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We have seen in events like the America«s Cup or the
Admiral«s Cup that sailmakers are taking a more and
more scientific approach in their sail development-programs.
WB-Sails has studied 470-sails with the help of the
most modern computer tools.
This article first appeared in "470 Times" in June
-92.
WB-Sails chose the 470-rig as one of the first testing
grounds for their aerodynamic analysis-program MacSail
for several reasons: Firstly, with our long experience
of the 470-class we were confident that the shapes of
these sails represented well today«s state-of-the art.
Also, we had accurate shapes available through normal
photography as well as photogrammetry. Another important
factor was that the level of racing in the 470-class
represents (both internationally & nationally) the highest
standards.
The MacSail-calculation
program is based on a system called "vortex-lattice
method". In this method, the sails are replaced by a
3D-grid, dividing both mainsail & jib into approximately
100 elements. Flow velocity is calculated in the center
of each element, on both leeward and windward sides
of the sails, taking also into account the effect of
the jib wake (backwind) on the mainsail.
From flow velocity, the pressure can be deduced, and
by adding the windward side pressure to the leeward
side suction, the pressure difference in each element
can be obtained. This pressure acts perpendicular to
the element surface, forming a small force vector. Summing
up all these small vectors in each element gives the
magnitude and direction of the total force acting on
the jib & the mainsail.
An example of the results can be seen in the
illustrations below. These examples are calculated
in a wind of approximately 6 m/s (13 knots):
Jib Main Both Driving force in kp 10,5 11,5 22,0 Heeling
force in kp 28,5 55,5 84,0 Heeling moment in kpm 58,5 174,0 231,5
As you can see, the jib seems to contribute much more
(relatively) to boatspeed than the mainsail, but don«t
get it wrong: it only does this because it is working
in conjunction with the main. In fact, the jib is receiving
a strong "lift" from the main, while the mainsail is experiencing
a bad header due to the jib (the jib, after all, is in
a "safe leeward position"). Take the main out, and the
jib will lose its magic. Note how very small the actual
driving force pulling the boat forward is, some 22 kilos.
In this light, it is easy to understand, how important
every little detail, from aerodynamic clothing to an open
bailer or dragging spinnaker sheet, not to mention hitting
a wave badly, is to getting the maximum out of your boat.
470 Main & Jib
Pressure difference at
- apparent wind speed 8 m/s
- apparent wind angle 23 degrees
For the purpose of the calculation, the sails are divided
into approximately 100 elements each. Different shades/colours
represent a different surface pressure according to the
table on the left. You can tell from the colours that
the pressure is far greater in the upper part of the sails,
and even more so in the jib than the mainsail. This is
partly due to the triangular shape of the sails, and partly
to the intensional shaping of the sails to minimize vortex
induced drag.
The jib represents 40% of the total force of the sails,
although it is only 30% of the area. Nearly 50% of the
forward drive comes from the tiny jib.
Flow velocity on the leeward side
The difference in flow velocity on the windward &
the leeward sides creates the pressure difference. The
pressure difference is the cause of the driving & heeling
forces excerted by the rig onto the hull. Here we can
see the calculated flow velocity on the leeward surface
of the sails , in a free stream velocity (apparent wind
speed) of 8 m/s.
A flowchart such as this gives the saildesigner a
wealth of useful information. You can see, how the flow
is accelerated on the surface of both sails so that
the speed of the flow is nearly everywhere greater than
the wind speed. Near the head of the sails the speed
is almost doubled, ie. 14-15 m/s. The influence of the
jib can clearly be seen on the main surface - the flow
is decelerated in the slot between the two sails, not
accelerated, as you often hear claimed.
The velocity on the surface of the jib is larger than
on the main, and even in most of the leech area it is
higher than the wind speed. As a result, the flow is
less prone to "separate", and thus the jib can produce
a greater drive in conjunction with than mainsail than
it would alone. The flow separates, if it is forced
to slow down too much too fast. The jib maintains higher
velocities all the way down, because the foot lies on
the deck, while in case of the mainsail, the pressure
difference tends to even out under the boom, thus making
the lower part of the sail less efficient.
The flow has to slow down to the free stream speed
in the leech of the mainsail, which makes it prone to
separation. Separation always occurs first in the head
of the sails, where acceleration & the following deceleration
is big. In the front part of the main, flow velocity
is nearly the same as wind speed - there«s hardly any
pressure difference, the sail is close to backwinding.
Although calculations such as these give a wealth
of useful information to the saildesigner, it would
be wrong to expect a miracle shape to come out of the
computer as a result. We can only optimize the sails
for one given wind condition at the time - and although
we can simulate mast bend and forestay sag in the computer,
the true art and skill of the sailmaker comes out in
the intricate balance between the seam-induced, moulded
shape and the luff curve shape, combined with the rig
characteristics and the fabric strech, to produce a
suit of sails versatile enough to power the boat from
the lightest zephyrs to the choppy seas in heavy breezes.
So, even in this computer-age, the ultimate proving
ground of a new suit of sails remains the race course,
the way it should be.
Copyright © 1995 WB-Sails Ltd. All rights
reserved.
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