Moore on Mainsails - Shore Sails LTD RI
Moore on Mainsails - by Geoff Moore
When you consider that we have been riding around on sailboats longer
than we have been riding around
on wheels is hard to believe that sailboats still offer us a
challenge. Just peer into any modern yacht
designers office and you will quickly discover that the workings of
sailboats are anything but simple. By
comparison, aircraft designers have it easy, although their failures
are a little more catastrophic.
Sailboats have to work in two different fluids with different
viscosity's, and they have to float on a bumpy
surface. Aircraft only have to fly through air. However, both
modes of transport are bounded by the
basic forces of lift, thrust, and drag. These similarities have been
a great advantage to sailboats. An
incredible volume of sailing innovations can be traced to the
aerospace industry.
So with all this high technology entering our sport how can we
mortals hope to keep up? The answer of
course is that we don't need to know everything about sailboat
physics to successfully race a sailboat. So
lets explore what information you really need. More specifically
lets focus on the mainsail because
mainsails are a good place to discover some fundamentals about how
sailboats work.
But first we have to have a little knowledge about masts. Masts come
in all shapes and sizes. They can be
carbon, fiberglass, aluminum, or wood. They can have spreaders,
in-line , swept back, or adjustable.
They can have jumpers, diamonds, diagonals, runners, checks and
permanents. Whatever contraption of
tubes and wires, masts do two things. They hold the sails up, and
when they bend, they affect the shape of
the sails attached to them. Controlling mast bend is the first step
towards controlling the mainsail, and
controlling the mainsail allows us to fine tune the delicate balance
between sails and keels.
In general masts bend too much in heavy air, and they stand up too
straight in light air. So by control we
usually talk of restricting, or inducing mast bend. There are so
many ways to induce or restrict mast bend
that it would be absurd to try to mention them all here. Instead,
lets try to understand what the great
mainsail trimmers are looking for, regardless of how they achieve it.
Most sailors understand that bending a mast flattens the mainsail.
But, how do you know when the
mainsail has exactly the right amount of depth? There are two
steps. First you have to find out what the
best depth is, then you have to have some way of duplicating it.
Determining the best mainsail depth for your boat is a trial and
error process. At some point your yacht
designer tried to place the sail plan (center of effort) directly
over the keel (lateral resistance). But that is
where the yacht designer's job ended and the crew took over. Since it
is virtually impossible to get the
mast and the keel in exactly the right place for all conditions
there will be some inherent design flaws.
Good crews will always find these minor flaws, and then optimize
the sail trim to compensate for them.
For example lets imagine you win the lottery, and you decide to
build a custom 45 footer. New sails were
ordered and the boat launched. Over the course of the season we
might notice that our mainsail trimmer
has gravitated toward setting the traveler much lower than usual. We
might also discover that the boat
performs better if the sheet is eased and the upper leech is
"opened". Using this information I could make
a fairly educated guess that the keel is too far forward, or the mast
is too far aft. We could not utilize all
the power available in our mainsail because when the main was trimmer
for MAX power the boat became
unbalanced. (too much sail power in the back of the boat). So the
boat just sails better with some power
twisted out of the mainsail.
So the following season you might ask how you could improve
performance. One option might be to
move the keel aft. Another might be to move your mast forward.
Moving the mast means all the sails
would have to be recut. So a more reasonable solution would be to
lengthen the headstay a little and then
recut the mainsail to be a little flatter. A flatter main can be
trimmed much harder and will therefore be
more efficient.
My point is that there is no magical sail shape that works perfectly
on every boat in every condition. At
least not in the real world. The only reasonable method of proving
what works is by experimentation.
The difference between a great mainsail trimmer and mediocre one is
the time it takes discover the best
trim.
Since sails are three dimensional objects it is difficult for the
human eye to consistently evaluate sail
shape. So many mainsail trimmers are absolutely obsessive about
things like draft stripes, tell tails, and
number sizes, colors, and positions. They want to make sure that
their carefully trained eye is looking at
exactly the same things.
Luckily for us mortals there are a few more obvious clues. Mainsail
luffs are cut with a curve that has to
approximate the mast bend. When a mast bends about 30-40% more than
the designed mainsail luff
curve the sail will start to develop diagonal wrinkles. These
"overbend wrinkles" are a very consistent and
visible indicator of how much depth the mainsail has at that point.
They serve as a reference point. Any
further bend will only increase the unfairness of the mainsail and
performance will start to drop. At the
point where we start to see overbend wrinkles all the "external
shape" has been pulled out of the sail.
What we are left with is "internal shape". Internal shaping, or
broad seaming, is the shape built into your
sail by putting together curved edges of flat panels. Some
sailmaking companies advertise that they are
accomplishing internal shaping through large three dimensional molds,
but this is misleading. Mylar
film, the foundation of all laminate sails, is only available in flat
sheets. So all laminate sails must have
broad seaming seams, whether they are difficult to see or not.
The other end of the spectrum is a mast that is too straight for the
luff curve. (or a main that is too curvy
for a mast). In this case the indicator is more subtle. The sail
will look too deep and too draft forward.
When this happens the sail has too much external shape. There is
only so much external shape you can
jam into the front of a sail before you end up with a gutter just
aft of the mast.
So the area in between these extremes is the working mast bend.
Every mainsail will have a MAX power
shape and a MAX depower shape. During most sailboat racing the mains
will be in one of these two
settings. When you find yourself in between these settings then
angle of heel, and rudder pressure are the
best indicators of how to trim the main. Constant tinkering with the
mainsail controls can pay huge
dividends in this condition especially if the wind pressure is
erratic.
Once the overall depth of the mainsail is set there are still plenty
of adjustments. Cunningham tension
pulls the draft forward. As the mast bends, more cunningham is
needed to keep the MAX draft in place.
