A Shelf Foot is a mainsail construction in which the shaped body of the sail does not run straight down to the boom. Instead a panel of flat cloth, the shelf, is sewn along the bottom of the body and connects it to the foot bolt rope or slides on the boom. When the outhaul is eased the shelf opens like a hinge and the lower third of the sail bellies into deep, powerful curve; when the outhaul is pulled hard the shelf folds flat against the boom and the foot becomes as flat as a loose-footed sail. A beginner can think of it as a built-in gear change for the bottom of the mainsail. The experienced sailor and sailmaker know it as a way of getting a very wide depth range from a sail whose foot has to stay attached to the boom, and as a design closely associated with the flattening reef.
Anatomy of a Shelf Foot
Look at a shelf-foot mainsail from beneath the boom with the outhaul eased and you will see a horizontal seam running from a little aft of the tack to a little forward of the clew, roughly at the height of the boom plus the depth of the shelf. Below that seam is the shelf, a strip of cloth cut flat and attached along the boom by a bolt rope or slides. Above it is the body of the sail, cut with normal broadseam and foot round. The shelf seam is where the two meet. At the tack and clew the shelf tapers to nothing, so the corners are conventional. A flattening reef cringle is often placed on the leech just above the shelf seam, so that hauling it down removes the shelf entirely and pulls the body of the sail down to the boom.
How It Works on the Water
In a conventional mainsail with the foot attached to the boom, the depth of the lower third is limited by how much foot round the sailmaker can build in without the sail tripping over the boom. The shelf removes that limit. With the outhaul eased the shelf stands up almost vertically and the body of the sail above it is free to take its full designed depth, much as a loose-footed mainsail would. Pull the outhaul on and the shelf is drawn flat along the top of the boom; the foot of the body is now stretched along the boom and the lower third of the sail flattens. Pull the flattening reef and the shelf disappears altogether.
The result is a mainsail that can be very full for light air, chop and reaching, and very flat for heavy air upwind, without changing sails. Because the shelf is attached along the boom, the boom acts as an end plate, reducing the leakage of pressure from the windward to the leeward side under the foot and making the lower third more efficient than an open, loose-footed foot at the same depth.
Why UK Sailmakers Offers It
The shelf foot came out of racing, where crews wanted the depth range of a loose-footed sail on boats whose class rules or boom fittings required the foot to be attached. It was widely used on offshore racers in the era of the IOR and remains a standard option for many one-design classes. Paired with a flattening reef, it gives a mainsail a genuine two-mode lower section that a trimmer can switch in seconds. On a cruising boat with a conventional boom it also offers something useful: a full foot for light-air sailing and an easily flattened one when the breeze fills in, all with the sail still firmly captive on the boom.
Trim Tips
- Mark the outhaul: the shelf gives a big range, so put reference marks for light, medium and heavy settings.
- Ease it downwind and in chop: let the shelf open fully to power up the lower third.
- Flatten before you reef: outhaul hard, then the flattening reef, before taking the first slab.
- Watch the shelf seam: it is a long horizontal seam near the boom and collects chafe from the vang, lazy jacks and the sail cover.
Frequently Asked Questions about the Shelf Foot
Is a shelf foot better than a loose-footed mainsail?
It depends on the boat. A loose foot is simpler and lighter; a shelf foot gives a similar depth range with the end-plate benefit and a foot that is supported along the boom, which matters where class rules require an attached foot.
Does a shelf foot need a flattening reef?
Not strictly, but the two work together. The outhaul flattens the shelf against the boom; the flattening reef removes it entirely and gives a truly flat foot for heavy air.
Is a shelf foot only for racing?
No. It began in racing, but any boat with an attached foot that wants a big outhaul range benefits from it, including cruising boats that sail in light-air venues.
The shelf foot is a small piece of cloth that does a large job: it lets a mainsail attached to its boom be as full or as flat as the day demands. Understand what the outhaul is doing to it and you have gained a gear you did not know you had. See also outhaul, flattening reef, foot and loose-footed.