Blake's hitch and a pulley for hauling uphill

[peds]

Evening all,

 

It's raining, otherwise I'd go outside and have a play with this, but I was wondering if anyone with a history of Blake's hitches could help me with an experiment.

 

I'm a volunteer with my local mountain rescue team, we've got one popular location that requires hauling gear up, first, a 300m 45 degree grassy slope, then a rocky traverse, then another 100m haul over 45-55 degree grass, gravel, and rock. For the first 300m stretch we usually have an anchor at the top of the slope with a pulley and a prusik, or a progress capture pulley if we've brought it along, or a Petzl ID if there are people attached to the stretcher (not ideal because of the additional friction), and two or three team members attached to the other end of the rope, running downhill as a counterweight, which makes hauling the stretcher and all the gear on it uphill a hell of a lot easier. For the second stretch, we do a prusik and pulley, and a 3-1 haul system to make life easier, as there isn't the space to have a counterweight running downhill, and the amount of loose rock makes it unsafe. This is all for the uphill, as soon as we need to send the stretcher downhill we switch to a Petzl ID.

Now, there's been discussion about whether or not it is within acceptable levels of redundancy to have the whole thing secured by a single prusik for the uphill journey. Personally, I'm all for a simple prusik and pulley as a progress capture device instead of some heavy bit of metal, especially when you've got to drag it, plus several hundred metres of 11mm rope and other assorted kit, up close to 500m of mountain before doing anything. I have nothing whatsoever against the humble prusic, but some of the more belts-and-braces members of the team argue (quite correctly, of course) that the prusik loop has a lower breaking strength than the rope it is holding, and needs to be either backed up with something else, or replaced by an unwieldy and expensive bit of metal.

 

So here's a thought that came to me suddenly this afternoon: could you imagine this system, a 3-1 haul on one side, and a pulley with a Blake's hitch as the progress capture, instead of the prusik loop, on the other, the Blake's tied with either the spare tail of the haul rope, or the loose end of a 50m used to build the anchor, same diameter as the haul rope. Can a Blake's hitch in this scenario provide enough friction to safeguard the uphill progress? At what force does it start slipping? Can you add additional wraps to help it out, and in what configuration... 4-2, 5-2, 5-3... 6-3?!

I've not tied a Blake's hitch since my CS38/39 assessment, but it's something that sprung to mind during discussion on this topic, and I was wondering if it would be worth getting more practice with. I'm just looking for a quick, simple, safe, and lightweight alternative to 1.1kg of Petzl Maestro.

 

Any thoughts gratefully received.

 

Pic related.

 

[Dan Maynard]

Tie the prussic with 10mm friction cord? It's doubled so load shared in two legs of cord - I don't think the prussic snapping is a risk then.

 

I think the risk with a prussic on its own is if it starts to melt, then the friction reduces and you're in a runaway.

 

As an idea if the rope also went round a figure 8 then it would limit runaway.

[peds]

22 minutes ago, Dan Maynard said:

As an idea if the rope also went round a figure 8 then it would limit runaway.

I shudder at the thought of getting the kinks out of 100m of rope fed through a figure 8. It'd be a birds nest before we are even halfway. A secondary device of some kind would of course be ideal, but it can't interfere with the 3-1 in any way, and if it adds any significant friction we might as well just stick with the ID instead of the pulley and prusik, and suck up the additional work it creates.

 

With regards to melting, there'd be a real risk of that if it were being used to lower, with the weight of stretcher, casualty, and barrow boy easily being upwards of 150kg, but on the upwards journey the prusik is only holding a fraction of that weight, and only when static, as the prusik is obviously relaxed during each upwards pull on the 3-1 haul. This is why we switch to the Petzl ID for the descent.
I suppose there'd be a bit of a risk of glazing if the 3-1 team were really efficient, but so far, there's enough downtime between resets on the haul to dissipate the heat a little, I think. The prusiks are, regardless, inspected before and after each use, and treated as disposable if there's any obvious wear and tear.

 

We currently use Sterling 8mm orange friction cord for this application. I'll bring up the idea of using even thicker string and see if that satisfies the worriers.

 

...but I'd still like to know if the Blake's hitch might be an appropriate solution, too. Seems like a nifty retro application for it.

[AHPP]

Rope wrench(es)? Those ISC pulleys with one way bearings if they need further convincing.

Edited June 24, 2022 by AHPP

[Dan Maynard]

Ah I see what you mean. I don't see why not a Blake's, that would be the preferred knot for a split tail I reckon.

https://honeybros.com/shop/climbing/prusiks-split-tails/split-tails/marlow-gecko-split-tail/

 

I never bought one of these, used to have 2m of rope attached to a biner with double fisherman's. I prefer the Blake's to prussic, smoother and easier to grip, not used it since switching to hitchclimber though.

[Dan Maynard]

I think I'd stick with prussic though, it's simple to tie and simple to check. Under stress, dark, raining etc you could get the Blake's wrong.

 

I don't see the MBS of the prussic being a problem at all, you're not in a vertical fall situation so there is no risk of shock load as you reach the end of the slack as with climbing.

[slack ma girdle]

Thin cord prussic always has and always will be an absolute git to release under load. Blakes hitch can always be released under load.

Why don't you set the system up and test it to destruction to see what breaks first ,  i.e. with a tractor in a field.

Edited June 25, 2022 by slack ma girdle

[Craig Johnson]

i would put a figure 8 in with the blakes as it only has quite a small contact area and if used in anger on long runs that your talking of could easily weld on

[Dan Maynard]

9 hours ago, slack ma girdle said:

Thin cord prussic always has and always will be an absolute git to release under load. Blakes hitch can always be released under load.

Why don't you set the system up and test it to destruction to see what breaks first ,  i.e. with a tractor in a field.

There are some prussic video tests that I found earlier, the prussic was slipping rather than breaking in the one I saw but think 11kn. Plenty to stop you rolling down a slope.

[peds]

6 hours ago, Craig Johnson said:

i would put a figure 8 in with the blakes as it only has quite a small contact area and if used in anger on long runs that your talking of could easily weld on

A figure 8 would put way too much friction in the system and make hauling an absolute pig, kink the rope too much, and provide no benefit over our standard belay device, the Petzl ID. Failing other options, we'd just stick with the ID.