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Old 17-04-2011, 09:03   #1
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Knot Question - Spectra Life Lines

I am putting some new spectra life lines on. I know 'the proper' way to terminate them is with splices w/sailmaker thimbles on both ends and then lashings to the pulpits. BUT if I do that I can't ever remove them as the splices will not pull out thru the stanchion holes. The reason I want to be able to pull the life lines is if I shrink wrap the boat next winter I don't want them exposed to the guys waving propane torches around doing the shrinking.

So I am thinking of putting the splice/thimble/lashing on one end, and on the other end taking say three round turns around the pushpit tube (large 1.25" dia) and then tying it back to itself with one of the constrictor type knots - something like a buntline or uni hitch. My thinking is that the turns around the tube will take a lot of the load and the constrictor will not put a sharp bend in the loaded part of the line - so this will not weaken the spectra as much as say a bowline does.

Does that make sense or am I fooling myself? Anyone ever seen a pulled test of this sort (several round turns plus a constrictor) of termination?
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Old 17-04-2011, 09:19   #2
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Re: knot question - spectra life lines

I haven't seen any tests but was wondering about finishing off the round turns with a clove hitch. It the hitch is tied in the correct position on the tube the bitter end of the line would just continue forward into the constrictor hitches without having to make any 90^ bends and would serve as extra security should the constrictor hitches become undone. In reviewing the buntline hitch it is a clove hitch performed on the working end of the rope.
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Old 17-04-2011, 09:49   #3
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Re: knot question - spectra life lines

Your plan is not a good one IMHO, if I'm understanding correctly, because you'll be putting too much perpendicular force on the "anchor" stanchion you'll be using for a tie-off. Also some knots don't hold as well with slippery lines like Spectra or Dyneema.

If you do it "right" the splices are easy if you use single-braid 12-strand, and they can be removed and re-done seasonally. There are two variations of splices -- a modified brummel splice or a simple tuck finished with a lock stitch. The latter is easier to take out and re-do.... but instead of the lock stitch, make your tuck longer than "standard" and use chafe tape to hold the splice (instead of stitch). A good quality chafe tape will not gum up the line excessively after one sailing season, and you will replace it each season.
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Old 17-04-2011, 10:00   #4
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Re: knot question - spectra life lines

How about a short length of wire rope at one end with a turnbuckle. Then you could use a buntline hitch to tie the Spectra to an eye at the end of the wire rope, and tension with the turnbuckle. Should all disassemble easily.
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Old 17-04-2011, 10:09   #5
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Re: Knot Question - Spectra Life Lines

Quote:
Originally Posted by estarzinger View Post
So I am thinking of putting the splice/thimble/lashing on one end, and on the other end taking say three round turns around the pushpit tube (large 1.25" dia) and then tying it back to itself with one of the constrictor type knots - something like a buntline or uni hitch. My thinking is that the turns around the tube will take a lot of the load and the constrictor will not put a sharp bend in the loaded part of the line - so this will not weaken the spectra as much as say a bowline does.
Evans,
I think if you put enough turns around the pushpit tube it's got to be secure (same principle as the winch). I might take, say, 6-10 turns instead of three, since the tubing is smooth and there's not much friction.
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Old 17-04-2011, 11:07   #6
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Re: Knot Question - Spectra Life Lines

Rather than a clove hitch use a rolling hitch and then take the bitter end and finish it off with two-half hitches. Clove hitches over a slippery surface such as a stanchion require constant tension to remain secure.
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Old 17-04-2011, 12:30   #7
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Re: Knot Question - Spectra Life Lines

My main concern/question is that a simple knot (like a bowline) reduces the dyneema strength by 50%. I an trying to come up with a tied solution that does not have the big a reduction - by first using the round turns to carry most of the load and second using a knot that does not turn the loaded part of the line.

