Rappel Backups (beginner here)

I recently bought the Beal Jammy 50cm prusik cord for rappel backups, but the info sheet it came with said that only the 60cm and 35cm models can be used with ropes >8.6mm. I have 9.8mm ropes, why can't the 50cm Jammy be used with larger ropes? Should I return it and replace it with the 60cm or 35cm? 

No reason the length would affect rope diameter. Can you post a picture of the info sheet? 5.5mmX50cm would be fine, if a little long to be a convenient backup for rapping. But you can always shorten it with an overhand.

Most strange....why would the 50cm cord not be able to be used with large diameter ropes, while the 60 and 35 can be.  With smaller diameter ropes you need to make more than the "normal" 2 wraps with the prussic cord (or use a smaller diameter cord itself), and thus your prussic cord usually needs to be longer, but Beal isn't saying this.  Maybe it's the diameter of the cord itself??

I'd go back to where you bought it and talk to them.

However,don't be surprised if they don't take it back in return (if you decide to try to return it) most "soft" climbing gear is non-returnable, with (perhaps) the exception of a still-in-the-bag-and-still-originally-wrapped rope. 

Thanks for the replies! I also thought it seemed pretty odd that the 2 other lengths are considered ok. Here's the the info sheet, hopefully I'm not just misreading it. 

My *guess* is it may be specifying the correct length for a prusik or autoblock friction hitch on your belay loop. Different diameter ropes (Or if you are on one or two ropes) will mean different amount of "tails" left after tying the hitch.

Personally I like an autoblock on my leg loop, not on my belay loop, and just use a standard 24" tied cord sling.

The length of the tails left does a lot to determine how these hitches function.

I dont know what the people above mean with tail length as the product Its a sewn loop, has no tails. Also, tail length wouldnt affect the "grpping power" of the hitch.

I think the table is indicating that the 50cm loop is not long enough for the number of wraps a French Prusik needs to achieve enough gripping power, in those rope diameters.
For example, the 8.1-8.6mm ropes need 6 wraps of the 5.5mm - 60cm prusik, so I assume it would need 6 wraps of the 5.5mm - 50cm prusik. But the latter is just too short.
Probably still works as a "third hand" after the rappel device, buy possibly does not bite well enough for higher loads.

The tail, or legs are the section, or sections that come from the hitch and are clipped with carabiner, doesn’t matter if it’s eyes or loops.  It’s a balancing act of enough friction to bite reliably, but not too much, which isn’t the easiest thing to do with fixed lengths of cords or slings.

Fran M wrote: I dont know what the people above mean with tail length as the product Its a sewn loop, has no tails. Also, tail length wouldnt affect the "grpping power" of the hitch.

I think the table is indicating that the 50cm loop is not long enough for the number of wraps a French Prusik needs to achieve enough gripping power, in those rope diameters.
For example, the 8.1-8.6mm ropes need 6 wraps of the 5.5mm - 60cm prusik, so I assume it would need 6 wraps of the 5.5mm - 50cm prusik. But the latter is just too short.
Probably still works as a "third hand" after the rappel device, buy possibly does not bite well enough for higher loads.

Instead of "tails" I could have wrote "the resulting loop(s) to clip the biner into". That would have been clearer. A prusik will end up with one loop, an autoblock two.  

They should do a better job explaining the intended use of this product. I googled but couldn't find more accurate descriptions or instructions. I think you're right something to do with the number of wraps needed.

I've always just used 6mm 24" tied (double fishermans) cord slings. Seems to work on all my ropes, but I don't have any super skinny ropes. 

I don't know why the 50 is a no go and the 60 is ok but ...oh wait figured it out see below!

The thing about the "tail" or the extra loop needing to not be too long is because if it's too long the hitch could contact the rappel device which could cause it to slip and defeat the system.  

This is one of the reasons it is recommended to extend your rappel device (which I personally never do, I just do the leg loop thing)

Oh I think the difference is that the 50 and 60 are shown as for the "French prussic" whatever that is.  And the 30 is for a standard prussic.  I'm guessing the French prussic isn't good enough for some stuff?  I mean 5 wraps??

Probably the 50 isn't long enough for so many raps and the 30 is a different hitch so can be shorter only 2 wraps

French prusik = autoblock. It doesn't grip as strongly as a standard prusik so needs more wraps taking up more cord. Many climbers prefer to use an autoblock over a prusik for a rappel backup because it doesn't clamp down so hard and slides more easily. https://www.vdiffclimbing.com/prusik-types/
I would play with it with your rope and see if it works or not. 

"French Prussic" (Google it) seems to be similar (if not identical) to an autoblock.  i.e. there are two "loop ends" that the biner goes into, vs/ a normal prussic with only the one free "loop end",  Thus, I would guess the  "NO's" have something to do with the length being too long or too short.

 

Robert Hall wrote: "French Prussic" (Google it) seems to be similar (if not identical) to an autoblock.  
 

yep, different names for the same hitch.

Jackson Barr wrote: Thanks for the replies! I also thought it seemed pretty odd that the 2 other lengths are considered ok. Here's the the info sheet, hopefully I'm not just misreading it. 

I don’t understand why the frenchie only needs 5 wraps to grip the 7.3-8.1 but needs 6 wraps to grip the 8.1-8.6     Then 5 again for the fatter rope. Space needed between third hand and device doesn’t answer it because that’s a function of extension length. I am assuming the different lengths are the same diameter cord. 

It’s an Autobloc if the tails, or resulting loops don’t cross each other in the back when loaded, if they do it’s a French Prusik, or the Valdotain, more correctly.

Brocky wrote: It’s an Autobloc if the tails, or resulting loops don’t cross each other in the back when loaded, if they do it’s a French Prusik, or the Valdotain, more correctly.

Typically the auto block is tied using a cord which is sewn or tied into a loop configuration. I believe the valdotain tresse has to be tied using a cord which has an eyelet on each end. The Beal jammy is a loop so I assume Beal’s version of the French prusik must be more like an auto block than the valdotain. But the number of wraps they are recommending is still unclear. 

Greg R wrote:

I don’t understand why the frenchie only needs 5 wraps to grip the 7.3-8.1 but needs 6 wraps to grip the 8.1-8.6     Then 5 again for the fatter rope. Space needed between third hand and device doesn’t answer it because that’s a function of extension length. I am assuming the different lengths are the same diameter cord. 

I think it's telling the user what is possible with the cord, not what's necessary. It makes sense that you can wrap the 60cm loop around an 8.1-8.6mm line 6 times, but can only get 5 wraps around a larger diameter line.

FWIW I have the 50cm version and i use it as an autoblock on my 9.9mm rope all the time. I thought that i was gonna die, but alas...

The Valdotain Tresse does have to be tied with a cord with end eyes, with a loop it’s the Machard, but it is a different hitch with added braids, or crossed round turns.  The other two hitches aren’t necessarily dependent on what is used to tie them.

No a French Prusik is like a regular prusik but you also use your tongue.