Mainsails are straight forward in their construction. Quoting is
simply based on the vessels P and E design specifications.
We normally quick quote sails for expected wind speeds of 12 to
20 knots. Cloth weight is very important to overall sail
performance. Racers generally prefer light weight cloth and
Charter Boats generally use heavier cloth to compensate for
inexperienced sailors. Too heavy a cloth will sacrifice boat
speed and windward performance. If you sail in winds 15 to 25
knots you should go up an ounce. Winds greater than that, go up
2 ounces. Please note that this is based on average sailing, not
just a windy day.
Both the luff and the foot of the sail need to be taught to
produce great sail performance. A sail that is 2 inches too long
will sail poorly. A sail that is 2 inches too short will sail
perfectly. Also, be wary of trying for too much luff. Remember
that the mainsail has a headboard with a width of about 8
inches. You don't want that headboard interfering with the
backstay. Some vessels have black bands to specify how high the
designer wants the maximum luff. This is particularly true with
wood masts that often have the backstay attach directly to the
mast as opposed to a T-shaped mast head fitting on aluminum
spars. Normally, we consider maxluff to be 8" below the halyard
Normally a sail will stretch about 1.5 percent over the useful
life of the sail. Thus, if you stated that maximum luff was 40
feet (480 inches) you could assume the sail would stretch 7.2
inches over it's life. We would normally design a sail that was
39' 5" for the max luff of 40 feet. Measurements are
always sail edge to sail edge. Never attempt to use pin to pin.
On a mainsail, keeping the luff and foot taught, requires a
correctly located tack pin on the sail. That pin is specified by
the M1 and M2 measurement on the measurement form. That position
is to the center of the tack pin. Sailmakers often characterize
that position as tack cut-back and tack cut-up. Please pull this
measurement to insure peak sail performance. This measurement is
never standard and varies from vessel to vessel in the same
model year. It is the easiest of all measurements to take. It is
to the "center of the pin". If the A measurement is too great,
the sail will require a "jack line". The Jack Line readjusts the
luff and foot sail loads. Normally, sails with more than about
5" on the A measurement require a Jack Line.
The distance from the mast to the backstay tells the sailmaker
how to design the roach for your sail. Always include that
measurement so that your sail can have optimum roach. The
backstay measurement is taken at boom level from the aft face of
the mast (not the end of the boom) to the backstay.
Normally, a mast has some rake to it. The tack angle refers to
the angle created at the junction of the mast and boom. If a
mast had 1 degree of rake, and the sailor wanted the boom
parallel to the waterline, the mainsail would need a tack angle
of 89 degrees. If the mast had 2 degrees of rake, the tack angle
might be 88 degrees. However, as the vessel sails, the main
fills with wind and the boom rises a bit, thus when sailing, an
89 degree tack angle might be more like 87.
Vessels that have dodgers often require a reduced tack angle,
such as 86 degrees. If you have a dodger to clear, the best way
to be insure your sail is correct is to pull a leech measurement
with the boom in the lowest safe position to clear the dodger.
The measurement is from the fully hoisted sail shackle position
to the boom at the outside edge of the sail just past the clew,
at boom level, while the boom is in its lowest safe position. If
not specified, we would design a sail with 88.5 degrees of tack
angle. Most mainsails have tack angles of 88.5 degrees.
Reef positions are normally positioned by the sail design
software. If you wish to specify a reef height, that is normally
specified as a height above the boom.
Most common problems with mains are too long a luff or foot
and/or poor tack pin measurements. Always pull your luff
measurement with the sail shackle on the halyard.
When we supply the computer drawing of your sail you will have
an opportunity to double check the luff and foot and adjust if
necessary. Every properly designed sail must go through this
basic process. It is these measurements that generate that extra
knot you want.
We normally supply luff and foot slides and slugs for no charge.
If you wish free slides and slugs, you must specify the ones you
If you have questions, feel free to email us at