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.

We all agree to certain generalities about jibstay sag:

  1. 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.}


  2. The greater the wind velocity, the more the jibstay tension that is required to counteract the sag (and vice versa).


  3. The smoother the water, the more the jibstay sag that should be permitted (and vice versa).


  4. 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.


  5. 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)