In this blog we explore the differences between Torsional Ropes and Custom Torsional Cables, the pro's and cons of each and provide guidance on the specification boundaries between the two.
Just to be clear: a composite Torsional furling cable and an Anti-Torsion cable are exactly the same thing. In some peoples minds they “resist” torsional or twisting forces (the “Anti” lobby), whilst others see these cables as “transmitting” torsional forces effectively. Whichever camp you are in, here we are going to focus on the differences between Ropes and Cables, which is another subtle but important distinction!
The difference between Cables and Ropes
Torsional Ropes are produced in long lengths on spools, and are finished to the required length on site, by yourself or a rigger / sailmaker. To produce a finished torsional rope, it is cut off the roll and needs to be terminated, around a thimble at each end (to fit your furling unit/swivel). Due to the construction of the rope, it is too dense to splice and the ends are commonly secured with a metal clamp.
Torsional Cables are custom made to a specific length, in the factory, by winding individual high performance fibre threads (commonly K49, SK99 or PBO) around two end thimbles. This fibre core is then over-braided, multiple times, and some manufacturers add specialist resins to the covers to improve the torsional load carrying capability of the finished cable.
The Limits for Torsional Rope
Torsional Ropes are mass-produced and built to industry standard diameters (7, 9, 11, 13 and 15mm), whilst Torsional Cables are custom designed and built to any diameter. A finished torsional rope is significantly less expensive than a custom cable, but it is important to know the limitations of rope, and when a cable can provide better value for money.
Diameter / length
Torsional performance is directly related to diameter. A torsional rope/cable acts like a torsional tube and it is the outer layers of braid which carry the majority of the torsional loads. As standard torsional rope sizes stop at 15mm diameter, this is a natural transition from ropes to custom cables.
The relationship between diameter and length is also important. A 1m long, 9mm diameter torsional rope will be impressively stiff – you might only be able to twist it by less than 5 degrees. However, that same 9mm rope over a 20m length will twist up like a rubber band! As length increases, so does the diameter required and again you reach the 15mm maximum threshold for torsional rope.
There is also a big difference between bottom up and top down furling cables and the torsional loads they experience. For bottom up code zeros the cable is inside a luff pocket and from the first full rotation, the sail luff starts carrying some of the torsional load. However on a top down cable the torque has to be transmitted over the full length of the cable and at much higher speeds. Therefore, for the same length a top-down cable will need to be bigger diameter than a bottom up cable. At Upffront, for a 15mm diameter rope, we recommend a maximum length of 15m for top down and 18-19m for bottom up furling. Above those lengths we strongly recommend a custom cable.
Recommended cable length vs diameter for Torsional Ropes
TD = Top Down BU = Bottom up
This can be a consideration for performance code zeros, where tensile stiffness is an important consideration to minimise luff sag. The majority of torsional rope manufacturers use Dyneema® SK75 as a core material. For a performance code zero you are looking for minimum diameter, to reduce windage, and maximum stiffness. The best core material to achieve these characteristics is PBO which therefore requires a custom cable.
As boat size and cable length increase, so do the physical torsional loads exerted on the cable. Torsional ropes are very stiff and bending the finished rope around the tight radius of a furling thimble can be difficult. Therefore, it is hard to get a good, snug fit between rope and thimble and some movement is common. With a custom cable, the core fibre is wound onto the thimbles and then the braided jackets are wound along the cable and up onto the thimbles. Finally, a polyurethane boot is formed over the end, encapsulating core fibre, thimble and braid into a solid end fitting.
Again, this is particularly important for top down cables where the high speeds mean loads increase significantly in the event of an issue / obstruction during a furl. It is not uncommon for a thimble to get ripped out of a torsional rope and this is another reason for our recommended 15m cut-off for top down cables .
Availability of furling thimbles
As the standard torsional rope diameters range from 7 - 15mm there are a number of furling thimbles available on the market, up to the 15mm max size. In general these fittings work in furling drums up to approx 5T Safe Working Load. If you have a 7T SWL furling unit there is not a commercially available furling thimble on the market that will fit your drum and you are definitely looking at a custom cable.
A 9mm diameter custom cable is 9mm, as soon as it exits the end fitting. A torsional rope is bent around an end fitting and then has to be clamped back on to itself. The diameter is therefore doubled for approx 300-700mm at the ends and can look bulky. In addition, clamps themselves are large, heavy, ugly and can cause a snag hazard. At Upffront our preferred finishing method for a torsional rope is the S-Splice (see blog here). Whilst this is lightweight and low profile, you cannot get away from the increase diameter at the ends which can also cause an issue getting the finished cable into a tight code zero luff pocket.
Unless you take your racing seriously, under 15m cable length, torsional ropes are generally an excellent, cost-effective solution. For top down you should switch to a custom cable above 15m and for bottom up we believe 18m is the cost vs benefit transition point.
If you have any questions about torsional cables please do not hesitate to mail us at firstname.lastname@example.org or use the Enquiry form below.