Torsion rope

What is a Torsion Rope?

Torsional ropes have the benefit that they can be finished on the dock and there are a large variety of clamping methods used in the industry

• Sailmakers often sew together the two legs of the rope
• A variety of specialist clamps have been developed by code zero furler manufacturers e.g. Ubi Maior and Karver Systems
• Or a true DIY solution is to use wire rope clamps which are very inexpensive

However the main downside of Torsional ropes is the effectiveness of the clamping system and the bulky nature of the terminations. On Code zero sails there can be issues of getting the clamps/double rope thickness into sail luff pockets. And with Asymmetric’s you have to be thinking about the snag hazard of your clamping system. One of the nicest solutions we have seen is the S-Splice system used but Kohlhoff rigging. This uses a heavy duty Technora braided sleeve to encase the end of the rope and uses the constrictor principle to hold the rope in place. This is the lightest weight and lowest profile system we have seen on the market.

How is the Torsion rope made?

As their name suggests, they are built in the same way as normal ropes. They have a core of braided or uni-directional composite fibre. Historically this has been SK75 but as the use of torsional rope has been pushed to higher performance limits, some manufacturers are introducing SK99 or Aramid cores.

Be careful using a rope product with an SK75 core for a code zero torsional over 11-12m. This is a very stretchy core fibre and you can have problems maintaining luff tension and overloading the sail luff. Torsional stiffness of the rope comes from having multiple braided covers (minimum of x2) which are glued together with a flexible resin system. These ropes can be coiled to relatively small diameters to allow the cable/sail to be stowed in a sailbag, when not in use, but they are torsionally very stiff to allow effective furling of code zeros, gennakers and asymmetric spinnakers. 

What are the limits of torsion ropes

The industry has settled on a relatively standard range of rope diameters: 7mm, 9mm, 11mm, 13mm and 15mm, however some manufacturers offer 18mm ropes. As rope diameters increase the furling thimble interface with the furling drum becomes an increasingly big issue and we rarely use 18mm furling rope.

There are two limiting factors which limit the use of torsional ropes:

• The strength of the clamped terminationю Both in terms of tensile strength and the ability of the thimble to stay in the end of the rope under high torsional loads – they can literally get ripped out of the clamped eye of the rope.
• The Torsional stiffness of the rope. These ropes are built to generic specs for standard use o boats from 25-45ft. For more performance applications cable specification can be optimized by playing with the core material and number of torsional layers to increase tensile or torsional stiffness for a given length.

It is important to understand that top down furling exerts significantly larger torsional loads on the cable than for bottom up furling. The bigger the rope diameter, the higher the torsional load it can take. Therefore, a 14m cable used for top-down should be at least 15mm in diameter compared to a 14m long bottom-up cable which could comfortably be 11mm, or even 9mm in some circumstances.

Manufacturer Guidelines

It is important to refer to manufacturer guidelines on what diameter torsional rope you need for a given length of cable and the application you are using it for (top-down or bottom-up)

• Kohlhoff Rigging

• Armare Zero Twist

How do I find the right torsion rope for my needs?

Using the Manufacturer recommended guidelines:

• Step 1 - Determine cable length & furling configuration (top-down or bottom-up)
• Step 2 - Use the manufacturer table to select rope diameter. To get the best performing cable, bottom-up cables can be the smallest possible diameter available at that length, whilst top-down should be the max.
• Step 3 - Check furling jaw & fitting width. If the furler jaw is 0.2-0.3mm larger than the fitting width, it will fit comfortably. Often the cable fitting is 1-2mm narrower than the furler jaw.

• Step 4 - Check pin diameter compatibility. If your furler pin is too big for the cable fitting, the cable simply will not fit. If the pin is too small, this can be accommodated with a bushing.