What string is best for the pole downhaul on a modern twin pole system?
Would 2.5mm dyneema D12  be OK?
Is most of the load taken by the guy or the downhaul?

In terms of strenght, 2.5mm dyneema will be plenty, many classes actually elasticate the downhaul, its purpose is to try and persuade the pole from sky-ing, and little else, loads are generally less than 500N, which fits well within the 5000N typical breaking strain of 2.5mm dyneema The driving load is taken by the guy.

Personally, although 2.5mm is strong enough ... I would opt for at 4mm simply because it handles better ... I suspect the 2.5mm will be twisted and snagged around pole ends and the like more often than you would hope ... the 4mm will tend to kink less and behave itself. I use 2.5mm for the uphaul, mainly because in  a twin pole system, it is always under load from the weight of the pole, so tends to stay straight and keep itself out of mischief :)

I'm not sure what the current string is, but it is tied to the plastic ring through whichg the guy runs. It came undone today, so I thought I would splice it on, which would mean the thickness of the splice running through the hole in the bottom of the carbon pole. Hence I think it needs to be thinner to run nicely.
A spliced loop would also have the advantage of getting the lengths the same both sides.

On the other end of the pole, I've just pulled apart a terrible mess resulting from mixing aluminium with carbon fibre. A corrosion product was growing on the aluminium, crushing the carbon tube so that the pole end plunger did not want to move!

Next project will be to change some control lines, too many red strings!

Still we got out on the water and managed some gusty conditions without mishap, that's progress!
Cheers, Chris Iles 3442.

I'd go for 3 or 4mm as it is easier to splice. However i do prefer to tie them on myself! I think the ones on my current boat are 2.5mm and while wooley and in need of replacement they have lasted thne winter

Pole downhauls (snodgers) are never elastic on a modern Merlin!!!

no, I know, I was just using the fact that many classes elasticate them to illusrate that the forces are typically low.

Loads are not particularly low, or rather they are on a run but not on a reach, a tight reach is where you need low stretch in the whole system and a good cleat or the ring is not held to the end of the pole, which then bends on the forestay, not good.

aye, thats true you are right there.    

Point taken on the low stretch thing, I'd forgotten how important that is on a dual-pole flyaway system.