Top down furler and Bottom up furler - Differences and Options

What are the key differences between bottom up and top down furler 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 here.

Dedicated Top down furler unit

The primary difference between a bottom-up and a dedicated top down furler drum is that a top down furler 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.

Image courtesy of Karver Systems

A top down furler 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 furler 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. 

Furlers for 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 furler 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.

Above: Two examples of a tack swivel from Facnor(left) and Karver(right)

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 adapter 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 increase in weight, over a dedicated top-down drum, and a slight decrease in available luff length. However for most cruising, and a growing number of racing sailors, the advantages of using a single drum for both their code zero and downwind asymmetric spinnakers outweighs the disadvantages.

Furlers for 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 padeyeon 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 bottom up vs. top down furler

Separate, dedicated bottom up and top down furler 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

If you have any questions about bottom up and top down furlers, please feel free to email us at [email protected], or click the link below to see our full range:

FURLING SYSTEMS