Jibstay Sag
Stuart Walker 1/1/2001
It has become evident that the most - or at least one of the most - important determinants of speed to windward
is jibstay sag. Slight variations from the optimal sag for the conditions make major differences in performance.
Everyone admits this and the Sailmakers recognize it and cut their jibs to accomodate it, but most Soling sailors
take a rather cavalier approach to it. Most attempt to control jibstay sag only indirectly (by backstay, mainsheet
and shroud adjustment) and with very little precision.
Sag is determined by three factors: a). the essentially horizontal tension transmitted to the stay by the airflow
over the jib, b). the essentially vertical tension in the wire/rod, and c). the resistance of the wire/rod to stretching.
- The horizontal tension is determined by wind velocity and increases (by the square) with increasing velocity and
operates to displace the jibstay laterally and aft thereby putting more fullness into the front of the jib. In very light air
there is little or no horizontal tension, the jibstay is much too straight, and the jib is much too flat and the jibstay
needs to have its sag increased by every means possible. In moderate air the horizontal tension rapidly increases, the
jibstay sags to the desired degree, the jib acquires its optimal fullness, and the jibstay sag needs to and can be
limited by direct adjustment. In stronger air the jibstay sag becomes excessive so that
- the jibstay
itself must be tensioned,
- when this is insufficient to limit the sag the backstay must also be tensioned, and
- When the combination is insufficient, the shroud cars must be moved aft.
- The vertical tension is determined by the tension in the jibstay control lines, by the stiffness resistance to bending below the hounds) of the mast, and by the pulling of the mast aft at the hounds chiefly by the backstay). If the jibstay tension is divorced from the backstay tension by prebend (which correlates with tight upper shrouds led through stiff
spreaders to a deck position forward of the midline of the mast) and a mast
base cut at the upwind rake angle, mast bend/backstay tension can be optimal
for the mainsail and unchanged from very light air to “two hiking”
conditions. Some backstay tension is always required to limit jibstay sag
and should (until hiking can no longer keep the boat upright) result in the
amount of mast bend for which the mainsail was cut - usually 2-3 inches
(depth of chord in the bend). Between 6 and 12 knots only the jibstay itself
need be tensioned (gradually) to keep the jibstay sag from becoming
excessive. Between 12 and 18 knots the backstay and the lower shrouds should
be tensioned (gradually) so as to simultaneously tension the jibstay and
decrease its progressively increasing sag and so as to flatten the main.
Somewhere between 12 and 18 knots the shroud cars need to be moved aft
(sooner in waves) so as to stiffen the mast additionally and permit the
backstay to exert a greater tensioning effect on the jibstay. Above 18 knots
further tensioning of the backstay (beyond about 7 inches of mast bend) only
increases mast bend and has essentially no effect on jibstay sag.
- The elasticity of the jibstay itself, of course, determines how much
it stretches and therefore sags for a given horizontal or vertical tension,
but the effect is only relative and means that the above adjustments must be
applied sooner or withheld longer as the wind velocity increases. Rod
rigging reduces sag for a given horizontal force and vertical tension and
therefore in heavy air permits the use of a lesser mast bend and results in a
greater jibstay tension and a lesser jibstay sag.
We all agree to certain generalities about jibstay sag:
- Jibstay sag is good - when in doubt go for more. Sag displaces the
leading edge of the jib to leeward (which permits higher pointing), increases
the overall draft (which generates more thrust) - the greater the sag, the
fuller the jib - and moves the center of draft forward. Dave Curtis says
that in smooth water (in which most of us usually sail - offshore winds and
light air) the jib cannot be too full (nor the mainsail too flat). {Well, it
can be too full - but, in light air at least, it is difficult to make it too
full. Excessive jibstay sag (with the jib’s leading edge 10 inches or more
off the midline) causes the sail to become flatter rather than fuller.}
- The greater the wind velocity, the more the jibstay tension that is required to counteract the sag (and vice versa).
