Wednesday, November 2, 2016

Sewn Eye Splices

rev. 11-2-2016

June 2012:

Splicing is the gold standard for forming permanent eyes and joining lines; unfortunately used double braid generally lacks the flexibility required for splicing; the cover won't open and the core won't slide. Knots are a standard solution and work in most cases; yes, there is some loss in strength, but lines generally die from chafe and I can't remember having one fail at the knot, other than in testing. But sometimes there simply isn't enough space or a knot will snag.

Seizing is traditional, but it doesn't apply to modern ropes.Nylon get so skinny under load that the seizing gets loose and falls off. Even nylon is far too strong; test shows that 15-20 rope diameters are required to reach full strength. It worked on hemp 3-strand, but polyester is smoother and stronger.

Hand sewn splices, on the other hand, has been proven proven on my boat for decades, in industry for decades, and I even did a long series of articles for Practical Sailor, pull testing samples to failure (subscribe for the details). It works, and it is WAY easier than splicing old line.

Four years ago I needed new genoa sheets. The old one failed from fatigue (Kevlar core snapped at the clew where it flogged) and I needed a second set to go with my new inside sheet tracks. I found some good line for free (big boats replace halyards with a single wear spot in the center), but knots would hang up and splicing old line is basically impossible.

First I remove about 1 1/2 rope diameters of core. This will allowed the end to be stitched down to create smooth taper.

How much stitching is enough? Select a whipping twine that is 10-15% of the line diameter; for example, 7/16-inch line is about 11mm, so #10 whipping twine is a good match. Doubled that suggests about 12 round stitches (doubled, 2 passes, 2 sides, 56 strands total) on each side to reach 8000 pounds. Sure, it is not loaded in-line, but most of the load (about 35-55% in testing, depending on the roughness of the line) is actually carried by line-to-line friction, just as in a seizing. Also remember that due to friction of the eye around the shackle or fitting, the free end is only carrying about 25-35% of the load, depending on line stiffness. The results is that the stitching is only carrying a working load of about 1000*0.30*(1-0.45)=165 pounds and a line failure load of about 1350 pounds; not nearly as demanding as you would guess.

 I hate guessing. I broke over 100 samples during this project.

Forget the theory.Testing revealed that the total thread strength  is approximately equal to the failure strength. Since the stitching is scattered, distributing the force to every part of the core, the splice is as strong as the line*. However, because wear and UV are real, so Increased it to 20 round stitches on each side of 7/16-inch line, not counting those that secure the taper.

Whipping Twine Summary
Twine Number        Diameter       Strength
#4                            0.4 mm         35 pounds
#8                            0.8 mm         70 pounds
#10                          1.0 mm         90 pounds
#15                          1.5 mm         130 pounds

I went through lots of Robline product...

Product Details
 But leather sewing twine is a cheaper alternative...

Candora 260M Sewing Waxed Thread 1MM For Chisel Awl Upholstery Shoes Luggage Set 3 Colors (Beige)

I also lab tested Spectra and Kevlar thread sewn eyes. It turns out that that Spectra and Kevlar twine make a slightly weaker joint because it is too strong, does not stretch enough to equalize strain, and cuts right through the polyester fibers. On the other hand, Spectra is more abrasion resistant. Either one works.

Tip: it is easier to push the needle against a board than to use a palm.

Then cover it with something for UV and chafe protection. Heat shrink is fast and poor choice (doesn't last). Webbing is better in severe applications... like winching a sheet along a shroud.

I tried heat shrink, but after just 2 days it tore while winching across a shroud. The webbing has been in place for 4 years and is doing fine, if a bit faded.

I like soft shackles for sheets. Less steel to flog, removable, and as strong as the larger line because it is doubled.If the eye is small it can't fall out easily, but a lashing makes sense. The failure point is always the same; the loop cuts off the stopper knot.  

I have also switched to soft shackles for spinnaker sheets after a few failures. They are lighter, less prone to coming off, and less damaging to decks and foreheads.

* This discussion is about polyester and high modulus lines. Nylon is a completely different animal, because when it stretches under load the distribution is thrown off. Imagine sewing elastic--only the first row of stitches actually holds anything. In a nutshell, the stitching needs to be crammed into the smallest practical length, not spread out like you might think. I have sewn nylon line and use sewn nylon tethers, but my procedures were tested. I don't suggest trying and I warn you that most sewn nylon webbing is considerably under strength due to poor understanding of the correct methods. Even sailmakers fail, since they are used to working with low-strech materials. Climbing gear is the guide. Look for bar tacks, such as used on climbing gear, as a sign they know what they are doing.

1 comment:

  1. Thank you for this report. I have wondered about sewn splices ever since I saw them used on an Amel I was crewing on. Al

    (Re-post: I deleted the submission accidentally)