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Mylar
sails
Notes to test sailors from Frank Bethwaite.
17th March 2005
The design of the Tasar rig of 1974 was driven by –
• The then-new ability to adjust sail shape and the awareness that
crews who adjusted on a gust by gust basis sailed faster than those
who did not adjust.
• The belief that while the wind speed within gusts was stronger
than in lulls, the wind speed within gusts and within lulls was
relatively steady.
A group of us researched mast and sail shapes in wind tunnels ashore
and in the steadiest winds we could sail in on the water. The resulting
rig is flexible enough and controllable enough to be manually adjustable
over a wide range of camber. It is too stiff to yield significantly
to change of wind speed. It is probably as good as any and superior
to most contemporary rigs in steady air.
In the years since 1974 we have learned -
• That the wind speed changes within gusts and lulls every few seconds.
• That the wind direction also changes frequently.
• That the changes in wind direction are relatively small when the
air is cooled, and much greater when it is heated. (Water or adjacent
land warmer than air.)
• That rigs which are flexible enough to begin to yield automatically
to the changes in wind strength and changes in wind direction sail
faster than more rigid rigs.
• That rigs which yield and flatten from the top down sail faster
- sometimes much faster - than rigs which yield first in the middle
- and
• Both children and adults eat more and exercise less than they did,
so weigh more than they did.
My response to the request to re-image the Tasar with mylar sails
is to move as far toward the 59er or Byte C2 rig as is possible
with sails which will bend onto the present spars.
•
I have put together a mast which is non-standard in that I have
put nyloc nuts and washers on the diamond stay adjuster screws
inside the mast just above the mast base fitting so that the diamond
stays can be run as slack as desired (within reason – say
diamond stays to mast 6 to 9 inches above whisker pole fitting)
without fear that the screws will drop and project below the mast
base fitting and foul the screw heads of the mast step.
There is nothing I can do to make the topmast as flexible as I
would wish, but allowing the lower mast to “work” rather
than be held rigid may make a significant and favourable difference
to the flexibility of the whole rig. We will see.
• I have put together a boom with a rotation stop notched so that
the cage will remain in the slot when the mast tries to anti-rotate,
which it always does with a slack sheet.
I would encourage crews who trial the new rig to start by sailing
as they always sail, say for the first race. (Immobilise the lower
mast by tying the diamond stays together.) Then progressively try
sailing with a more flexible mast and the traveller further to
windward, a slacker sheet and sufficient vang (constantly adjusted)
to control twist and to trim the upper leech, and to hold the sheet
more lightly on springy muscles.
The leech of the mainsail will become much more mobile - like a
49er. Trialing will establish whether there is performance difference,
and if so what is its degree, in unsteady and rough winds.
Comments about ease or difficulty of handling, as well as about
relative performance, will be welcome.
If there is a consistent performance difference, it will presumably
be associated with an optimum lower mast bend (diamond stay tension.)
This, once established, may call for adjustment to the luff curve.
Once we have these factors optimised and handling pleasantly, it
will be time to trial the cuff. This, I believe, will primarily
affect windward-going performance, particularly in stronger winds.
Trial Kit.
I have put together -
Mast and topmast and halyard with vang and downhaul as above.
Boom with whisker pole ears aft and shock cord forward.
A fixed forestay.
Jib halyard arrangements.
Mainsail, jib, both battened.
Feather.
Tester provides –
Boat,
Shrouds (may need to adjust)
Jib sheets, mainsheet.
Whisker pole.
Pencil, paper.
Frank Bethwaite
Posted 2005/03/21
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