By Kim Miller. Wagga Wagga, Australia.
(First published in “On The Wire, 1996)
A catamaran is a highly efficient way of transforming sail power into
boat speed. Most Hobie sailors are quite satisfied with the power available
from their normal rig 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 catamaran,
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 on the water 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 on the water. Does the water today look like you need mild batten
load, or wild batten load? Batten tapering adds control over another aspect
of sail trim.
The Hobie battens are a heavy design profile, probably reflecting the
desire to be durable. Because the Hobie battens are a class standard nobody
has any say in what they get with the boat. They are not the best design
for modern racing, but if you want to race in official Hobie events you
are stuck with their battens. After all, “This is the way we do it
here”, say the guys who sell the stuff.
Batten design has become very high tech over the years. Since the Hobie
was designed, batten materials are lighter, have different ratios of resin
to glass, different types of resin, and different profiles for different
applications. It is from developments in catamaran sail design that most
monohulls have now accepted the advantages of full length battens and having
lots of them.
The stock Hobie batten shape, and examples of other profiles commonly
used. (Not to scale)
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 my old Hobie 14 Turbo varying
the batten taper and load has a profound effect on the power of the sail,
even with the old stock Hobie battens.
So how do battens make so much difference?
Sail curvature, or camber, produces the power in a sail. The camber
is designed into the sail by the sailmaker 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.
Batten 1a. Circular shape, full trim. Drive point centred in sail.
Batten 1b. Circular shape, flat trim. Drive point remains centred.
Batten 2a. Airfoil shape, full trim. Drive point well forward of
centre.
Batten 2b. Airfoil shape, flat trim. Drive point remains forward.
The goal of sail design and trim is to have the airstream attached to
the leeward side of the sail for the greatest distance possible. When the
airstream 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 airstream (stalls) more easily under varying
wind conditions.
Untapered Batten. The drive point is centred. Actual trim under wind
load will depend on the cut of the sail.
Lightly Tapered Batten. The drive point is pushed forward close to
where it matches the design of the sail.
Heavily Tapered Batten. The drive point is further forward and now
overides the original cut of the sail.
Although a 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 boat 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 airstream yet still
giving maximum power to the boat.
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 sailmaker wanted.
Let’s Look at Tapered Battens.
Tapered battens have the foreward 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 foreward
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.
2) Material stiffness.
Some resin/glass combinations are stiffer than others. Bigger boats
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, vinylester resin, plastic, or something else.
3) Profile.
This is the cross section shape/size of the batten. The Hobie batten
profile is like a rectangle superimposed on an ellipse. Others can be thin
or fat, round, oval, rectangular, like an extended plus sign, or a similar
variety of hollow sections.
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.
Batten under load. The deflection load is read from the fishing scale.
The drive point is indicated.
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.
My lower batten is 2390mm long. When compressed to deflection load it
is 2340mm long and the drive point is 810mm from the forward end. This
represents 35% of the compressed distance. The load it takes to get there
is 3 pounds.
The batten therefore has a drive point of 35% and a deflection load
of 3 pounds.
Doing the Work
If you check out a Hobie from a commercial hire place you will see it
(probably) has different battens from your own. (Unless yours is an ex-hire
boat) Hire boat battens are generally non-tapered and are made of cheap
but durable plastic which comes off long rolls. They are made to be cost
effective for knockabout sailing but have little performance enhancement.
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 boat 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.
* The drive point for most Hobies will range between 33% for the lower
batten to 45% for the upper batten.
* The deflection load will range between 2 to 4 pounds for the lower
batten to 3 to 5 pounds for the upper batten.
These figures will vary for different boats and different sail designs
but they are a guide to what is “within normal range”. Check
out the requirements for your boat from a local sailmaker.
My H14T battens are as follows. I have the older style 5 batten sail.
Batten
Length
Loaded Lgth
Drive Point
Percentage
Deflect.Load
1
920mm
880mm
350mm
40%
3.5 pounds
2
1510
1460
580
39
3.5
3
1940
1850
710
38
3
4
2210
2100
780
37
3
5
2390
2340
810
35
3
These figures come from the advice of a national champion cat sailor
and the batten maker who makes the battens for Hobie Australia. I suspect
that the deflection loads could be lower for the light winds of my local
lake. Even down to 2 pounds. However, my sail, though in good condition,
is old and has probably stretched a bit over the years. Slightly heavier
battens keep it flatter and counteract the tendency to belly out under
load.
You can see that the upper batten has a heavier deflection load that
the lower one even though it is made of the same original material. Even
sanded full length to a thinner profile it is a heavier batten (measuring
deflection load).
Getting On The Water
When you prepare to sail you give the battens a certain pre-load when
you set each batten’s V-lock. Different wind conditions require different
loads. To be able to look at the water 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 water.
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 water builds upon the on-shore decisions
which you have already made concerning batten taper and pre-load.
The control of the sail on the water is a broader issue altogether.
You have done enough reading, go sailing!
Kim Miller