Bowers MC Tuning Guide
Bowers MC Tuning Guide
Most recent update: 8 June 96
Congratulations on your purchase of a Bowers MC sail! We all know racing is more enjoyable when you are going fast. To achieve superior boat speed , we have developed fast sails that are easy to set and trim for changing wind and wave conditions. This sail design and tuning instruction will help you go fast and have extra time to concentrate on tactics and strategy.
Table of Contents:
Spreaders
Johnson and Melges spreaders are different lengths. Johnson spreaders
are 15.25" from the mast attachment point to the hole for
the sidestay. Melges spreaders are 16.50". The trend is
for less spreader sweep aft. Measure from a straight line connecting
the spreader tips (you can use either of your long battens turned
on edge) to the back edge of the mast. Johnson's sweep measures
4 "to 4.50 " while Melges's is 6.5" to 7"
due to their longer spreaders. Less spreader sweep aft results
in a stiffer, more controllable mast in medium and heavy winds.
Light air bend can be promoted by using loose sidestays.
Mast Rake
Measure your mast rake by attaching a steel measuring tape to
your main halyard, and hoisting it to the maximum height locked
position. The distance from this locked position to the intersection
of the transom and deck should be between 28' 4" and 28'
2". If overpowered in heavy air, rake back to 28' 2".
Sidestay Tension
First, make sure your sidestays are equal by hoisting a steel
tape measure, locking the halyard, and measuring with firm tension
to the deck on both sides. We recommend tight sidestays in winds
over 8 mph. You should use the Loos Tension Guage Model A to
get a reproducible number for sidestay tension. The best all-purpose
settings are between 260 and 300 pounds. However, if your spreaders
push the center of your mast to leeward in heavy air the boat
will feel jumpy and over-powered. Rake aft 1" - 2"
and/or loosen the sidestays by 1/4"to 1/2". Less sidestay
tension can also be faster in light air of 1-6 mph. You should
use settings that allow the leeward sidestay to be slightly loose
when sailing upwind. A loose rig puts less tension on the spreaders
and helps to bend the mast in light winds where all MC sails have
excessive luff curve and a draft-forward shape.
Outhaul
Outhaul tension controls the amount of shape and power in the
lower quarter of your sail. Upwind in light air and smooth water
the outhaul should be tight enough to flatten and open the lower
leech. But, if there is choppy water caused by other MC's or
motor boats (more waves than wind) ease the outhaul slightly to
get a deeper, powerful shape.
In medium air and flat water, tighten the outhaul again to open
up the leech. This shape lets you trim harder without closing
the whole leach. A crease along the boom will appear when you
have enough tension.
For medium or heavy winds when there are large waves, ease the
outhaul slightly to get a slightly deeper sail down low. You
can often use a full foot at the wavy bottom of the race course
and a flat foot as you approach the weather end where the water
is flatter.
Reaching and running, ease the outhaul 1"-2" to get
the fullness and power into the foot.
Traveler
Mainsheet tension and traveler position are related and should
be thought of as helping to position the
boom relative to the boat's centerline. The traveler car should
be centered and the sheet should be eased to position the end
of the boom toward the aft corner of the boat. The resulting
twisted leach will accelerate easily in the little wind available.
In medium winds (7-13 mph), keep the traveler on centerline and
trim hard once you are going fast to point high.
Once the wind is above 13 mph, single or groups of boat-stopping
waves should be avoided by heading up or down slightly. For those
wave sets which you cannot avoid, drop your traveler 2" -
4" and ease sheet slightly to build speed before you cross
these large waves. After you are over the waves and your speed
is back up, pull the traveler back up to center and trim to maintain
good pointing. Remember, drop the traveler only if you are already
hiking hard and using the correct mainsheet trim.
In very heavy air with large waves you can drop the traveler as
much as 6" to keep speed and the proper angle of heel. Below
that point the MC just will not sail fast enough to make up for
the extra distance sailed with a low traveler position.
Mainsheet Tension
Correct mainsheet tension is one of the biggest subjective variables
in sailing. It helps to become more sensitive to changes in both
wind strength and direction and their effect on your boat's heel
angle. Sheet tension, traveler position, vang, outhaul and Cunningham
tension are all interrelated. Yet, once these variables are "in
the ball park", correct mainsheet tension accounts for 80%
of your sail trim attention span. The general rule in medium
wind is to trim so that the last 10" of the top batten is
parallel to the boom. Unfortunately, you need to look up at the
sail from under the boom - a difficult position when sailing single
handed! Take your crew out in medium air for practice. They
can steer while you look at the batten/boom relationship from
the cockpit and halfway back under the boom. Then mark your sheets,
go back to the skipper's position and try to memorize the look
of the sail at the same wind speed you observed from the cockpit.
The wind will change up or down and you will trim and ease accordingly,
but the shape of the leech should remain the same. If you trim
too hard for the wind strength, an inversion wrinkle will develop
running from halfway up the luff back at around forty-five degrees
toward the clew. The mast is now bending too much for the sail's
luff curve, your power source is cut in half and boats around
you will be going faster. The cure is to ease mainsheet tension
until the inversion disappears and tighten the sidestays (both
of which will straighten the middle of your mast).
Exceptions to the parallel rule exist in very light and heavy
air. The best performance results from trimming the top batten
angled from five to fifteen degrees outboard of the boom. However,
in very light winds it may be impossible to keep the upper batten
from hooking to weather because of the weight of the boom and
lack of wind pressure on the leech. A second exception is in
medium air and choppy conditions when you want to accelerate quickly.
Ease your mainsheet approximately 3" from the reference
mark to again open the upper batten slightly. Easing straightens
the mast and makes the main fuller.
You can also use the top leech telltale as a guide to proper sheet
tension. In light air it is often hard to get the top leech telltale
to flow straight back so try to "key" off the second
batten telltale. In medium air (6-12 mph) the top telltale can
be stalled as much as 80% of the time once you have accelerated,
trimmed hard, are pointing high and going fast. Conversely, in
winds above 12 mph the MC's top telltale will almost never stall.
Think of the mainsheet as your accelerator! You should ease
to accelerate around waves and after tacks. On the other hand,
trimming hard is like your car's third gear - to be used when
you are already going fast to help you sail even faster and higher.
Boom Vang
Up to around 12 mph usually no vang is necessary. Just trim the
main normally and take up the slack in the vang just in case you
need to ease at the high end of the medium wind scale. Above
12 mph, if you cannot sail between 15 - 20 degrees of heel, tighten
the vang hard to kick the spar forward and flatten the sail.
Cunningham and outhaul pressure should also be increased . The
added vang tension now controls mainsail twist so you do not need
to adjust the traveler (which will be set from on centerline or
6" to leeward, depending on the wave height and wind speed).
This technique, called vang sheeting, bends the mast proportionally
more down low so it is useful to look at how wrinkles develop
in the lower third of the sail. Excessive sheet tension causes
long inversion wrinkles starting two to three feet above the spreader
and running almost to the clew. With correct vang sheeting it
is fast to have diagonal wrinkles from about two feet below the
spreaders to about two thirds of the way back on the boom. This
shows you are getting maximum low bend for the sail's design.
More Cunningham tension will reduce some of these vang sheeting
wrinkles. Used properly, vang sheeting allows you to ease sheet
in strong gusts while maintaining upper leech tension. It also
speeds your boat's response time so you can keep sailing at a
constant angle of heel as conditions change.
Boards
Upwind in light to medium conditions, the top of the leeward board
should be within 1" of the deck. You can pull the top of
the board 2 - 3" above the deck to reduce weather helm in
heavy air.