When the mast is straightened the cunningham needs to be eased to
elevate that knuckle forward shape.
Remember the overall depth of a sail is always more important than
precise draft placement. However,
correct draft placement will help reduce drag and also help balance
the boat. As sails age they need more
cunningham tension because the draft migrates aft as the sail
material becomes more elastic.
Outhaul tension controls the lower mainsail, and therefore the lower
batten exit angle. The best place to
view the lower batten exit angle is from the windward side of the
mast looking straight aft. From there
you can make fair assessments about the lower leech versus the center
line of the boat. The most common
mistake is to have the lower leech stalled in light air, but it is
just as devastating to have the lower leech
too open when the traveler is down.
That brings us to the traveler and mainsheet system. These two
controls allow us to adjust the upper leech
or "twist" in the main. It is important to note that a foil with
no twist is more efficient than a twisted
foil. You will never see a keel, or an airplane wing with twist.
Twist works in lumpy conditions because
we are trying to reduce pitching by lowering the center of effort in
our sail plan. In heavy air, efficiency
is not what we are after. We already have all the power we need.
The challenge in heavy air is to reduce
drag. A twisted mainsail is actually less drag than reefed
mainsail.
In light air a straight mast, and the weight of the boom both serve
to eliminate all twist so that the upper
leech stalls completely. In this case the traveler should be pulled
to windward and the sheet eased until
the top batten tell tail starts to fly. Keeping the boom on
centerline up to the point where power needs to
be shed is usually a good plan.
Many boats have booms strong enough to utilize the boom vang instead
of the traveler in heavy air. This
technique is called vang sheeting. Vang sheeting works best when the
traveler system is either too short
or too difficult to control. In very puffy conditions there just is
not enough time to play the traveler and
the sheet. So the vang roughly sets the twist and the mainsheet is
used to keep the boat balanced. One
benefit to vang sheeting is that the extreme vang pressure forces the
boom forward and adds lower mast
bend. This helps to depower the lower mainsail, and opens the slot.
Excessive vang pressure can damage
the mast or boom. The risk can be reduced by easing the vang before
rounding the windward mark.
Mast are built to take a lot of fore and aft load, but they
definitely do not like to be severely side loaded.
Another important consideration is that traveler sheeting allows you
to place considerable more tension on
the mainsheet. This pressure is transferred up the leech of the
mainsail and eventually helps to pull the
sag out of the headstay thereby flattening the headsail. With all
these contradicting forces how do you
know when to vang sheet or traveler sheet?
Traveler sheeting allows you to be much more precise, but it slows
down the reaction time. Vang sheeting
is very quick, but it is a crude way to adjust your twist. Make your
decision based on which priority the
conditions warrant. Often, a combination of both methods works well.
Mainsails are just one component of a modern sailboat. So it is
unwise to trim a mainsail without some
consideration as to how the headsail interacts with it. The most
obvious interaction is "backwind". As
wind flows between the main and genoa it gets compressed slightly.
The Genoa also bends the wind
forward. The effect is that the mainsail is always sailing around in
a slight header relative to the genoa.
As more air gets compressed through this so called "slot" the back
wind increases. Back wind is not
desirable. The sail plan would be much more efficient without
overlapping sails, but without huge bow
sprits there is only so much length to pile on sail area. So some
interference has to be tolerated.
When we start looking for ways to shed power the first obstacle we
hit is that the main can't be eased
because too much back wind results. Eventually there will be so much
back wind that the mainsail will
become difficult to fly. Stiffer battens help keep the leech stable
under these conditions, but once the
mainsail starts flogging performance will drop quickly for two
reasons. One, flogging mainsails generate
a lot of drag. And two, the genoa will drag the bow down and the
whole boat will become unbalanced.
Balance is the overriding goal. If the main has too much backwind
then the balance shifts too far forward
in the boat. So you have to open the slot. Usually this means
easing or twisting off the headsail, but
flattening the main also helps. As conditions get windier a smaller
headsail will become desirable
because a genoa will have to be eased too far.
On well sailed boats there is an intimate relationship between the
helmsperson and the main trimmer.
One reason is that the mainsail trimmer operates the safety relief
valve. When a crash happens all eyes
fall on the helmsperson, but in ducking situations it is really the
mainsail trimmer who controls the boat.
Another reason is that a good helmsperson is constantly testing the
balance of the boat, and it is the
mainsail that fine tunes that balance.
Unlike jibs and genoas mainsails are kept up when we turn down wind.
This deserves some
consideration. Swept back spreader rigs have a special problem
because the boom can not be let all the
way out, and the spreaders poke into the leeward side of the main.
Sometimes the vang can be eased
slightly so that at least the top part of the main can go out all the
way. However, many of the newer breed
of race boats never sail with the apparent wind aft of the quarter.
These light, powerful boats drag the
apparent wind far enough forward to sail reasonable gibe angles. In
their case they get flow across the
mainsail. The important difference in these boats is that the
mainsail has to be trimmed much more
aggressively. The goal is to keep the mainsail at MAX power, or just
before it stalls. For the rest of us the
goal to sailing dead down wind is to create the maximun amout of
drag. The boat with the most drag
mushes fastest. Many boats switch between both these methods
depending on wind and sea conditions,
but that is a topic for a different article.
Good Luck, and remember, practice, patience, and perseverance wins
races.
by Geoff Moore
Owner of Shore Sails Ltd
7 Merton Road
Newport, RI 02840
401-849-7700
F) 401-849-7952
********************************************
* Geoff Moore *
* Shore Sails Ltd *
* 7 Merton Road *
* Newport, RI 02840 *
* 401-849-7700 *
* fax 401-849-7952 *
* gmoore@shoresails.com *
********************************************
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