for a luff like that your would need a 40cm extention, or there abouts, i would say u have no where neer enouph

should be OK if it's a nice neat fit in the mast. The sloppier the fit the more likely to split the mast.
Saying that I'm not sure how happy I'd feel using one of those cranks to downhaul a stiff sail.

Disagree... a little. If your extension at its longest setting has 13cm "up the mast" then I think that is fine if it is a good fit.

As for needing 34cm extension according to the sail manufacturer and having an extension that goes up to 34cm according to the extension manufacturer.... prolly not good enough. I have always found that you need about 3-6cm more extension due to the fact that you cannot pull the tack pulley (the downhaul bit on the sail) all the way to touching the extension pulleys.... and also the fact that their measurements can be a bit crook.

Some extensions are better than others in that regard: my cheapo needed about 6cm more and my Chinook is much better, maybe about 3cm more.

Also remember that too long an extension is bad as on your smallest sails you may have 35-40cm of ally pipe up the mast which will change it's bend curve. I now have a short one I use for 'no extension' and my long one is used for 10cm - 40cm. Partly cos I've got too many sails

better buy another extension and have piece of mind, than not and having to buy an new mast!
And now if it really goes ballistic you can over downhaul it

Skinnies are good: standard waterpipe and BSP fittings work .... bit heavy though

If weight is a consideration go Carbon, what type of base are you using? Euro pin or double pin cup? If you use something like the fiberspar or the Chinook then you should be able to get the pulley to within 1cm of the base provided you thread it right. I have seen many sailors with the downhaul all twisted at the bottom because it is not threaded right.
Does your sail have the tack pulleys, paralel with the foot of the sail or at right angles? this makes a big difference to how you thread the pulleys!!

David

NotWal - No, it's actually a 34cm extension

http://www.neilpryde.com/matrix/technology/2008-mxt-extension-base-system.html

The X-Combat MXT34. As can be seen in the RDM extensions right at the bottom of the page, with it set at 34 cm there is very little extension remaining. I keep thinking they wouldn't make it as such if it wasn't safe to use at that setting. Still seems to be too little IMO.

Gybesports - Yeah, a tack pulley, parallel with the foot. I'm pretty particular with my rigging and the lines are nicely threaded as per the instructions from the sail manufacturer and not twisted at all. My base is actually the newer MXT mast base so I guess my choice of extensions is limited short of buying a new base. I'm not concerned with the weight of the extension so much but it seems most over 40cm in the skinny range are made of carbon anyway, I assume for strength.