Please select your home edition
Edition
Leaderboard FD July August September 2023

Torsional Ropes Versus Cables

by Phil Anniss 16 Oct 2020 01:00 PDT
Torsional Ropes Versus Cables © Karver

In this blog we explore the differences between Torsional Ropes and Custom Torsional Cables, the pros and cons of each and provide guidance on the specification boundaries between the two.

Nomenclature

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

Core material

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.

Cable Construction

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.

Aesthetics

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 our blog). 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.

Conclusion

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 or use the Furling Enquiry Form.

Related Articles

Soft shackles are a win-win solution
Upffront look at their use across multiple applications Soft shackles have surely penetrated the mainstream sailing in recent years. They are a popular alternative to traditional metal shackles. Made from single braid Dyneema® soft shackles offer several advantages over their metal counterparts. Posted on 8 Feb 2024
Vakaros Atlas 2 - sailing with ultimate precision
A careful review by the experts at upffront.com Recently upffront.com have introduced the latest Vakaros Atlas 2 sailing instrument to the shop and compared it in a review of other top sailing instruments. Posted on 25 Jan 2024
Sailing instruments from Velocitek/Sailmon/Vakaros
An introduction and comparison from Upffront.com The term 'sailing instrument' is broadly applicable to just about any onboard system with a sensor and a readout. While wired instrument systems are common, in this blog we discuss portable GPS sailing instruments from Velocitek, Sailmon and Vakaros. Posted on 11 Jan 2024
Introducing Ronstan Orbit Winches
QuickTrim feature allows line to be eased without removing handle or line Ronstan have just launched an all new range of Aluminium Self-tailing Orbit Winches in three sizes plus an innovative new QuickTrim feature which allows the line to be eased without removing the handle or the line from the self-tailer. Posted on 7 Jun 2023
Performance Code Zero Furlers
Options from KZ Racefurlers put under the spotlight We are pleased to introduce KZ Racefurlers to the upffront.com website. Racing sailors will need little introduction, as KZ have been gracing the bows of grand prix race boats for almost 30 years. Posted on 28 Jan 2023
How to Choose a Spinlock Deckvest
The main consideration is usage and application Lifejacket pioneers Spinlock are renowned for their sleek, comfortable to wear, high-spec lifejackets. In a previous article, we introduced the full range of Deckvests and took a look at the features and benefits of each. Posted on 16 Sep 2021
Best Sailing Boat Blocks in 2021
Review covers soft attach, snatch looper, open ring, beckets and more In this article we take a quick look at what is new and/or interesting in the sailing block world in 2021. Harken continue to develop soft attach blocks. Morfrac Systems are developing their product line to include becket versions of their blocks. Posted on 7 Sep 2021
Make Waves with nke Marine Electronics
Innovative systems for racing or cruising It's the age of information, and global technological advancements have revolutionised our lives and industries alike. As with many others, the sailing industry has benefitted from the introduction of marine electronics that make it easier. Posted on 23 Nov 2018
Introduction to Core Fibres in Modern Ropes
The majority are polyester or Dyneema There are many core materials used in rope making (e.g. Vectran®, PBO, polypropylene, hemp, nylon etc) but the vast majority of modern ropes are based on a polyester, or increasingly, a Dyneema® core. Get a basic introduction here. Posted on 9 Nov 2018
High Performance, Low Hassle Family Cruising
The radical - but not too radical - 39BEN How do you design a cruising boat that's fast enough to get your competitive juices flowing, but without scaring the family? That's been the challenge for Alex Vrolijk, who has drawn the lines for the radical - but not too radical - 39BEN. Posted on 26 Oct 2018
North Sails Loft 57 PodcastAllen Dynamic 40 FooterMaritimo M75