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Mainsail
Diagram
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1. LUFF
Hook your tape measure up to the main
halyard (along with the head of the old mainsail
if available). Hoist it under normal luff
tension. If your old sail is not available then
hoist to the maximum height you want your
mainsail to stretch up to. If there are bands on
the mast, measure to the band. Then measure down
to the top surface of the boom itself. (Not the
tack pin on the boom). This will be the
“LUFF” (or “P”).
2. LEECH
While the tape measure is still hoisted, swing it
back to the aft end of the boom and measure to
the point where you want your clew ring to be
located. This is the "LEECH".
3. FOOT
Lower the tape and attach it to the out haul line
along with the clew of your old mainsail if
available. Pull the out haul to the maximum point
of draw. Take the measurement from there to the
aft face of the mast. (Not to the tack fitting on
the boom). This will be the "FOOT" (or
"E" dimension). If no old mainsail is
available pull it out to where you want your clew
to stop. (If there is a band on the boom, measure
to it).
4. LUFF & FOOT HARDWARE
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Internal Slide
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Slugs and
Slug boltrope
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External Slide
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Note the shape and size of the slides, slugs, or
bolt rope and how they attach to the mast. These
are easily changed on mainsails but it is helpful
to know anyway. Flat internal and external slides
are measured in width while cylindrical slugs and
bolt rope are measured in diameter.
5. TACK PIN SETBACK
Determine the location of the Tack Pin on your
boom "gooseneck". This is measured in
inches behind the mast and above the boom and is
used to locate the tack ring on your mainsail in
the right place so it doesn't pucker. This is not
necessary for used sails since they are already
fixed. You can add shackles if needed to smooth
it out. This is called "TACK
PIN SET BACK".
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1. MAX. LUFF
Hook a long tape measure up to your jib halyard
(or top furling swivel on a furler) just as if it
were the head ring of the sail. Hoist it all the
way until it stops at the top. Measure down to
whatever point you intend to attach the sails
tack to. This will be your MAX. LUFF dimension or
"MAX. HOIST".
2. "J" DIMENSION
Measure the horizontal distance between the tack
fitting and the mast. This is known as
"J" dimension and will be the number
the "L/P" is compared to for percent
overlap.
To Figure %
Overlap
Multiply the boat's "J"
dimension times the headsail percent you
want. That should be equal to the sail's "L/P."
Luff Perpendicular is the length of a
line drawn from the clew to the luff,
intersecting the luff at 90 degrees.) If
your "J" is 10 feet and you
want a 150% the L/P must equal 15 ft. |
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3. FURLING SAILS
If you have a furling unit, hook the tape measure
up to the upper furling swivel (as if it were the
head of the sail). Hoist the tape all the way up
and measure down to the tack fitting on the
furling drum. For both furlers and foil type
headstays you will need to determine the diameter
of the luff tape needed. (Luff tape is the small
bead that feeds into the headfoil).This can be
measured on an old sail with calipers or a fine
scale ruler. The size is usually is provided with
furling unit documentation too. We do have a
chart that lists most manufacturers sizes.
Measuring
for Spinnakers:
Two dimensions are needed. First the
"J" dimension; the distance from the
tack to the mast as pictured under measuring
headsails. Second, the "I" dimension as
measured from the upper intersection of the
headstay vertically down to the deck at the chain
plates (not the cabin top) also pictured on the
diagram above under "Measuring For
Headsails". The spinnaker luff should
roughly equal "I" and foot should be
roughly equal to "J" X 180%.
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