The ups ...
A mast that is soft sideways
makes life a lot easier in a chop - the rig becomes
more forgiving and the "dynamic response" to the
waves seems to be better. The same goes
for softness fore-aft down low, just above (and
even under) the deck.
... and downs of side bend
The nuisance with mast bend down
low, and particularly side bend is that you loose
some pointing ability, especially in mid-air and
flat water. Very light air and very heavy air
seem to be OK. Side bend at the tip of the mast
may be the best solution - it tends to flatten
the head without twisting off the sail as a whole.
To help you to understand what
side bend does to the sail, take a look at the
mastcam photo with excessive mast bend illustrated
on it. If you study the photo for a while, it
should be easy to understand that side bend
for an unstayed, cat-rig mast is essentially the
same as sail twist.
While mainsails on rigs with
spreaders & shrouds twist from the leech,
cat-rigged mainsails like those of the Finn and
the Europe twist mostly from the luff as a puff
hits, or as the boat hits a wave, the leech of
the sail remaining more or less in the same position.
This is the "dynamic response" mentioned at the
beginning.
All this is a little more true
for the Finn, whose sail has no roach. The Europe
sail with its very large roach at the top batten
also does exhibit some "normal" twist, as the
top batten acts like a lever.
Aerodynamically, there is no
difference whether it is the luff moving to windward
or the leech moving to leeward - the angle of
attack of that part of the sail is decreased,
which essentially is the definition of twist.
Boats with shrouds & stays
Side bend works much in the same
way as for the cat-rig, but with spreaders you
have more to play with. Shorter spreaders allow
the mast to bend more easily to the side. As the
middle of the mast moves to windward, the slot
between the main and the jib is opened, depowering
the rig efficiently.
Many sloop rigged boats use the
spreaders in light winds to create "negative"
side bend, i.e. to push the middle of the mast
to leeward. This introduces negative twist in
the middle of the sail (the mid-sections are at
a higher angle to the wind) and closes the slot
- a very efficient way to add power in the light
stuff. |
For an unstayed, cat-rig mast sideways bend
means increasing the twist of the sail.
Side bend is an efficient way
to depower a normal rig with shrouds, as
well. In contrast to the cat-rig, the mainsail
of a sloop twists mainly from the leech.
Side bend at the luff accentuates the twist.
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The ice-boat rig
Ice boaters have a very different
approach to side bend and twist. They want to
mast to bend "the wrong way" (see photo) as much
as possible, and as light in a wind as possible.
This makes a huge difference in boat speed and
pointing for these skimmers that easily fly faster
than 70 km/h.
As pressure increases on the
sail, the shrouds (no spreaders) coupled with
the mainsheet put a tremendous compression on
the mast. Under the compression the mast pops
into its peculiar bend, and the iceboat is off.
What's the reason for this?
Killing twist with negative bend
While sail twist is the friend
of the salt water sailor, it is the enemy of the
ice boater. With very little resistance between
the skates and a good, slippery ice, all drag
in the air is detrimental, and twist does no good
to the lift/drag ratio of a sail. With the wind
pressure at apparent wind speeds around 40+ knots,
there is no way to keep the leech from twisting
off.
So, the witty ice boaters use
negative mast bend to eliminate twist from their
sail. The mast follows the curve of the sail
leech, providing essentially zero twist. Seagulls,
as birds in general, do the same - as the feathers
at the trailing edge of the wing twist out under
pressure, the seagull bends down its wing to
eliminate the harmful twist.
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The DN
iceboat. Ice boaters use negative mast bend
to eliminate twist from their sail.
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