I've just been doing a bit of 'testing' at home on a new length of Spanset Gold. Tying the double version of the knot around a ring does take a fair bit of dressing but it is very neat once its been set. Tying it underground on a more likely stiffer length of rope would probably result in the knot not being as neat as the one they tested to destruction on the video so I would be wary of those breaking figures until I was sure sufficient testing had been done on a variety of ropes, e.g. thick, thin, soft, stiff, worn and wet and with varying degrees of dressing and setting.
I've seen what a lot of people call well dressed Fig 8 on the Bight knots, the problem is, the difference between a badly dressed Fig 8 and a badly dressed Jonny knot could be the difference between life and death.
Having said that the breaking strength the knot achieved on the video isn't that much different to most knots. A round turn and two half hitches would probably achieve that.
One of the important things to test, particularly if its being used on the end of a cow's tail, is the dynamic capabilities of the knot. You never know it might be really good but I doubt very much, based on the shape of the knot, it would be anywhere near as good as a double twisted scaffold knot, which also holds the carabiner in the correct orientation. I wouldn't be surprised if a round tun and two half hitches was a better energy absorber than a Jonny knot.
I've also been playing with the cow's tail version on the same new length of low stretch Spanset Gold. If you don't make a very good job of dressing and setting the end that you pass back through the carabiner and poke back through the knot you can get it to undo itself with not much more than a shake and a wiggle.
I think I would recommend having a bit more of a tail than shown on the video and probably stick a half hitch or two around the live part of the rope with the tail, just to be sure. Trouble is you are now having to tie two knots to be safe, a bit like a bowline. I'd rather tie one knot that I know is good and has been properly tried and tested over a long period and taking account of foreseeable misuse, i.e. poorly dressed and badly set.
There does seem to be a thing on a few of these forum threads about saving on weight by using knots that use less rope and lightweight carabiners but I bet most of those people will have a full English before they go caving. Are a few crabs and M/R's really that heavy?
When tied around a ring, e.g. at a rebelay, the knot uses about 2" more rope than a Fig 9 on the Bight and nearly 6" more than a Fig 8 on the Bight with the same loop and tail sizes.
The cow's tail version of the knot IS likely to affect the safe functioning of some of the smaller more lightweight carabiners as discussed on other similar topics and mentioned earlier on here.
I think Kenilworth sums it up nicely.
Mark
