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©
2008 Capital Blokart® Club |
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website
designed by
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battens
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recommended skill level for this guide |
advanced |
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A
blokart® is a highly efficient way of transforming
sail power into moving speed. Most blokart® sailors
are quite satisfied with the power available from
their rigs in even moderate winds. In heavy winds
many sailors find that power quite difficult to keep
under control. However, there are always those times
when we try to find a bit more power and a bit more
speed. If you sail in light winds, or if you just
like the speed of a blokart®, then you are probably
looking for another place to crank on a bit more
power. Your sail trim is the most obvious place to
look.
The
most dynamic controls over sail trim when in the
blokart® are mainsheet and downhaul. However, when
you set the battens into your sail each day you are
already making the first adjustment for sail power,
even before you get into the kart. Does the
conditions today look like you need mild batten
load, or wild batten load? Batten tapering adds
control over another aspect of sail trim.
The
blokart® battens are a heavy design profile, probably
reflecting the desire to be durable. Because the
blokart® battens are a class standard nobody has any
say in what they get with the blokart®. They are not
the best design for racing, but if you want to race
in production blokart® events you are stuck with
their battens.
Batten design has become very high tech over the
years. Since the blokart®
was designed, batten
materials are lighter, have different ratios of
resin to glass, different types of resin, and
different profiles for different applications.
The
stock blokart® batten shape at left, and examples of
other profiles commonly used in sail battens.
The control of the sail which full length battens
give is quite significant and the greater use of
battens in sail design should be seen as indicative
of their potential for performance tuning. On your
blokart®
varying the batten taper and load has a
profound effect on the power of the sail, even with
the old stock blokart®
battens. |
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so how do battens make so much
difference?
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Sail curvature, or camber, produces the power in a
sail. The camber is designed into the sail by the
sail maker according to the type of boat, expected
wind range, and mast bend characteristics. The point
of maximum camber is called the drive point and is
expressed as a percentage of sail width fore to aft.
Whether the sail is trimmed full or flat the drive
point of the sail will remain in approximately the
same relative fore/aft position for similar wind
conditions.
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Batten 1a.
Circular shape, full trim. Drive point
centred in sail.
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Batten 1b.
Circular
shape, flat trim. Drive point remains
centred.
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Batten 2a.
Airfoil shape, full trim. Drive point well
forward of centre. |
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Batten 2b.
Airfoil shape, flat trim. Drive point
remains forward.
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The goal of sail design and trim is to have
the airstreams attached to the leeward side
of the sail for the greatest distance
possible. When the airstreams has to turn
too sharply around the bend in the sail it
loses contact with the sail (delaminates),
causes turbulence, and stalls. The wind will
attach best to a sail which curves smoothly
at a constant radius, a circular arc.
However, more lift comes from a true airfoil
shape which is not circular but elliptical
and having varying radius. A circular arc
will hold the wind better but gives less
lift. An airfoil shape gives more lift but
loses the attached airstreams (stalls) more
easily under varying wind conditions.
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Untapered
Batten.
The drive
point is centred. Actual trim under wind
load will depend on the cut of the sail. |
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Lightly
Tapered Batten. The
drive point is pushed forward close to where
it matches the design of the sail. |
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Heavily
Tapered Batten.
The drive
point is further forward and now overrides
the original cut of the sail. |
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Although an airfoil shape
gives more power it can stall more
easily. When an aircraft stalls it has
the advantage of being able to increase
engine power. But a blokart® only has the
power of the wind across the sail and in
a stall that is nil. This means that the
sailor has to be watchful when
approaching the stall zone. A sail which
has inappropriate batten taper or
tension will stall more readily.
When a sail is under wind
load the drive point is mostly forced
aft. The cut of the sail might put the
maximum camber at 40% but the wind load
can force it to 50% or beyond. This
means that the power of the sail is
reduced.
The airflow will always
try to control the shape of the sail.
However, the sailor wants the shape of
the sail to control the airflow, that's
where power comes from. The battens
extend the range of control over the
shape of the sail beyond what is
possible by sail cut alone. Efficient
trim allows the sail to fall into a
shape suitable for holding the
airstreams yet still giving maximum
power to the blokart®.
The wind in the sail tries to move the
drive point closer to the rear of the
sail, ie 50% or more aft. By having the
battens tapered closer to 35% or
40% they counteract this influence and
return the sail to the shape the sail
maker wanted. |
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let's look at tapered battens
Tapered battens have the forward end thinner than
the aft end. Where a batten starts to taper, and how
quickly it varies in thickness, is up to the
designer. When the batten is loaded it bends more at
the forward end and the aft end stays almost
straight.
