Please select your home edition
Edition
upffront 2018 Ronstan shock blocks Leaderboard

Top-down Furling versus Bottom-up Furling Cable Specs: What to Look Out For

by Phil Anniss 3 Dec 2018 09:10 GMT
Bamar furlers © Bamar

Have you got any idea what's the difference in design between a top-down or bottom-up torsional cable? Find out what makes a good torsional cable and the important differences between the two applications.

They might look the same, but a top-down furling cable requires a fundamentally different specification to that of an equivalent length bottom-up cable. Top-down furling cables are more expensive and there may be a temptation to go for the cheaper, bottom-up option. But you do so at your peril... as failures can be expensive!

What makes a good torsional cable?

The secret to torsional performance is in the covers of the cable. A variety of techniques are used by different manufacturers but, in essence, the covers are designed to act like a torsional tube, around the core fibre. Torsion is a function of the force applied by the distance (r) from the axis of rotation, which for a cable is its centre. Therefore, torsional force increases relative to the diameter of the cable. The trick with a good torsional cable is to maximise the torsional stiffness but still retain overall cable flexibility to allow it to be stuffed into the bag with the sail when not in use.

Bottom-up

Bottom-up cables are highly loaded to ensure good luff tension in your straight luffed code zero or staysail. Therefore, cable stiffness is a key performance parameter together with minimum diameter to reduce upwind drag. This is why PBO is the optimum core material for bottom-up furling cables on GP race boats, being the stiffest, lightest and smallest diameter soft composite fibre available on the market.

Good torsional performance is obviously also required, however, as the furl starts at the bottom of the sail, within a few turns the tack of the sail has rolled onto itself and the sail luff starts to assist in the transfer of torque up the cable. As the sail luff adds significant diameter to the cable, it actually contributes significantly to the effective torsion of the overall cable/sail furling system.

Top-down

Top-down cables used on loose luff, downwind sails, spend much of the time slack and should only be fully loaded during furling and unfurling. As the furl starts at the head, the torque has to be transferred from the drum, along the full length of the cable without any assistance from the sail. In addition, due to the larger sail area of gennakers, speed is a critical success factor for top-down furling. High torsional loads and high speed mean that torsional performance is THE most important cable characteristic for a top-down cable.

With relatively low working loads, the core fibre's only role is to be light and fat to provide diameter to support the cable covers which are doing all the work. This is why Dyneema® offers an excellent choice for the core of a top-down cable. It is one of the lightest and strongest fibres on the market which allows you to build diameter without a weight penalty. Gottifredi Maffioli goes one step further in the pursuit of building low-load, lightweight, large diameter top-down cables by using a hollow filament called Aircore at the centre of the cable, surrounded by Dyneema® and then the outer covers to deliver the torsion.

Summary

  • Bottom-up cables need to be stiff, thin and light whilst still delivering reasonable torsion
  • Top-down cables are all about torsional stiffness and need maximum diameter without adding too much weight.

To get a complete guide to specifying and purchasing continuous line furling systems please download our Furling System guide.

Related Articles

Karver's double whammy at the DAME Awards
Compact winch and carbon handle applauded in Amsterdam The Dame Awards are the leisure marine industry's equivalent to the Oscars, awarded every year at the Marine Equipment Trade Show (METS), held in Amsterdam each November. Posted on 19 Feb
Local Ropemaker to America's Cup Supplier
How did little Italian rope producer Armare make it to the top? Despite being a small, family-run business, Armare Ropes are operating at the top of their game. Posted on 13 Feb
Robline Rope Cruising Ranges Explained
How to choose which range you need The success of Robline as an international rope brand is based on decades of expertise in manufacturing fibre ropes, the experience of professional sailors in the company, and the intensive collaboration with Olympic gold medallists. Posted on 5 Feb
nke Depth and Speed sensors explained
nke are the standard for the majority of Mini-Transat and IMOCA 60 classes The marine industry has been awash with new and exciting technological innovations in recent years and standing at the forefront are companies like nke marine electronics. Posted on 30 Jan
Tylaska Cone and Plug Fids explained
How to release loaded fittings remotely Tylaska Marine Hardware is an American manufacturer of high quality- marine hardware for the sailing and marine industry. Tylaska's stated aim is to make the best product based upon physics and not economics. Posted on 16 Jan
Velocitek - get your electronics strapped in
Brackets for every possible mounting point Simple, clear and extremely functional. Velocitek electronics are popular among sailors from beginners to competition veterans thanks to their functional design and ease of use. Upffront are big fans of the Velocitek range of electronics Posted on 8 Jan
Tuff Luff head foils care and maintenance
A guide from upffront.com Tuff Luff's twin-grooved headfoil allows for easy and fast headsail changes. It is a must-have system for any club or competitive race boat. Tuff Luff is owned by Schaefer Marine Inc. who has been in marine hardware design since 1966. Posted on 2 Jan
Soft Attachment Blocks - a guide to the basics
There has been a quiet revolution going on in the last 5 years There has been a quiet revolution going on with block attachments over the last five years. Soft attachments are universally accepted as the preferred method of securing a block. Posted on 25 Dec 2019
Bamar's RLG EVO - Code Zero Furler
Sleek, performance-orientated furling designs investigated Hailing from Northern Italy, Bamar are over 40 years old and on a mission to create sleek, performance-orientated furling designs that aim to 'partially automate sailing operations'. Posted on 19 Dec 2019
Trogear Bowsprits
A good enough reason to drill a hole through your bow Here at Upffront, we talk a lot about asymmetric sails and code zeros, and with good reason! Faster and more manageable, these are a popular choice among many sailors, with one caveat: they require a tack point. Posted on 11 Dec 2019