A question of stiffness?
The mast is one of the most important parts of the equipment
setup, affecting early planing, control and manoeuvrability of the
rig. The mast is so critical to the rig performance that sails are
designed around the curve of the mast, rather than the other way
round. Getting the “right” mast for the sail will unlock the
performance of the rig, giving potentially better performance in
all conditions, as well as increasing the range of the sail.
STIFFNESS
The main property of a mast is the stiffness,
which determines how much it bends when
downhaul is applied to the rig. All mast
manufacturers use the IMCS value to indicate
the stiffness of their masts; standard IMCS
values are shown in the table. In general, short
masts are softer than long masts. It is rare
though to find a modern mast that differs too
much in stiffness from the values shown in the
table. Although sail manufacturers recommend
a mast length and IMCS for each sail, it is
possible to use a shorter (and hence softer) mast by using an extra-long mast
extension, or longer (and hence stiffer) mast by lengthening the head strap.
Using a too long mast in a sail uses a stiffer IMCS value, and also sets the sail
on a thicker lower section of the mast, which is stiffer. This makes the mast
harder to deflect, both statically, so a stronger arm is required to reach the
recommended downhaul, and also dynamically, so the mast doesn’t bend
enough to allow the leach to spill wind in gusts and when hitting chop. This is a
nightmare to control, typically with small sails in gusty, rough conditions when
you might be tempted to use a 400cm instead of buying the recommended
370cm. Heavier sailors might handle the combination a little more comfortably,
as they tend to use a given sail size in stronger winds, so the extra stiffness in
the mast gives more tension in the leach. For lighter sailors the rig can feel
more like a solid door, with not enough flex to give a smooth ride.
Using a shorter mast with a long extension gives a softer spar in the sail, so the
tension in the sail is less. Downhaul tension to reach the spec value is less, and
the rig has a softer feel. Gusts cause the mast to bend more, so the leach twists
off and exhausts more. This suits lighter sailors, as well as the rig feeling softer;
less strength is required to apply downhaul tension. The twisting leach can
make the rig feel gutless for heavier sailors though, losing bottom end. Using a
long extension can cause other problems: extra leverage on the bottom of the
mast which can cause a breakage low down, the boom not setting on the
reinforced section which some lightweight masts have, and some rotation
problems with the bottom batten catching as the mast hasn’t tapered enough.
BEND CURVES
Although the IMCS value defines the stiffness of the mast, it has nothing to say
about the way the mast curves, whether the top or the bottom bends more or
less. Most modern masts are described as “constant curve” but the name is
misleading, the masts bend more at the thinner tip than at the thicker base.
Constant curve covers a range of average curves, the any mast with a tip%-
base% (see IMCS panel) in the range 10%-14% range can be described as
constant curve. Outside the constant curve range, masts can be described as
hard-top or flexi-top. Virtually all modern masts fit into the constant curve bend
range, with a degree of either flexi-top or hard top character.
Sail designers cut their luff curves with almost the exact opposite curvature to
that at the mast, the luff tube bends more at the bottom than the top, which
pretensions the bottom of the sail to give a stable lower sail. The upper luff tube
is set relatively straight, so that the upper mast is under less tension and will
bend off more to exhaust power when
a gusts hits. The matching of the luff
curve cut to the mast curve is the
most important part of sail design, if
the luff curve is right for the mast then
the sail will be at worst OK, but if the
luff curve is wrong then the sail will
always be a dog. Some sail makers
like their masts to flex off in the head
(Neil Pryde are an example of this
with their progressive flex masts, with
a tip%-base% just outside the
constant curve range), while other
prefer the whole mast to breathe
(Maui sails). Manufacturers try to get
uniform bend curves over their mast
ranges, but differences between
different lengths and even individual
masts mean that giving more than
the vaguest of guides to bend curves
is difficult.
