
The wire leader introduces weight at the top of the mast where you want it least since it adds to the pendulum effect
The solution is to replace the wire leader with a Dyneema leader which is attached to the mainsail shackle at one end and then spliced to a regular rope which leads down to the cleat at the foot of the mast.
A knot in the Dyneema is used to catch in the V-cleat.
Dyneema is lighter and stronger than wire and also cannot come undone
The Dyneema leader can easily be replaced if it becomes worn.
The rest of the halyard can be made from any rope that can be spliced to Dyneema but Dyneema core is recommended.
Peter Martin’s spliced system above uses a 1.75mm Dyneema leader spliced to a 4mm halyard line with a Dyneema core.
Instructions for making the splice on the Animated Knots website here.