Tuning Shroudless Rigs
- by Ben Morris
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It seems there are countless articles on tuning rigs on sail
boats though most are specific about a particular yacht type.
Shroudless rigs as used by most Marblehead yachts have quite specific
requirements and need to be treated differently to most rigs.
Unlike rigs with shrouds and spreaders there appears at first
to be little that can be adjusted on a shroudless rig. Those
adjustments that can be done then must be critical to getting the best from
you yacht. Most of this discussion will assume a carbon mast stepped
through a tube at deck level to the bottom of the hull
Mast Rake
This is the easiest adjustment to be done. It is most
easily controlled through the forestay and backstay adjustments. The
main reason for adjusting the rake is for balance. As a good starting point,
your mast should have a slight rake aft (15-20mm) when initially setting up
your rig. The when testing your yacht under sailing conditions, if it
has lee helm and tends to fall off the wind particularly in light airs then
the mast may be raked further aft to shift the centre of effort aft and
re-balance the boat. The reverse is true if the yacht has excess
weather helm particularly in moderate winds the mast may be raked a less aft
to rebalance the yacht. It is probably unwise to move the mast too far
from vertical though a range of 50 mm or so is quite reasonable.
Backstay and Forestay Tension
How much backstay tension is enough? In any rig,
the backstay tension is added to bend the mast slightly to match the luff
curve built into the mainsail. At the same time it will
tension the forestay. Forestay tension is necessary to ensure the luff
of the jib does not sag too much when beating to windward as it will distort
the carefully designed sail shape your sail maker has built into it.
Forestay tension is also necessary to ensure a reasonable amount of jib
leech tension such that the jib boom does not lift until the wind speed is
near the maximum for that rig. This is really the place to start
to see how it all works! Make sure the pivot point for the jib boom
lies about 20% to 25% of the distance between the jib tack and the clew.
Try sailing or better still set the yacht in a stand in winds
near the maximum speed for the rig. This corresponds to an angle of
heel of about 30-35 degrees. Observe the jib leech and the jib boom.
If the boom lifts and frees the leech well before maximum wind speed for the
rig then more forestay tension and therefore backstay tension is needed.
If the boom does not lift at all then there is too much tension in the
forestay so the backstay tension needs reducing. Play with the
backstay tension until the jib boom lifts just at the maximum wind speed for
the rig. Do not be concerned about the mainsail at this time just
concentrate on the jib first.
Having got the jib set up to lift and free the leech
correctly it is time to see what is happening to the mainsail. The
chances are that with the required backstay tension, the mast is now bending
too much (greater than the built in luff curve) and distorting the sail
particularly in the top third showing diagonal creases and even inverting in
this region. If so then forestay tension needs to be obtained in an
alternative way using a mast ram near the deck level.
A mast ram produces an aft force on the mast by pushing it
backwards at a position near deck level. The effect of this is similar
to the 'pre-bend' that one metre sailors give to their aluminium masts.
Test it by removing all backstay tension and using forestay adjustment,
force the top of the mast to bend forward 10mm or so pushing against the
ram. Already there is a tension in the forestay even before the
backstay tension is added. If backstay tension is now added it will
increase the forestay tension.
The optimum situation then is to set the backstay tension
sufficiently to bend the mast to match the main luff curve and to use the
mast ram to generate additional forestay tension to get the jib boom to lift
only near the maximum wind speed for that rig. Remember that adding
pressure on the mast ram will produce a slight 'S' bend in the mast though
as the ram operates near deck level it is working on the thickest, strongest
part of the mast and will bend it only a small amount. You can go too
far though and particularly if your carbon mast is not made of the hi
modulus carbon, too much 'S' bend can occur and cause additional distortion
of the mainsail.
A small amount of fine tuning can occur if the pivot point
for the jib boom is moved within the range 20% - 25%. Moving it
forward reduces leech tension while moving it back increases it. Don't
come back beyond the 25% though as the sail will become unstable and not
work effectively.
If your yacht does not have a mast ram then it may have a
filler at deck level which determines the mast rake. These are often
replaceable while some rotate through 180 degrees to give a variety of
rakes. These can act as the ram by pretensioning the forestay against
it initially with no backstay to start with bending the mast forward.
Then add the required backstay tension to get the mast bend matching the
main luff curve.
Clearly the ram pressure, forestay adjustment, backstay
adjustment and mast rake are all interdependent so all need to be adjusted
together to get the correct jib leech tension and mast bend and mast rake.
Matching the Main Luff Curve with mast Bend
I have mentioned this a few times. Generally with
simple sail ties around a round mast, the mast needs to be curved
sufficiently so that when the sail is setting in mid strength winds for that
rig, the luff of the sail sits slightly off the mast and parallel with it.
The distance away from the mast will be determined by the small amount of
slack in the sail tie. Clearly these should be the same for each
position. I always insert a small dowel between the mast and the tie
and do the not tight when attaching the sail to the mast. Removing the
dowel gives a constant slack.
It may be desirable to allow more fullness in the sail in
lighter winds by reducing the mast bend slightly allowing the middle and
upper parts of the sail to belly out more as there will be an excess of sail
cloth in that area. In heavier winds with more mast bend the top
sections of the mast will bend more readily pulling the curve of the sail
out there. Only small amounts of bend is required to do this!!
Again, practice while the boat is on a stand and see the effect of
additional mast bend on the shape of the sail and notice how little movement
is required to flatten the sail.
Please feel free to E-mail comments to me at
benmorris@senet.com.au
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
