Prusik for thin double ropes?

I've started using a pair of thin double ropes (7.8 and 7.9mm) and my current cord is too stiff to fully 'grab' the rope without me pinching it/pushing it upward to initially engage it. Obviously this is no good for the purposes of a 3rd hand when rappelling, which is even more crucial with such thin ropes that are already super low-friction in a non-alpine ATC.

What are y'all using for your prusik cord when dealing with uber-thin ropes? Should standard 5-6mm cord do the trick?

I’ve been using a Beal jammy, which I think works great. A little more expensive than just cord but I feel it’s worth it, the sheath is pretty grabby so it’s been great on smaller diameter rope. 

Sterling hollow locks are fantastic. Super grippy even on skinny cords!

Try add more friction to the whole rap setup so as not to overly relying on the third hand. Lots of options like tighter device, addition biner, different friction hitch...

The Edelrid Aramid Cord loop is good once you massage it to make it more flexible. 

Edit: If you can splice (brummel lock), then a 4mm hollow braid kevlar loop is ideal. 

I just use 4mm, always have done. 

Three wraps on a Sterling Hollowblock (just wrapped, not as a prussic or climbheist) works great. For reference, I normally do 2 wraps when rapping on a 9.X-10.X single rope rappel (double strands).

Also, as Noel noted above, adding friction to the rap set up really helps. Consider ATC Alpine guide, Petzl Reverso4, and/or doubling up the rappel biners.

4mm cord probably works fine too, but for me the 14kn strength of the hollow block makes it a bit more versatile and they are quite durable. 

The difference in diameter between rope and prussik cord changes how well the prussik grabs. Use a smaller diameter prussik for more grab. Use a klemheist instead of autoblock hitch for more grab. I prefer longer prussik loops for more options.

The short sterling hollow block isn’t good in my opinion. It’s only long enough for three raps where four raps is much too tight but three doesn’t grab well. A longer cord allows more wraps without being too tight.

I also prefer a regular accessory cord (cheapest, available anywhere) over any of the pre-sewn options.

Sterling Hollow Blocks don’t like sand. I broke one with my bare hands once after a season or two of use. 14KN my ass. And versatile for what exactly?? You never trust your life hanging by a friction hitch without a true backup so what exactly is that “strength” useful for?

Sounds like the hollowblock or beal pre-sewn loops make the most sense. $15 isn't going to break the bank anyway.

EDIT: maybe not, haha, just saw the above comment

The 'adding friction' suggestion is a good one - I use a fat round-stock locker for belaying my 2nd (trying to minimize friction since guide mode can be an elbow-eater) and small lockers for everything else, so I should just swap the big one and one/two small ones when rapping. Won't fix the prusik problem but makes rapping less dire without a prusik. 

The key to any good prussik cord, pre-sewn or not is that it is soft and supple. Stiff cord will not do the job. 

I love hollow blocks, BUT do not commit solely to them. It was news to me when I was taking my SPI course when I heard that they shouldn’t be used as a primary connection point. Once I re-read the manufacturer paperwork it says right in there that they shouldn’t be used without a backup.

I would recommend some 5mm cord. Also a alpine draw with a Bachman hitch works well on half ropes in a pinch. 

(as many people) I ve been using the Beal Jammy for years with my Beal Gully (or any other ropes really), no issue winter or summer. It's "bitting" super good enough.

Alex Fletcher wrote:

(...) Sterling Hollow Blocks don’t like sand. I broke one with my bare hands once after a season or two of use. 14KN my ass. And versatile for what exactly?? You never trust your life hanging by a friction hitch without a true backup so what exactly is that “strength” useful for?

We still sell them, but I personally have moved to the BlueWater 7mm VT Prusik exactly for this reason. If you search there are a few failures of Hollow Blocks reported in various forums. I try to always point out the flaw that there is no core with the Hollow Blocks, so you really don't have any kind of strength reserve and you can't estimate how strong it is after use.

Saddle hunters have been using Sterling 6mm TRC which has a nylon core for friction hitch loops and eye-to-eye hitches for single strand 8mm rope. Perhaps this would work well for thinner ropes. I have been wanting to try it out. There are a few sites you can order sewn loops of it from, though Sterling doesn't have it listed on their site.

I've seen the "too stiff to fully 'grab' the rope without me pinching it/pushing it upward to initially engage it" problem, and I don't believe adding friction to the device does a darn thing for it (it does solve other problems - I'm not disputing that)

What matters here is the cord's flexibility. Smaller diameter helps, but https://www.mountainproject.com/forum/topic/110791723/whats-a-good-rope-to-use-to-make-prusiks recommends Sterling and New England.