Please select your home edition
Edition
upffront 2018 GFM 728x90

Top-down and Bottom-up Furling Units: Differences and Options

by Phil Anniss 28 Dec 2018 01:00 PST
Top-down and Bottom-up Furling Units: Differences and Options © upffront.com

What are the key differences between bottom-up and top-down furling units and do I need a dedicated unit for each type of furling? For a definition of the expressions, "top-down" and "bottom-up" furling - please read our related blog article.

Dedicated top-down unit

The primary difference between a bottom-up and a dedicated top-down furling drum is that a top-down drum has the addition of a free floating tack swivel mounted on the furling drum. When the drum is furled, the fork and cable are rotated whilst the tack swivel remains static. This allows the torque to be transmitted along the full length of the cable to the head of the sail where the furl starts. With the free floating gennaker tack, the bottom of the sail is then the last part to be furled around the torsional cable.

A top-down drum is heavier than its bottom-up counterpart due to the addition of this tack swivel. It should be noted that some manufacturers specify a separate working load for the tack swivel. For example: the Karver KSF8 and KSF8R top-down furling units essentially have the same overall Safe Working Load (SWL) of 8T, but they have tack swivel SWL's of 3T and 6T respectively. Having said that, with top-down furling, SWL is rarely an issue because the cable is only tensioned during furling and unfurling and even then, with just enough load to hold the cable straight during the furl rather than under serious tension.

Note: a top-down furler can be used as a standard bottom-up furler, it is just the tack swivel becomes superfluous.

Other options?

Cruising

Furling units are a reasonable investment and so most will be pleased to hear that there are some options, other than buying a dedicated top-down furling drum.

Most common of these, and popular amongst cruising sailors, is to use a top-down adapter. This is a stand-alone tack swivel that has an eye to connect into a standard bottom-up furling drum with an upper fork into which the torsional cable is connected.

The top-down adapter can be set up and left permanently on the sail, with the cable connected and tack lashing in place. With a swivel similarly dedicated and stowed with the sail, when it comes time to hoist your gennaker, simply attach the top-down in the jaw of the drum, attached the halyard to the swivel and hoist out of the bag.

The only disadvantages with the top-down adapter are the significant increase in weight, over a dedicated top-down drum, and a slight decrease in available luff length. However for most cruising sailors the advantages of using a single drum for both their code zero and downwind gennakers outweighs the disadvantages.

Racing

Another, slightly more complicated, option is used on a number of big boats, with the crew and brain-power to pull it off! In reality, the tack of the gennaker can be completely independent to the furling drum. Some boats set up a 2:1 gennaker tack line directly onto a padeye on the deck, close to the drum. For the hoist, the swivel is attached to the halyard, the bottom of the torsional cable to the fork in the drum and then the tack to the line onto the deck.

The advantages of this setup are that it is the lightest option and it allows the crew to quickly and easily adjust the luff tension on the gennaker. The downside is that you do need to ensure the tackline trimmer is paying attention during the furl/unfurl manoeuvres.

Summary of available options:

  • Separate, dedicated bottom-up and top-down furling units
    • Expensive
  • Use a top-down adapter on each of your gennakers together with a single, standard bottom-up furler
    • Heavy, but a very flexible and efficient cruising set-up
  • Use a dedicated top-down furler for both bottom-up and top-down furling
    • Slow sail changes but otherwise perfectly acceptable
  • Use a bottom-up drum with a separate gennaker tack line
    • Need to have experienced crew

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)