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I haven't seen any tests but was wondering about finishing off the round turns with a clove hitch. . . In reviewing the buntline hitch it is a clove hitch . . .
Yes, my second question is going to be what's the best knot - one that will not jam and that I can untie in a year or two and being under pretty decent continuous loading? But I wanted to get question #1 answered first - do we think I can do this in a way that does not cut the line strength by 50%.

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Originally Posted by SailFastTri View Post
you'll be putting too much perpendicular force on the "anchor" stanchion you'll be using for a tie-off. Also some knots don't hold as well with slippery lines like Spectra or Dyneema.

I am not too worried about either. I have quite a bit of experience with knots and splices in dyneema single braid and whipping the tail to the standing part does in fact prevent it from slipping. The whipping is easy to cut off if/when I want to remove it. And I am not worried about the load on the pushpit tubing. Its a very strong construction, and its has taken the wire lifeline load just fine for 12 years.

If you do it "right" the splices are easy if you use single-braid 12-strand, and they can be removed and re-done seasonally. There are two variations of splices -- a modified brummel splice or a simple tuck finished with a lock stitch. The latter is easier to take out and re-do.... but instead of the lock stitch, make your tuck longer than "standard" and use chafe tape to hold the splice (instead of stitch). A good quality chafe tape will not gum up the line excessively after one sailing season, and you will replace it each season.
I am not sure what you are suggesting with the tape - where exactly in the splice are you suggesting it go - do you mean around the outside of the 'throat' (where the tail enters the standing part) of the splice? I am having trouble envisioning a splice that I will both trust in use and that will also 'be easy to undo'!

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Originally Posted by steve_hendry View Post
How about a short length of wire rope at one end with a turnbuckle. Then you could use a buntline hitch to tie the Spectra to an eye at the end of the wire rope, and tension with the turnbuckle. Should all disassemble easily.
I am not worried about getting tension. The lashing on the front end with the spliced thimbles will go that just fine. I am worried about strength reduction and if I understand your suggestion correctly, a buntline hitch to a wire rope eye seems worse (smaller radius) to me that tying directly to teh 1.25" tubing.

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Originally Posted by Ziggy View Post
Evans,
I think if you put enough turns around the pushpit tube it's got to be secure (same principle as the winch). I might take, say, 6-10 turns instead of three, since the tubing is smooth and there's not much friction.
Good thought. I can take 10 turns if it will help - I have plenty of space on the tubing and plenty of line. I could also make the surface less slippery in a variety of ways - either wrap it with rubber tape or glue on sandpaper or putting contact cement on the tube and letting it dry. Do you think any of that would help?

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Rather than a clove hitch use a rolling hitch and then take the bitter end and finish it off with two-half hitches. Clove hitches over a slippery surface such as a stanchion require constant tension to remain secure.
OK, we are back to the question of which knot(s) Of the 'rolling hitch' style knots my personal favorite has been the icicle hitch. We use that when adding a bridle line to our drogue rodes and its always worked well. But I as leaning toward the uni knot because it seems like it might be easiest to untie a year from now - although I don't have a lot of experience with it's behavior after being highly loaded.
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Old 17-04-2011, 12:51   #8
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Re: Knot Question - Spectra Life Lines

can you find a quick and easy way to just cover the lifelines before the shrink wrappers have their way with them? Some cheap PVC, split and opened up to go over the spectra? Garden hose?

It seems to me the problem is the shrink wrap process, not the way the spectra lifelines are terminated.
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Old 17-04-2011, 13:01   #9
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Re: Knot Question - Spectra Life Lines

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Originally Posted by estarzinger View Post
I can take 10 turns if it will help - I have plenty of space on the tubing and plenty of line. I could also make the surface less slippery in a variety of ways - either wrap it with rubber tape or glue on sandpaper or putting contact cement on the tube and letting it dry. Do you think any of that would help?
It might, but whatever you choose as the interface needs to be really securely bonded to the rail tubing. I think the best way would be to find a way to make a cross hatch pattern on the tubing without weakening it. if the walls are thick enough you might be able to just use a file.
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Old 17-04-2011, 14:18   #10
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Re: Knot Question - Spectra Life Lines

I like Doug's idea. Maybe even leave it to the shrinkwrap guys and put preservation of the lifelines in the shrinkwrap contract.

