In-Mast Furling Mainsails

Kemp Sails Furling Mainsails

Ever since Ted Hood’s “Stoway” In-Mast Furling system brought Mainsail Furling to the mainstream, it has made great strides and earned a ‘refined and reliable’ rating throughout the industry. What are the options for sails?

Classic ‘In-Mast Main’ – Easy Simplicity


Kemp Sails recognises that rolling up cloth that’s not designed to lay flat can create pinches of sail material that may jam during the furling or unfurling process. To avoid this, we shape our Furling Mainsails a little flatter and build them with a hollow roach. Kemp Sails only make In-Mast Furling Mainsails from stiffer Yarn-Tempered Polyester or a High-Modulus material, that resists stretch and wrinkling.

The Foot and Leech of Furling Mainsails can come under a great deal more stress than ordinary sails, which is why Kemp Sails also reinforce these edges with a specially created Kevlar Tape.

Adding Battens

short leech battens

Adding Short Leech Battens to an In-Mast Furling Mainsail, allows us to support a straighter Leech profile.  This maintains more of the sail area which would otherwise have to be 'hollowed out' without the use of battens, therefore improving Light Airs Performance.  The use of battens also helps control the Leech and reduces the potential for vibration and flutter.

‘Vertimax’ – Ultimate Vertical Battened Reefing

VBM full length battens

The Vertimax In-Mast Mainsail Reefing system accomplishes that simple function which many other battened In-Mast Reefing sails absolutely fail to achieve,
and that is: Long-Lasting Furling Performance.  The reasons for this are simple – and two fold:

Firstly, Kemp Sails recognise that space is at a premium in most Mainsail Furling mast chambers and therefore the batten configuration is critical. Kemp Sails use 4 Full Length Vertical Leech battens to support the increased Roach Area, placed to be as accurately aligned as possible when furled.

Secondly, the life of such sails is largely governed by the quality of the fabric used in the manufacture, so Kemp Sails only offer these sails in one of 3 options: Premium Dacron Sailcloth; Laminated Cruising Sailcloth; or ‘Hybrid Wovens’ (where the fibres are mixed with an exotic material such as Vectran or Dyneema). In our experience, other fabrics with less Tenacity simply don’t offer the long term support required.

Unfotunately, to add Full Length battens is only possible if the mast design itself will accomodate the battens, so the chamber needs adequate size and the sail entry gap must be wide enough to allow the battens and their patches/reinforcing.

Kemp Sails are always happy to talk about Vertimax and other In-Mast Sail systems and recommend the best solution for your In-Mast Furling Mainsail.

 Area Increases

The Operation of In-Mast Furling Mainsails

Selden's Linedrive operated Furling masts have asymmetric tensioned luffspars. This reduces furling friction. The linedriver must be rotated clockwise when furling or reefing, and the sail must be wound on tightly. To achieve this, there must be some residual tension in the outhaul line when reefing or furling.

Note: Many other Furling Masts do not have not tensioned Luff spars and so they require extra care and vigilance in use and to get the best operation from them.

Before any sail area change, slacken the kicker & mainsheet, then adjust the boom vertical angle using the topping lift or gas spring powered kicker. Adjusting the boom slightly allows the loads in the sail to be spread more evenly between foot and leech, promoting a neat furl. The degree ofvertical boom adjustment required depends entirely on the individual sail’s shape. The sail should never be furled or reefed with a slack foot or leech.

For deploying the sail in light conditions, when the full main is required, slacken both sides of the endless reefline and pull the outhaul. The linedriver slips under the slack reefline, and has almost no friction. Adjust the outhaul to give the correct sail camber for the conditions.

If a reef is required immediately, deploy the sail completely as above, then reef to the required size before tensioning the outhaul.

To reef, adjust the boom vertical angle slightly, and ease the outhaul until the sail has a large “S” shaped camber. Take in on the reefline (Stbd side of linedriver), and allow the return side to be pulled forward. Secure both ends tightly. Slacken the outhaul again and repeat. It is best to reef in a series of “bites”, as the sail will be wound more snugly round the luffspar. Finally, secure the reefline tightly and tension the outhaul.

For increasing sail area from a reefed size to less reefed, a different technique is required.

Obviously the sail wants to unfurl by itself, so leave the outhaul in a stopper, have the "Return side"(Port) reefline line in hand, and the "Reef side"(Stbd) line ready to surge round the winch. As the Reef side is surged, ensure that the slack in the Return side is taken in immediately. If this is not done, a bight of reefline can emerge in the 11 o’clock position of the linedriver, and then it may slip out of control. The sail would then unroll completely. Once the handling technique has been learned, linedriver operation is simple and very effective.  

For most applications, the control lever above the linedriver is set to the “Free” position, allowing free rotation in either direction. If on a very long leg with a reefed sail, the left hand “Ratchet” position may be selected, allowing the reefline to be offloaded. The most likely time for selecting “Ratchet” is when the vessel is left unattended. In this case, the ratchet is a good safety feature in that it guards against unintentional deployment.

In light airs on the wind, it pays to ease the outhaul to match the genoa camber and also top up the boom to induce a little twist. The mainsheet must be used carefully, and not pulled directly against the topping lift. At all times when sailing off the wind (not close hauled), increasing mainsail camber is effective.