- The smoother the water, the more the jibstay sag that should be permitted (and vice versa).
- In increasing light to moderate air to counteract the increasing
jibstay sag and consequent movement of the draft forward, the luff tension
should be proportionately reduced (tack up, wrinkles along luff). Indeed,
from 2-14 knots as the sag increases the luff tension should be progressively
reduced and only above approximately 14 knots when techniques that increase
jibstay tension halt the progressively increasing sag should luff tension be
increased.
- If the jibstay pumps, it is (probably) sagging too much. Pumping
means that the jibstay is changing tension - tightening and loosening - as
the mast moves back and forward with pitching. John Kostecki introduced the
concept of “The Kostecki Wobble” believing that optimal performance was found
close to the amount of sag that permitted the jibstay to begin to pump. I
would modify this by saying (at least for modern V-1 jibs) that the optimum
is (probably) just short of (the jibstay tension slightly tighter than)
pumping. Because pumping is increased by pitching in waves, this means as in
(2) above that smooth water permits more sag.
Sagging the jibstay until it starts to pump and then tightening it to
just eliminate that pumping is a good basic approach to judging optimal
jibstay tension, but it obviously depends on the size of the waves present
and in smooth water does not occur. I try for greater precision in
controlling jibstay sag by measuring jibstay tension directly (with a strain
guage) and marking the jibstay and backstay control lines to reproduce the
desired tensions.
The now-common technique of cutting the mast base so that it sits flat on
the deck at the upwind rake angle permits this greater precision. If the
mast is so modified (and reinforced to maintain the flat-on-the-deck angle),
the jibstay and the backstay become (at least partially) divorced and jibstay
tension and sag can be controlled directly. In this condition tensioning the
jibstay reduces jibstay sag without changing mast bend - at least initially -
and tensioning the backstay bends the mast without reducing jibstay sag - at
least initally. Under these circumstances one can control jibstay sag by
tensioning the jibstay control lines directly and, by marking the jibstay
control lines and the backstay, reproduce the desired jibstay tensions with
precision.
After a day of sailing (or a race or a portion of a race) on which
performance was particularly good, I set up the rig ashore (without sails) on
the same jibstay/backstay/ shroud mark/tensions that were in use and measure
the jibstay tension directly. After a number of such experiences I have been
able to establish a set of jibstay and backstay markings/tensions and their
associated jibstay tensions that I know to have been fast (and have
interpolated the appropriate settings for intermediate wind velocity/wave
conditions). The jibstay tensions discovered in on-shore testing are
certainly not the tensions the jibstay is under when sailing (although
testing has demonstrated that they are close), but they correlate directly
with those tensions. What is important is that once I have discovered the
amount of sag and the tension needed to maintain that sag for the best
performance at a given wind velocity and sea condition, I can reproduce them
at another time when conditions are similar.
Summary:
0-2 Kts - Jibstay - loose Backstay - eased Shrd cars - fwd
Mast sag - 0 *Jibstay tension -20 Jibstay sag - too little
Jib luff - tack up, wrinkles - 4+
2-12 Kts - Jibstay - tighten Backstay - optimal Shrd cars - fwd
Mast sag - 3-4cm *Jibstay tension - 0-25 Jibstay sag - increasing
Jib luff - tack up, wrinkles - 4+ (adjust for shortening jibstay)
12-18 Kts- Jibstay - max Backstay - tighten Shrd cars - slightly aft
Mast sag - reduce*Jibstay tension - 25-40 Jibstay sag - maximum
Jib luff - tighten - tack down, wrinkles removed gradually
18+ Kts - Jibstay - max Backstay - max Shrd cars - aft of mid-mast Mast sag -straight*Jibstay tension - 40+ Jibstay sag - maximum
Jib luff - max - tack full down, smooth leading edge *Jibstay tension - measured directly on Loos Gauge (40 = approximately 400
pounds)