Slipping Main Halyard

Hauling the sail up with the halyard running thru the cleat will quickly wear the cleat jaws. I make sure the halyard is running beside the cleat until the sail is up, then flip the rope into the jaws and finish pulling up the last inch or two. I have two clam cleats and a vectran halyard. If it's seriously windy I haul down on the cunno after hoisting the main, then let it off & re-tension the halyard. Stops the sail slipping down under cunno, but makes it a right nightmare dropping the main after sailing. I just hate those racks - I find them too fiddly. Next mast will probably have a halyard lock.

Of course the best way to get D12 to lock in a V type cleat is to give it a core, easily done by sliding in a length of the same rope in the area where it cleats, 2 min job, but essential to get a hollow core rope to cleat fast.

Nick, if you are thickening D12 with a core then you need to do it for the region in the cleat + an inch or two on either side. This is standard practice on spinnaker halyards but I have to say I dont know many people using D12 for main halyards.
I have found that boats which rely on standing rigging to provide mast pre-bend are suitable for hook racks and those which rely on main halyard tension for pre-bend need a cleat such that additional purchases can be added to the halyard once nearly up to assist with increased tension to induce bend. The RS600 being a case in point.
One solution sometimes used is to use two cleats or pack out the top end of the cleat to get a better bite angle.

If you thicken the halyard with an insert please remember to taper the insert by cutting away strands gradually

This makes the insert roll over you sheave block at the top easier and prevents creating a hard point where the rope thickens that massively reduces the strength of the rope.

I would have thought a Jav would have a wire halyard....I am sure mine did. Why not have an external halyard and the cleat at the top? Tasar has a wire with a 'pip' on it and a metal 'V' cleat 3" from the sheave at the top.There is only 6' of wire to stretch. Byte has a similar solution but with string and a cleat. The only problem is that you need to change your glasses to see up there and on the Byte which runs with loaded cunningham it tends to jam.