Tuning a One Metre Rig - by Ben Morris
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Not another article on tuning a one metre (IOM) RC yacht you
may ask? Well yes, but in this case the tuning follows the path of a
real sailor attempting to get the best from his IOM (me actually).
This is because I tend to build my own yachts and often the plans do not
include anything but the barest essentials about rigging and setting up the
yacht to actually sail well. I chose (well really my son chose) a
'Triple Crown' design from the early 2000's as it was available free on the
Internet and looked OK i.e. nice sharp entry and good flat run aft.
Regardless, my son built it in balsa, I made a mould and produced a couple
of fibreglass hulls. I even made a deck mould to give it a smooth
professional look.
The plans showed the key issues of mast and keel positions
which are critical to the balance of the yacht while sailing but gave no
information about rigging. I hunted up information on the net and
found a rigging plan for a TS2 and copied that. Perhaps this was the
start of my problems as the TS2 is a wide shallow hull with lots of form
stability and the ability to carry sail well into the top of the wind range
for a particular rig whereas the Triple Crown' is a relatively narrow hull
with little form stability and tumble-home on the middle-rear sections which
detract even more from the stability when pressed. I made my sails
based on experience with my marblehead (which is in fact a stretched Triple
Crown) - perhaps my second mistake! The marblehead has much more
stability due to its deeper and heavier fin/keel and a shroudless mast with
its built-in automatic depowering in stronger winds where the mast bends to
leeward twisting off and depowering the top sections of the sail.
Nevertheless, I was quite impressed with the Triple Crown
especially in the light winds and definitely downwind where it could match
it with any One Metre I have sailed against but in the mid and top wind
range of every rig, it suffered lacking drive and heeling more than most of
the other yachts. At first I thought it was just my technique as the
IOM has to be controlled much more than the bigger yachts like the
marblehead or ten-rater.
I proceeded methodically to deal with this issue and tried a
variety of fixes to improve the heavier weather performance. I tried
all the usual things - flattening the sails, easing the sheets, increasing
the mast bend etc etc. These all helped to the point where I was at
least at the back of the leading bunch most of the time.
Clearly tho there was a problem with the power in the sails.
They were too powerful and still tended to heel the boat too much. So
back to the building board where I made a set of sails that still had a
reasonable amount of drive down low but the curvature of the top two seams
in the mainsail (top one in the jib) were reduced considerably. At
last this seemed to get the right profile. Now when viewed from the stern
the top sections of the sails could be twisted off and depowered in
any sort of breeze and be flat enough not to backwind and flog. It
proved to be the case and the yacht performed a little better and at least I
could keep in touch with the top boats and feel part of the fleet.
I was still looking for more speed though and started once
again to read the copious and helpful pages from
Lester Gilbert and especially the page on short spreaders. The
last two diagrams on that page show clearly what happens with the normal rig
on a one metre. When healing, the normal rig causes the mast at the
spreaders to be pushed to leeward and forcing the top of the mast to curve
back into the wind. The effect of this is to increase the drive in the
middle and top portions of the sail exactly what is not needed! In a
full sized yacht with single spreaders, there is almost invariably a set of
lower shrouds attached to the mast at the spreaders. This prevents the
mast from falling to leeward and allows to top of the mast to act separately
falling to leeward if desired and depowering the top of the sail.
These lower shrouds are no allowed in the IOM rules so how was I to deal
with this problem. I did not like the short spreader answer suggested
in Lester's article as it explains other problems which result but perhaps
simply reducing the length of the spreaders might help. I checked this
out with my yacht on it's stand by using a mirror to sight up the mast when
the wind was in the higher range and sure enough, the middle of the mast was
pushed to leeward and the top to windward.
I boldly reduced the width of the spreaders to about 63% of
the hull (120 mm spreaders 190 mm hull), realising now that the length of the spreaders I had been using
were for a TS2 with a much greater hull width. Once more sighting up
the mast showed that the effect was dramatic. The mast now stayed more
or less straight in the gusts! Now I was thinking! I realised as
well that the shrouds I used went quite a way up the mast about the same
distance down from the forestay attachment as the sail went above. If
I reduced the height of the shroud attachment and dropped the spreaders as
well the bottom of the mast should be well supported and allow the top
section to flex off to leeward a little in the gusts. So I lowered the
shroud attachment by 100 mm and the spreaders by about 80 mm and adjusted
the shrouds accordingly. Once more onto the stand, sight up the mast
and yes! the mast remained straight but when the gust hit the lower section
remained straight but the top section of the mast bent to leeward a little.
The effect on the sails was significant. With a steady wind the sail
would sit nicely with the leech twisted off about 40mm from the centreline
but when the gust hit, the top of the sail responded by immediately twisting
off and depowering the rig. The lower half of the sail was still
supplying full power exactly where the drive should be in the heavier winds.