Battens are measured in: 1) length, 2) material
stiffness, 3) profile, 4) deflection load, 5) drive
point.
1) Length
This is as you imagine it, a little
longer than the pocket. |
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2) Material stiffness
Some resin/glass combinations are
stiffer than others. Bigger yachts
generally use stiffer sections, even
if they are smaller profiles. They
can have different percentages of
glass (65% is high glass content),
or be made of polyester resin, vinyl
ester resin, plastic, or something
else. |
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3) Profile
This is the cross section shape/size
of the batten. The blokart® batten
profile is circular. Others can be
thin or fat, round, oval,
rectangular, like an extended plus
sign, or a similar variety of hollow
sections. |
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4) Deflection load
The deflection load is the
pressure required so that an
increase in curve requires no
increase in pounds pressure when
supported on a horizontal surface.
When a load is put on
the end of a batten it bends. You
already know that. But how do you
measure that bend?
Take your batten, lay
it edge upwards (as if in the
pocket) on the floor with the
forward end touching the wall. Get a
fishing scale which measures up to
ten pounds. Hook the scale over the
free end and drag the batten
straight against the wall. As it
bends take notice of the reading on
the scale.
After it has bent
about one foot off line the scale
reading will not increase, although
the batten will continue to bend
more and more. This takes some
people by surprise. It seems
reasonable that more bend requires
more load. But that is not the way
it is. When the maximum deflection
load is reached, any increased bend
comes free of effort.
Sometimes a light batten might bend
initially at one load, and more bend
thereafter takes less load.
The deflection load
happens quite quickly. The batten is
only dragged in a few inches and you
are there. After that you can drag
the scale on the batten for more
bend but it won't register any more
load on the scale.
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Batten under load. The
deflection load is read from
the fishing scale. The drive
point is indicated by the
T-line. |
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5)
Drive Point
Drive point is
expressed as a percentage figure.
The percentage refers to the
distance from the forward end of the
batten to the point of maximum
camber when compressed to the
deflection load, relative to the
batten length.
To measure the drive
point of the batten drag it with the
fishing scale as before until the
batten bends and gives you the
deflection load. Now let out the
scale until the batten us almost
straight. Now drag it in slowly
until the earliest point where the
deflection load is reached. Measure
the distance from the wall (forward
end of batten) to the point of
maximum camber. This is the nominal
drive point. Calculate that as a
percentage of compressed batten
length.
The batten therefore
has a drive point of 35% and a
deflection load of 3 pounds.
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doing the work
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A
non-tapered batten will have a 50% drive point
because its maximum bend will be in the very middle.
When a batten is tapered at the forward end, the
drive point will move forward because the thinner
material bends more easily. If it bends really
easily then the deflection load will be low. If it
bends very close to the front then the drive point
will have a low percentage figure.
The
tapering of a batten will control the drive point
and the deflection load. Good batten design matches
these two values with the blokart® and the type of
sailing. Batten suppliers will taper new battens to
suit any application. But you can taper your own
battens to suit these two controls just by sanding
it fairly evenly on each side with an orbital
sander. Wear a good dust mask! A dose of resin dust
in the lungs is quite painful for a day or two.
When you taper the battens you only need to sand the
front two thirds of the batten. Start by sanding the
forward half then increase the sanded area aft as
necessary. You will be measuring the deflection load
as you work so finding how far aft to sand is no
problem. The upper battens might require a longer
sanded area than the lower battens as their shorter
length means they are harder to bend, therefore they
are a thinner section overall.
Battens in different parts of the sail have
different drive points and different deflection
loads. Lower battens have the drive point slightly
more forward than upper battens. Lower battens have
a lighter deflection load. |
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getting out on the track
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When you prepare to sail you give the battens a
certain pre-load when you set each batten's webbing
lock. Different wind conditions require different
loads. To be able to look at the conditions and set
the battens is an important skill to learn. No
matter how much time you have put into preparing
them, your battens will respond significantly to how
you use them on the track.
Generally, in light air put heavier pre-load on the
battens. In heavy air, light pre-load. The amount of
pre-load will depend also on your body weight and
sailing skill. Lighter or less skilled sailors use
lighter pre-load, heavier sailors need the extra
power of a heavier pre-load.
Although the battens influence the drive point at
their own locations, the downhaul, outhaul, and
mainsheet settings have control of the drive point
when sailing. Heavy downhaul moves the drive point
forward quite significantly, just as mainsheet
tension varies the flatness of the sail. This range
of adjustment on the track builds upon the in the
pits decisions which you have already made
concerning batten taper and pre-load.
The control of the sail on the track is a broader
issue altogether. You have done enough reading, go
sailing! |
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