Please select your home edition
Edition
upffront 2018 GFM 728x90

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

by Phil Anniss 3 Dec 2018 01:10 PST
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

Torsional Furling Cable - What's an S-Splice?
Another cunning use for the 'Chinese finger trap' principle When it comes to furling cables, custom top-down cables (for loose luff, asymmetrical gennakers) and bottom-up cables (for code zeros and staysails) can sometimes be expensive. Torsional rope can often be a viable alternative. Posted on 22 Feb
Cousin Trestec Constrictor - A Textile Rope Clutch
Based on the 'Chinese finger trap' so is kinder to your ropes At some point in our lives, we have all experienced the vice-like grip of the 'Chinese finger trap'; when pulled, the cylindrical, woven braid is designed to contract and constrain the finger. Posted on 18 Feb
Choosing the Right Winch
An educated choice will save you from unnecessary spend Choosing a winch can be daunting. The main decision criteria may be size or power ratio, but style, speeds, material and grip are also of a consideration. That's without going into powered options (electric/hydraulic). Posted on 13 Feb
Basic Running Rigging Terminology
Dekanewtons explained If you are looking at upgrading your running rigging, before you delve into the manufacturers catalogues there are a few key terms you need to understand: Single Braid, Double Braid, Rope Strength measured in daN (Dekanewtons). Posted on 8 Feb
Would you trust Dyneema with your lifelines?
80% lighter and 4 times stronger than traditional wire Lifelines are typically made of wire, however, as the sailing industry modernises, more and more people have been converted to the use of synthetic composite lifelines, such as Dyneema®. Posted on 4 Feb
A Guide to Mooring Lines
Arguably, one of the most important pieces of kit? Arguably, one of the most important pieces of kit onboard your boat is a mooring line; it's all well and good having a great boat, but if you can't safely secure it at the quay, harbour or pier then you're in real trouble. Posted on 30 Jan
Facnor FlatDeck Genoa Furling System
Fresh in at upffront.com with a unique webbing feature It's no surprise that the Facnor FlatDeck is the newest addition to the Upffront website: it's the latest in genoa furling innovation. We are excited to launch Facnor's revolutionary furling system on our website: the Facnor FlatDeck Furling System. Posted on 28 Jan
Why and When You Need a Code Zero
Andy Rice interviews Bjarne Lorenzen of sailmaker Doyle O'leu In a previous blog, Bjarne Lorenzen talked about the exciting new performance cruiser, the 39BEN built by Bente in Germany. While the sail plan will develop over time, the code zero was one of the first sails to go into the inventory. Posted on 25 Jan
What is a sailing dogbone?
They have been around for centuries, now re-engineered A dogbone is a small, light, dual tapered rod, which can simply and efficiently connect lines together. As the name suggests, its shape is comparable to that of a 'dog bone', where the middle section is tapered, and the two ends proportionally wider. Posted on 21 Jan
Nick Black Talks Main Halyard Locks
Final discussion in the series on lock systems for yachts The final in our series on lock systems for sailing yachts, Andy Rice talks to Nick Black of Rigging Projects about mainsail locks. Reef locks have taken off in a big way in recent years, mostly because clew loads have become higher and higher, Posted on 14 Jan
YY.com app (top)