Using a flexi-top type mast in a sail
designed for constant curve masts,
and vice-versa, will cause loss of
range, often at both the top and
bottom end, and can adversely effect
the feel and balance of the sail.
Tuning of outhaul and downhaul can
return much of the performance,
but the sail specifications printed on
the foot may not give the best set
and handling.
EXTENSIONS
Extensions at the bottom of the mast
should not affect the bend curve, as
the sails are designed around a mast
with extension. Some long carbon
extensions designed for formula rigs
state that they have the same bending
characteristics as carbon masts, but
the main worry with long extensions is
the leverage causing mast breakage,
rather than an adverse effect on the
bend curve of the mast.
Fixed length mast extensions (such as
North’s 50cm Carbon X.Tender,
pictured) are designed to fit into the
bottom of the mast to allow one mast
to cover a greater range of sail sizes.
Because of the snugly fitting ferrule,
they are not at as high risk of
breakage as long extensions. The
carbon construction bends along with
the mast, however, the shorter mast
plus extender combination is still
softer than a long mast, so will work
better for lighter sailors, or those who
want to buy a larger rig but don’t want
to splash the cash on a longer mast at
the same time. North say most of their
UK racers are happy with the
performance of the X.Tender and
shorter mast combo, so they don’t
need to cart as many masts around
with them.
RDMs
Because the stiffness of a cylinder is
proportional to the fourth power of the
diameter, slight variations in mast
taper have a great effect on the mast's
behaviour, an effect which is particular
noticeable for RDM masts. Previously,
some sail brands were claiming that
RDM masts would not work well in
their sails, as the bend curves were
not close enough to the bend curves
of the SDM masts that the sails were
designed around. If no changes are
made to the stiffness over the length,
then the reduced taper of an RDM
mast will give a bottom section which
deflects more relative to the top,
giving a hard-top bend curve. Two
previous RDM-doubters, North and
NP, have introduced new RDM masts
this year, which they claim reproduce
the bend curves of their SDM masts
exactly giving perfect compatibility
with their sails. Sails designed around
RDM masts are starting to appear,
Goya and Ezzy recommend their
ranges on RDM masts, but claim
SDM compatibility.
SUMMARY
Matching the sail to the mast allows
the sail’s full potential to be extracted.
Setting sails on the correct length and
stiffness mast is the most important
consideration, although the bend
curve can also have a large effect on
performance. This is most important
for racers, where a small advantage in
sail performance may give a huge
advantage around the course.
Freeride sailors may not regard
squeezing the last few drops of
performance out of their sails as that
important. However, some tweaking
during rigging can give decent
performance from the sail, provided
the mast bend isn’t too extreme.
Thanks to John at sailrepair.co.uk
for his contribution to this article.
1
2
3
4
SAILS ON THE “WRONG MASTS”
1 Here is a (custom)
sail cut around a
constant curve RDM
400cm mast. The owner
and designer of the sail
had broken two of the
RDMs, and replaced
them with a 2005
Gaastra RDM, which
was a slightly hard-top
in character*. It can be
seen that the leach is
too tight, but the lower
battens are rotated
nicely around the mast.
*Gaastra are using
constant curve masts for
2006, so would be more
compatible with the sail
shown in this example.
2 Rather than
accepting that the sail
doesn’t work with the
new mast and returning
it, the designer has
removed a sliver of
material (20mm at most)
from the luff panel,
giving a straighter luff
tube at the top. A little
(5mm) is also removed
from the luff panel at the
very bottom of the sail,
so that the battens are
not over-rotated.
3 The sail is now put
back together, and set
on the mast. There is
now a nice amount of
leach twist, but the sail
still has shape in the
bottom.
4 Setting the sail back
on a constant curve
mast results in too loose
a leach, and the sail
would be gutless.
We’re not advising
people to go out and
buy industrial sewing
machines and start
recutting their sails; if
you think the bend
curve of your masts do
not match your sails
then talk it over with
your local shop.