It would seem then that by experimenting with a variety of
shroud lengths and position of the shroud attachments, a rig can be designed
to be as stiff or soft as you wish. Maybe the rigidity of the mast
section plays a part here. I have been using the higher strength
aluminium section which is certainly more rigid than the normal aluminium
and this required (allowed) me to lower the shroud attachment and spreader.
But did it improve my heavy weather sailing ability? I will see
tomorrow!
Well, did it work? Actually it did. In the light
weather the yacht was and still is competitive. This I would have
expected as little changes in the light breezes - no mast bend etc.
when the wind picked up to 10 knots plus the mast bend effect was clearly
evident as the top of the main twisted off well and the yacht had a much
more livelier feel to it, punching through the chop etc and pointing just as
well. it is certainly not a world beater but there is a definite
improvement. Now I seem to have a problem with the middle range of
wind strength. Not that the problem is different from before but the
improvement didn't show until the upper range. Perhaps I need to
soften things down even more so the twisting effect begins earlier on both
main and jib. Ah well, there's always something to do and perhaps the
hull has a lot to do with it. (Good excuse anyway)
I will be trying some of these ideas next to extend the range
at which the yacht is competitive as I have only had one chance to try the
new rig out and may need some fine tuning to get the best out of it.
So what next? I have always believed the one metre to
be overpowered and suffers in the top wind range of each rig. Good
hull design can help here by allowing the heeled beating lines to change
minimally from the flat running lines. Even so, as the wind increases
skippers are seen frantically readjusting the back-stay, mast ram and
boom vang to increase the twist in the mainsail. But what about in
normal sailing conditions where the wind varies over a significant range or
there are gusts going over the course? It would be much better if the
rig could adjust to these variations automatically at least over some of the
range. If the mast ram were not used then when a gust hits, the boom
will tend to lift bending the mast forward into the ram position and
allowing the leech to twist off. Now if that movement could be
controlled perhaps this could be the way. To get this to work I needed
to adjust the prebend in the mast so that in light winds with gentle
backstay tension etc, the mast should take up a nice even curve matching the
sail luff without using any mast ram pressure. (I had been cheating
and using the ram to achieve the curve.) Naturally I had just got this
right - maybe a little more - and due to the many addition holes in the mast
from the previous shroud adjustments the b.... thing snapped. The next day
was a general sailing day and I was determined to test the arrangement so
gluing a carbon fibre tube into the mast was going to have to do. The
next task was to order a new mast section! To make the ram
adjustable I placed a compression spring between the adjusting screw and the
bulkhead so that when fully out there was no pressure supplied by the ram
but as the boom lifted so the spring assisted holding the boom down.
Adjustment of the screw then allowed more or less pressure to be supplied by
the ram while still allowing some movement to release the leech.
Out on the stand and view the result. Fantastic, the
sail could be adjusted to its normal light weather position and as the wind
increased the boom lifted pushing the mast forward against the spring and
releasing the leech. I could only guess at the strength of the spring
required but I used some stainless steel shroud wire wrapped with a
drill around a 4 mm rod and stretched it out to act as a compression spring.
Now I looked a little more closely at the jib as most books
will say the jib leech curve should always match the mainsail leech curve.
I had always believed in a need for considerable tension in the jib luff to
prevent the luff from sagging too much as this simply makes the top section
of the sail fuller ( a bit like the problem with the mainsail and the shroud
positions). This means that the leech tension is also probable too
great not allowing the jib boom to lift and free the jib leech the same as
the main. This was easy to fix. I simply moved the pivot
position forward on the boom so reducing the leech tension. I ended up
with the pivot at about 20% of the sail foot length instead of the 22%-25%
often quoted. When tested the jib boom lifted more readily and freed
the leech very much in sync with the mainsail. So I made minor
adjustments to the sheeting positions on the boom to match.
Next day revealed a markedly different yacht. Sailing
in the 10+ knots with the No1 rig the sails could be seen to be
automatically easing off in the gusts and coming back in the lulls.
The boat drove significantly better and I was able to keep up with the top
bunch and even pass a few. As the wind lightened in the day, I was
able to verify that the light weather performance did not suffer as the
changes really only reveal themselves as the wind increases. Yes I did
own up to having a piece of carbon fibre in the mast but as it was a just a
fun day no-one minded.
I guess the next step is to repeat the process with the new
mast taking a little more care with pre-bending and trying various springs
on the mast ram and minor adjustments on the jib boom pivot position.
I wonder as well what the effect will be on the 'B' and 'C' rigs and whether
different spring tensions will be needed?? That could be awkward.
Still at the moment I am extremely pleased with the improved performance of
my one metre with the 'A' rig in the upper wind range.
I will return
Please feel free to E-mail comments to me at
benmorris@senet.com.au
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
