Development
and Trialling of Mylar Sails - A Progress Report from Frank Bethwaite
The
background:
- At the Japanese worlds I was asked to trial an asymmetric
spinnaker. I did so, found that it didn’t work and
analysed it and realised that it couldn’t work, and
canned the idea.
- From the following ferment I distilled the Tasar Two idea.
Had that run to trial and proved as successful as I believe
it would
have been, the twin classes would by now be growing
strongly with the expectation of a forty to fifty year future,
because
there is nothing remotely like it even now on the market.
Those who
were
so
selfish as to oppose even a civilised trial, look in
the mirror.
- A Melbourne group inspired by Alistair Murray promoted the
Tasar locally to such good effect that interest and
demand from Victoria
has risen from zero to twenty plus, and as a follow-on
I have been asked to re-image the class with mylar sails.
The hope
here is
that this will spur the renewed interest beyond Victoria
and beyond Australia.
This sets the scene for a fourth try.
It is time to explain why I am doing what I am
doing.
My starting point is a relatively small, hard, manually adjustable
rig of extraordinary efficiency in steady air. Its area is right
for 300lb crews. It is too small
for today’s heavier crews.
My brief is to do the best I can with Mylar sails which will
fit on the existing spars.
My intention is to produce the fastest sails possible with present
knowledge within that brief for crews now averaging say 320 to
330 lbs. In the years since 1975 when we signed
off on the Tasar sails we have learned much. Of key importance
to this project are three
factors which we did not know thirty years ago
1. The wind speed fluctuates within gusts by about 18 to 20%
every 5 to 8 seconds. This makes a mockery of manual adjustment,
because manual adjustment for the gust and the lull every minute
or so can access only a fraction of the potential advantage.
This understanding has lead to the automatic rig. The Tasar spars
are too stiff to do the automatic job by themselves, but I will
try to create a rig which will be efficient when sailed by crews
who steer and trim sheet with a light and springy touch like
a fisherman with a big fish on a light line.
2. The turbine blade effect. In considering this let us look
at three factors:
a. The first is that most races are sailed in winds of less
than 12 knots.
b. The second is that what we want from our sails when sailing
to windward in winds of less than 12 knots is higher force,
not best lift/drag. (for the purist, optimum power factor.)
c. The third is that a sail which is set to exhaust into
the area of lowest pressure of the sail behind it acts
like a turbine
blade. The pressure at the leading edge of the jib is greater
than the pressure at the trailing edge (the jib leech)
which is just to leeward of the mast where flow speed is
highest
and pressure lowest. So the flow around the lee side of
the jib is
accelerated and the sail pulls harder and develops greater
force. Julian put the first cuff on the Eighteen footer “Looney
Tunes”. His object was to stop the spinnaker from
tangling around the mast base. The boat bolted. So did
all the other
boats to which we fitted cuffs. It was only some years
later that I
realised that the logic had almost nothing to do with the
mainsail. What was happening was that the cuff was extending
the area
of lower pressure just to leeward of the mast down to the
deck level
of the lower jib, and this lower jib was suddenly beginning
to pull much harder than it can when there is no cuff.
And this
low level is exactly where it serves best.
So I now look at the present Tasar
rig as three levels
d. From the deck up two feet to the boom, there is eighteen
inches of jib which is doing half the job.
e. From two feet to about 11 feet the sails are efficient
and powerful.
f. From 12 feet to the masthead the aerodynamics are good
but the dynamics are too stiff to yield properly.
3. Smoother transition from light air (0 to 5 kts at height
of 5m, laminar flow, with glassy water surface) to breeze (6
kts
plus at 5m, turbulent flow, with rippled water surface.) In the
lab and in the aerodynamicists’ wind tunnels the boundary
layer flow trips abruptly from laminar to turbulent. At the time
I designed the Tasar rig I thought it always tripped abruptly
over the water too, and proportioned the rig accordingly.
I now believe that there is a blending which is dependent on both surface roughness
and temperature. Over a hot rough beach the flow will trip turbulent abruptly,
Over cold smooth water the transition will be smoothed to the point where it
may need up to 8 knots to develop turbulence to masthead height. The practical
effect is that I now think that sails with more area higher will be faster
not just to 6 kts, but often through to 8 kts.
My object with the new rig is:
- That the whole jib from deck to 11 feet and the mainsail
up to 11 feet should be efficient and powerful.
- This two feet
of greater force at the lowest level should enable all crews
to sail faster and higher in winds up
to 12 knots, and determined crews to windward plane as a
routine in strong conditions with the upper sails twisted off.
- The upper mainsail will look like the top of a 49er mainsail.
Because the topmast is too stiff we will have to fudge
the flattening with battens, but we have done this before.
- When vang sheeted with light sheet tension the broader
upper sail will yield if it is given a chance with
light springy
muscles. It is possible that an internal halyard will
make a detectable
difference. I am advised that it often does - we will
never know if we do not try.
- Offwind and downwind the extra area up high will give
more speed particularly in light unsteady conditions.
It will of
course benefit heavier crews.
All positive suggestions welcome. If they are negative - stick
them onto your mirror.
Frank
Notes:
- This development project is line with the
motion passed
at the World Council meeting in Victoria in 2003, which endorsed
the establishment of a development committee and investigation
of possible changes to the
Tasar, followed by a report to the World Council. Any
changes to the class rules will go through the normal approval
process,
which now includes approval by ISAF.
- Franks thoughts on how to modify the rotation lever to work
with the cuff are here.
Richard Spencer
2005/02/05
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