/ Abseiling on two different diameter ropes (9mm dry - 9.8mm not)
The ropes in question are 9mm and 9.8mm, it is for rapping off mainly bolts (and some in-situ). What would you recommend?
Does it matter that one is dry treated and the other isn't?
On the black diamond website it says overhand knot is fine for unequal diameter? This would obviously be a lot easier but on UKC it says to use the double fishermans
I use overhand on different diameters and seems alright.
What's the worst that could happen?
I know it, I use it normally, unless too much doom arrives, I will be using it from this weekend on enequal ropes ;-)
FWIW I've failed to be able to roll an overhand knot made with a single rope and 6mm accessory cord so doubt you'd have any trouble with 9 and 9.8.
0.8mm is hardly a big difference. Usually advice about unequal diameters refers to say 11mm and 8mm. There's no absolute formula, also a lot of the old advice referred to hawser laid rather than kernmantle ropes.
In your case, overhand (with long tails) is fine. If you are really unsure/paranoid then double fishermans.
Why? All the joining knots mentioned so far are fine, what do you gain by overcomplicating matters?
The double fisherman's knot is the safest option, some people prefer the overhand knot as they say it snags less easily, but as they always insist on leaving long tails as the knot slips a bit it has never inspired me with confidence :-)
The choice is yours, I know what mine is.
Thanks very muchly
Overhands don't roll. If they did, no-one would use them
With same diameter ropes, I usually tie just a flat overhand like everyone else does with 30cm tails.
With different diameter ropes, I sometime use the "Doppelter T-Spierenstich" (Double T-Fischerman's Knot) shown here http://www.gudelius.de/spst.htm (you have to scroll down a bit)
Basically it's a double fisherman's knot but is not tied "inline". It's tied as a "flat knot". It's more complicated than the overhand knot but non more complicated than the regular double fisherman's. It's not much more bulky than two overhand knots. I've found it to be relatively easy to untie after loading. I've never seen or heard of it capsizing.
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