Brion Toss is an advocate of fiber lifelines (and standing rigging for that matter) and might have some thoughts. He has a forum on his own website at SparTalk - SparTalk
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Old 17-04-2011, 14:48   #11
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Re: Knot Question - Spectra Life Lines

I think we are over engineering this. Climbers have been tying off Spectra runners for years with triple fishermans knots (manufacturers recommendation) and then taking huge falls. Never a slip, very simple. Remember, it is a triple knot, but no backup is required.

The knot is very nearly the strength of a splice (85% at least). If you have a failure, it will be chafe at a pass-through.
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Old 17-04-2011, 15:40   #12
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Re: Knot Question - Spectra Life Lines

How about a "luggage tag" splice?
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Old 17-04-2011, 15:56   #13
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Re: Knot Question - Spectra Life Lines

Doesn't the strength of the line have to do with the number of diameters occur for the turn? For example a 1/8" line bending around a 1.25" tubing would have a diameter s equal to 10 diameters where as a 1/2" line would only have only 2 1/4 diameter turns. The higher the number of diameters the more strength that is retained. If the 1.25 is too small then you could have a plastic sheave made up to the correct diameter with an 1.25" hole in the middle and then cut it in half so that it can be attached over the pushpit. That way you should be able to retain the strength. The logic to me is fine b/c the load is taken by the round turns. As for a knot the only experience I have had is with leaving a taught line hitch on for about two years in manila rope and I was easily able to undo it with a marlin spike. Also I would think that the actual knot would not be taking a lot of load b/c the theory is that the load was taken at the round turns. Thus the not would not be under pressure.
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Old 17-04-2011, 17:47   #14
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Re: Knot Question - Spectra Life Lines

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Originally Posted by doug86 View Post
can you find a quick and easy way to just cover the lifelines before the shrink wrappers have their way with them?
Interesting idea. I can easily cover the lines for the shrinking but I am not sure how well that will protect them from the heat. Spectra melts at 150c and shrink wrap shrinks at 100c. Thoughts?

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if the walls are thick enough you might be able to just use a file.
Yes, I decided I did not want to do that - might create a stress riser and weaken the tubes. I may just spray on a little contact cement - can't hurt.

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I think we are over engineering this. Climbers have been tying off Spectra runners for years with triple fishermans knots (manufacturers recommendation) and then taking huge falls. Never a slip, very simple. Remember, it is a triple knot, but no backup is required.

The knot is very nearly the strength of a splice (85% at least). If you have a failure, it will be chafe at a pass-through.
Useful perspective. Quite different than the typical marine rigger thought that one should avoid knots in high modulus line at all cost. Not sure why the difference. The difference in perspective - thoughts?

I actually use the fisherman to make small spectra loops on the boat, when they are too small to make the bury. I have never had one break but don't use them in highly loaded applications. But the fisherman can jam under load and be hard to untie (in my experience), so I don't think its the knot.

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How about a "luggage tag" splice?
Don't think that helps, as the splice will not pull thru the stanchion holes, which is the objective.

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Doesn't the strength of the line have to do with the number of diameters occur for the turn? .
Yes, exactly. I have a decent radius . . . 32mm tubes and 8mm (top) and 6mm (bottom) lines so I have 4x for the top line, which is just about right. So, my 'only' concern was whether the 'knot' creates a stress riser where it joins the loaded part of the line.
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Old 17-04-2011, 23:00   #15
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Re: Knot Question - Spectra Life Lines

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...Don't think that helps, as the splice will not pull thru the stanchion holes, which is the objective...
Are you talking about a double-braid Spectra, with a normal (polyester) cover and Spectra core? In which case I would have thought that a luggage tag splice with the cover stripped would compress to about the same diameter as the base double-braid. I guess the "tricky" bit would be the slight increase in the diameter of the double-braid in the vicinity of the "bury"?
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