As others have pointed out, what you have re-invented is the UCR.
Originally Posted by GIDDYUP
Sorry to burst you bubble above though. For a 20' tree span and, say about 9' ridge line (structural or virtual), you will not be saving anywhere near what you have computed.
By the time you add up the fact that the UCR needs a much longer bury, has 2 fixed eyes instead of 1, and has 2 free tails instead of 1, the savings on rope for a UCR over the Whoopie is only about 25% (it may be less or more depending on the max desired tree span and hammock ridge line). Then add back in the rope needed for your Prussic and the savings are even less.
TeeDee and I have been around the tree many times with the UCR. By the time you add in the needed "doohickeys" like the Prussic to get the UCR to hold and not slip, your weight savings is usually very small and the amount of rope saved is very small also.
Now quite a few people here prefer the UCR over the whoopie and continue to like them for a few reasons which they have stated a few times. It's all about personal preference.
let's do math!
Hey TiredFeet...a lot of truth in what you say. How about we quantify it.
Let's suppose you want length L of cord between your hammock end and the tree. L varies by taste. Sneak preview of coming attractions...the longer L is, the more efficient is the UCR relative to the WS.
Let B be the length of the bury. Why you think the bury on the UCR has to be significantly longer than on a WS? I missed those posts. Mine are 10" on both types, and so far have done me just fine.
Bury length of an eye-splice...6" for 7/64" Amsteel, by the book, more or less. Call that S. Let's say the length of the eye is E. 'bout 2" in my riggings.
Figure below marks out where these lengths show up in a WS and a UCR. I'm not counting that a bury length is shorter after the rope goes through, for the lengths of interest that doesn't matter much.
Screen shot 2010-10-11 at 9.46.48 PM.jpg
So to have L feet from one end of the WS to the other you need 2L feet of cord. For a UCR you need L + B + 3*(E+S) feet. The fact that the UCR has one more buried eye than the WS (if you choose to use a bury eye...you could use a knot...)
is where the supposed efficiency of the UCR takes a hit. But that's a fixed cost, and the longer L becomes, that fixed cost is amortized, because the WS length is increasing by a factor of 2 while the UCR system length only by a factor of 1.
I didn't figure tails in because I just tie a stopper knot on those, just to impede slipping into the bury.
Let's do a spreadsheet!
Screen shot 2010-10-11 at 9.50.36 PM.jpg
So according to your scenario of 20' between trees and a 9' ridgeline, you're looking at a value of L---if this was the maximum distance between trees that you'd consider---of 5.5'. 11' of WS compared with a little over 8' of UCR. Yawn. I agree.
But you know, I can hang between trees that are 26 to 28 feet apart, 'cause I'm tall enough to push the tree huggers high. And the weight cost of Dynaglide, I don't mind one bit carrying a suspension with that much cord. That will add 3 or 4 feet of length to the suspension scenario, so we're then comparing almost 20 feet of cord for a WS versus a little over 12 for the UCR. That's a difference worth noting, if only because I have lots of uses for that 8' of Dynaglide other than being on a whoopie sling.
There are other differences, but its all personal taste. Frankly, I'm increasingly drawn back to just using a simple length of cord out to a Marlin spike hitch, then back to an SMC ring on the cord to be tied off. Hang your own hang, choose your own suspension.
Looked at your diagram of the whoopie and UCR now.
A few small niggles:
- we did our computations using the bury length before the bury for the constrictor part and then measured the amount buried after the bury to get the length inside the constrictor. The constrictor part gets shorter when the bury is completed. The longer the bury, the greater the shortening. My GF suspects it is a linear relationship and she said that TeeDee suspected the same.
- As we make the UCR, there should be another fixed eye on the end on the right in your diagram for attaching to either the hammock or the marlin spike hitch, whichever. We need 2 fixed eyes, one for the hammock and one for the marlin spike. That increases the UCR length needed. The shortening of the constrictor must be accounted for in computing the final length.
- we simply could not get the UCR to hold consistently with a bury length less than 12" to 14" on amsteel (1/8") or dynaglide. we needed that as a minimum for the UCR to obtain a consistent hold whether using a prussic or tubing. If we used neither, we had to increase the bury to about 18" to get a consistent hold and even then it was extremely sensitive to touching for us, I mean extremely sensitive.
- we follow Samson's splicing recommendations. They recommend 3.5 fids for the whoopie bury which comes to 9.6" for the 1/8" amsteel and 8.4" for 7/64" amsteel and 6" for 2 mm dynaglide. All of our computations and measurements of whoopies and UCRs are based on these dimensions. Any one of those is less than the 12" to 14" we needed for the UCR.
- samson's recommendations for the fixed eye bury is 2.25 fids which computes to 6.2" for 1/8" amsteel, 5.4" for 7/64" and 4" for dynaglide. So for the UCR as we make them, you double those numbers.
- doesn't really make any difference whether you bury a terminator or tie a knot for a terminator, just do the same for both the UCR and the Whoopie to get an apples to apples comparison. 1 terminator for the whoopie and 2 terminators for the UCR.
- we used 2" for the fixed eyes. That is 2" when pulled flat, or 4" of rope in the eye. That comes to 8" in fixed eyes for the UCR as we make them.
Not too sure, but my GF thinks you may have accounted for 2 fixed eyes in the UCR formula, but she isn't sure. She is pretty sure that you need to double E in your formula (it goes out and back). But that's another niggle.
Studied your diagram some more and I think you have the 2 fixed eyes, just not where I first assumed the second one would be. Ignore those niggles if so.
Hey Tired Feet--I think I have all my eyes accounted for.
One on the left goes to the hammock, the one on the right is on the UCR.
Wherever there is an eye and bury the cord is doubled. So on the left
half of the eye+bury is hidden in the term L, then there is E+S. On the
right there are two lengths of E+S, because the cord is doubled there.
That's where the 3*(E+S) term comes from.
I figure on the bury length shortening by at most a couple of inches
on a 12" bury. When we're talking about multiple feet a couple of inches
don't affect the overall comparison much, so I left that out. Believe I said so, but it was buried in a bunch of words.
Don't know what to say about your experience with UCR bury lengths except that mine is different. I do put a pretty good bite down on the prusik that puts tension on the end, perhaps that's the difference. I use Zing-It on that prusik, so it can take a few tens of pounds of force.
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