A Simple, Strong, Economical Tow Line [59]

By Scott Harris

There are a number of systems used for tow lines. I have seen a few and none looked really good. As usual life is a compromise.

Background

Firstly I was told Venetian blind cord was just the thing, very strong, compact and a little stretchy to absorb the shock of the tow as the line loads and unloads with the action of the sea. Then the next instructor complained that my tow line didn’t float and could tangle rock or weed. The problem is that Venetian blind cord doesn’t float and the thin, cord is prone to tangle. It is made of nylon, which has a specific gravity (SG) of about 1.14 (water is 1.0).

Technical Stuff

We need a cord that is thin and floats (SG less than 1.0). A check of materials on the net quickly narrows the field to polypropylene and polyethylene (don’t laugh, if the polymers are aligned, polyethylene is one of the strongest plastics). But nothing is anywhere near as “stretchy” as nylon.

There are high tech (high cost) materials such as Kevlar, Spectra and Vectran of which, Spectra (High modulus polyethylene) will float but has been developed for low stretch so will have almost no give as load takes up and drops off in a bit of a sea.

Polypropylene is a low cost, low strength, rope making material and therefore a little frowned upon, but it is much used in low tech marine applications especially for tow lines due to its buoyancy.

Finding a suitable source of small diameter floating cord was not so simple, the only readily available product was water ski rope (usually polypropylene but sometimes polyethylene) which is about 9 mm and makes for a bulky tow line. I am told an elite K4 can pull a water skier but the power you or I are likely to develop towing another kayak should allow a lighter gauge.

Solution

I use clothes line cord from the supermarket as a general purpose cord around the house and camping because it is cheap and easy to work with. The cord is 4mm polypropylene and sold in 15 metre lengths. Perfect – the club has standardised on 15 metres for tow lines.

Splicing a loop

The cord is 8 strand diamond hollow braid, the same construction as water ski tow rope. A loop (or eye splice) can be made easily. Just thread the line through the snap hook then on the standing part of the line at a point 1cm up the chord from the hook compress the cord lengthwise to loosen the weave and expand the strands, now insert the loose end of the line into the gap in the weave and up the centre 3 to 5 cm. This is easy with water ski rope but a little more difficult with this 4mm polypropylene. It just doesn’t expand to the same degree. If the end of the cord is melted and smoothed as you would to stop the end fraying, you can use a copper electrical wire to push the sealed end up the inside of the cord.. The result is a very neat, very strong buried eye splice. Remember – pass the cord through the eye of the snap hook first.

The Tow Line in Use

The first one I made, I attached floats on the ends to counter the weight of the snap hooks. The first time I used it, one of the floats fell off. No problems, the cord itself has more than enough buoyancy, only the last few metres of the line sank as I retrieved it. An alternative is to thread the line through a small float before you thread the snap hook.

The 4mm polypropylene braid is supple, easy to handle, and doesn’t tangle nearly as easily as nylon cord. With a little care, I have never had a tangle. I roll it around my hand loosely and progressively and place it into small bag

One word of caution though, polypropylene has very poor UV resistance. Don’t leave it lying around in the sun for long periods between paddles. Keeping it in bag should make it last a lifetime, although replacing it occasionally is no big deal.

Materials

  • 2 stainless snap hooks (50mm) from Whitworth’s $4.99 each.
  • 1 Clothes line (15m) from Woolworths $3.69, Total cost $13.67
Common Rope Making Materials
Material Specific Gravity Other Characteristics
Nylon 1.14 10-15% wet strength loss. Poor wet internal abrasion resistance. Moderate creep.
Polyester 1.38 Good wet internal abrasion resistance.
Polypropylene 0.91 Lighter than water, moderate creep, lower strength, low water absorption

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