There is occasionally some confusion with respect to mainsail area.
Most sailmakers, including us, compute mainsail area as if the sail were
a right triangle with three straight edges. With this method, area
equals the luff times the foot divided by two. Most mainsails are, in
fact, somewhat larger than that by reason of the extra area at the
leech, called roach, which falls outside the straight line triangle. The
promotional material distributed by boat builders and dealers sometimes
includes this extra area as part of mainsail area. If they’re using the
same luff and foot dimensions, they’re talking about the same sail in
spite of the different figure for area. When we make mainsails, we’ll
include as much roach, not only on the leech, but also on the luff and
foot, as is practical and proper for the particular sail.
Reefs
Mainsail reefs are used to depower a boat in strong winds. Reefing makes
the mainsail smaller. Each row of reefs consists of a heavily
reinforced patch with a grommet at the luff for the tack horn and a
grommet at the leech for the reefing outhaul. Between the reefed tack
and clew may be a row of evenly spaced smaller grommets (called reef
diamonds) to provide a means of gathering up and tying off the excess
sail. This row of holes is omitted in the case of a flattening reef
because there is not enough excess material to worry about. The number
of area-reducing reefs varies with the size and anticipated use of the
sail.
The Batmain
 We use the term Batmain to describe a mainsail with full-length battens,
meaning the battens extend all the way across the sail from leech to
luff. Full-length battens have many virtues for cruising sailors
including better shape holding ability, longer life and easier handling.
The sails last longer because the full-length battens prevent flogging —
the violent snapping back and forth that happens when you're tacking,
reefing, hoisting or lowering your sail. As conventional sails flog, the
sailcloth breaks down, especially at the front end of the battens where
the leech of the sail “hinges” back and forth. Flogging is the prime
culprit in sail wear-and-tear. The full-length battens induce a smooth
airfoil shape to the sail, which improves performance in all conditions.
In light air the Batmain holds a smooth airfoil shape instead of
sagging like a wrinkled bed sheet.
Also, the Batmain won’t slat as the boat rolls in the waves. In heavy
air the Batmain doesn’t flog when the sheet is eased out to reduce
heeling. The Batmain does not flail and snap when depowered, the silence
is a pleasant surprise. Full-length battens also restrict the draft of
the sail from moving aft in heavy air when sailing upwind. Downwind, the
full-length battens hold out the leech of the main giving you more
pro-jected sail area.
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All major rating rules, IMS, MORC, and PHRF permit sails with
full-length battens with no rating change at all. If your current main
is in good condition, its life can be extended by adding full-length
battens.
Shelf Foot and Loose Foot
The Shelf-Foot and Loose-Foot constructions are options that allow us to
add a substantial degree of extra fullness in the lower portion of the
mainsail in order to improve racing performance on reaches and runs. In
very light air, the improved performance is achieved even to windward.
The extra fullness is removed by tightening the outhaul. When the
outhaul is eased, the sail maintains an airfoil shape down to the boom.
With the Shelf-Foot option, a light piece of material connects the sail
to the boom. With the outhaul eased, the connecting material unfolds and
forms a shelf between the bottom of the main and the boom. When the
outhaul is pulled tight, the shelf closes up and lays against the side
of the boom.
Loose-Foot construction does away with the shelf material. The sail is
only attached to the boom at the tack and clew. This attachment method
is equally as strong as that of the foot attached to the boom. Many
boats are switching to Loose-Footed mains because they make it easier to
bend on your mainsail and to remove the sail from the boom — and it's
less expensive.
The Powerhead Main
The Powerhead main is our long-life racing design. Whether you are
buying a Tape-Drive, Tri-Radial or Cross-Cut main, the Powerhead’s
full-length top batten gives you many of the durability benefits of a
full-batten main (UK’s Batmain), while offering
performancebenefits. The Powerhead gives you more control over the
shape of your main, especially the draft's depth and fore-and-aft
positioning because the full length battens prevent the sail from
changing shape much. The Powerhead is easier to trim than a full-batten
main since the leading edge of the sail is free to luff. And the
Powerhead
is a lighter sail than a Batmain since it uses shorter battens and no special luff hardware.
 The long top batten stabilizes the highly loaded head area, and
stabilizes the leech allowing UK to build a maximum-roach sail that
gives you more sail area. The full-length top batten also increases
longevity by dampening the damaging effects of luffing and by preventing
the draft of the sail from moving aft over the years.
Along with the full-length top batten, Powerheads feature the new
longer-length battens in the lower positions. The longer battens support
the
leech better and eliminate a hard ridge that sometimes develops at the
inboard end of the battens as sails age. Powerheads that are used with
lazy jacks or the Dutchman ® flaking system can have the bottom batten
full-length to help catch the sail better as it comes down.
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