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clarcana: Projects:

Thimble Splice
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Background to the project.

Some of the bigger weeds.
Photo of very big weeds.

This is actually another minor 'sub-project'. The major activities involved with maintaining these premises include weeding the large patch of ground at one time called a garden. Because some of these weeds now exceed 55 feet in height, some seriously challenging logistics are required for their removal. One need is for some polypropylene ropes of half inch diameter; rated to 5850 pounds (2650 kilograms, or just over 2½ UK tons). To obtain their maximum performance, what is known as a thimble[1] has to be fitted to each end of the line. This is a steel strip bent into an almost complete, elongated ring, but with a concave outer edge in which the rope nestles.

Some terminology: the main length of rope before the splice is called the standing end. The loose, or free end that is manipulated into spliced form is the working end. The rope is said to enter the thimble along one leg, and to leave it by another. Some method to fasten each leg to the other is needed to prevent the rope slipping off the thimble when under load. The traditional way of accomplishing this is to splice the rope, forming it into an eye in which the thimble sits.

Such splices are described in the Ashley Book of Knots,[2] The simplest of these (#2725) is useable with the three-strand rope I bought from Amazon.[3] ABOK predates the introduction of polypropylene, so what modifications to #2725 are necessary for substitution of the original hemp material is unclear.[4] Other engineers new to ropework may care to note that the three strands comprising most ropes twist around in a clockwise fashion - like a normal (right-hand) screw thread.

That the ABOK has become the standard reference on knots is, perhaps, tribute to Ashley's skill as an illustrator. His achievements impress because, where necessary, he uses three drawings for each knot: a diagram explaining its topology, one demonstrating practical method(s) of forming it, and one displaying the finished (tightened) shape (e.g. #2636). None the less, a reader struggles at times to comprehend essentially three dimensional forms depicted by two dimensional means; hence this page. Another limitation of the ABOK is that, although a useful aid to comprehension, colour is unused. Well, whoopee-do, today we have it. Eventually, I will learn some CAD package thoroughly enough to draw knots.

One point of potential confusion: Ashley draws most of his splices without showing a thimble. Clarity is increased that way and, in any case, he forms the eye first as a separate task before hammering the thimble into the eye; unless the rope is 'thick'. I decided that ½ inch was 'thick' and spliced the eye around the thimble.

Lastly, a splice is not your only option. You might try a scaffold knot (Ashley #1120), saddle clamps (Amazon etc) or a crimp. Choose well, though, because thimble failures are embarrassing at best and fatal at worst.

Forming a thimble splice.

Knot on your life.

Table 1: Sequence of strand crossings.
Lower
leg
strand
Under/
over
Upper
leg
strand
1 under 1
2 over 1
3 under 1
1 over 2
2 under 2
3 over 2
1 under 3
2 over 3
3 under 3
1 over 1
2 under 1
3 over 1
1 under 2
2 over 2
3 under 2
1 over 3
2 under 3
3 over 3
Figure 1: Forming the splice.
 The splice eye being formed.
Photo 1: Starting the splice.
 The splice after the first few crossings.
Photo 2: Completed splice.
 The splice nearing completion.

Start by untwisting the rope into its (three) strand components until each straightened strand is at least 18 centimetres long.[5] That should be sufficient for about six 'tucks'. Before work can continue on the splice you need to prevent the strand ends fraying. Ashley doesn't show his method (whipping?) for the hemp used by tradition. Tubers favour electrical tape, which is probably better than the hot air gun I used to melt the ends. Heat shrink sleeves or ferrules might also work.

Next, secure the rope around the thimble. Align the start of the loosened strands with the point where the rope leaves the thimble. I used a nylon cable tie to hold the legs together. Ashley uses a traditional whipping of twine. In skilled hands, that could be somewhat better. See this in Photo 1 at right.

If, like me, you are inexperienced with this particular splice then colour code differently each strand of each leg. For the lower (exit) leg I used black, blue and red. For the upper (entry) leg, orange, green and yellow.[6] Figure 1 shows the idea.[7] If you use different colours then tweak the following description according to your choice.

Untwist the entry leg by one half turn where it goes into the thimble. This will slacken the strands sufficiently to allow the splice to proceed. Unfortunately, as soon as you let go of the rope it will spring back to its twisted state. If this proves bothersome, then grip the rope with a bench vice, a batten, ties or clamps. Ashley recommends use of a fid to establish the opening. Whatevs.

Identify the uppermost free strand (blue) then find the second strand behind that (black). Coax that, as shown, under the uppermost strand on the entry leg (orange). The next 'tuck under' is the strand immediately behind the blue: red. It, too, bobs under the orange strand. The third tuck is blue under green. After that, back to black.

Proceed in the same fashion, keeping the exit leg strands in their correct order. Do not cross one of these strands over its sibling. This means that they will resemble a left-hand thread within the splice, whereas the entry leg strands maintain their original right-handed progression. If you have done any platting then this process should be familiar. Said splice should be at least 15 cm long and around 1.8 cm in diameter when complete. The strands on the 1.27 cm diameter rope I used, in rotating around this increased diameter, impart the cost of a slight shortening of the line; but it is unlikely that you will need to take that into account with most reproductions.

The pre-terminated end of the line, as supplied, was finished with a covering of heat-shrink tubing. Ashley pre-dates that, and shows traditional whipping. That might be superior with some conditions of use. Because I had 1 inch diameter, adhesive lined shrink tube to hand, I used that. I suspect that was a mistake. Allowing adhesive to penetrate the outer fibres will impair the flexibilty of the line and, in the end, may cause fibre breakage. I will find out after I am clobbered by a three kilogram pulley block travelling at a couple of hundred miles an hour 😨

If you use shrink tube then use no more heat than necessary. Do not melt the poly fibres. Use a few turns of masking tape to help protect the bare rope where it leaves the tube. Also, apply the covering with the line under some tension; it will stretch a few percent, even with a reasonably tight splice. That is why the splice works so well. Increasing the line tension stretches it and thins it down, forcing the strands against each other more tightly.

I no longer have access to tensile test facilities. Those who do - please mail.