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Understanding Jib Sag

By Dave Starck
Kenmore, New York
Fleet 12, Buffalo Canoe Club
1987 Junior North American Champion

1990 Canadian Open Champion

(From Racing the Lightning)

Any of us Lightning sailors (or any One Design sailor) spend an exorbitant amount of time making sure our sailplan and rig is tuned properly using the adjustments available to us. Centering the mast side to side, making certain it is straight, butt position, and paying attention to rake are usually the criterion we abide by. Further, mast blocking for the existing conditions is also a common tuning step we take to induce prebend. There is howev­er another important ingredient which we may not always have on the top of our “things to tune” list...Jib Sag.

Jib sag may be the single most important gear we have on the Lightning to help power-up/depower the rig. Utilizing the tools available for power (backstay, main­sheet blocks, cloth, etc.) are essential to control the entry and fullness of the jib. Hence, jib sag may be that miss­ing ingredient to your tuning and racing success.

Wouldn’t it be great if we could add cloth to a sail when the winds were light and the seas choppy and, in an instant, reduce the amount of cloth when the weather turned. In essence, this is exactly what we are accom­plishing with the idea of jib sag. In light to moderate winds and chop, inducing jib sag accomplishes many other things on our “wish list.” First of all, in these trying conditions, it seems we are constantly hunting for power. That is, driving the boat through the water becomes more important than pointing and concentrating on gain­ing lateral distance on the competition. Secondly, induced jib sag allows the sail to be much fuller and more powerful as well as forgiving. Hence, your groove to steer through the water is wider and easier.

Now, the flipside to this is moderate to heavy winds and rough weather. When these conditions are witnessed, depowering the jib (and entire sailplan for that matter) is our primary goal. Straightening the headstay (jib luff) through the use of the tools available to us will “reduce” the amount of cloth in the jib, making it flatter and less effi­cient. Pointing higher will also come about at this time.

In understanding jib sag and how it relates to your setup, the two most important tools we have to control sag are mainsheet tension and backstay. Mainsheet ten­sion will suffice in conditions where you do not have to depower the boat. When sailing with two on the rail and sometimes three these conditions do not necessarily warrant backstay. If you feel your jib is too full and pow­erful, utilizing the wire and cloth controls can be helpful. Likewise, jib lead position can also be controlled without altering the shape on your mainsail.
In conditions where sailing overpowered is the norm, the entire sailplan must be changed. In this case, your backstay adjust now becomes critical. Using all your other controls to straighten and flatten the jib aren’t enough. Constant backstay adjustments will enable you to point high in the puffs with a straight luff entry and bladed sails, and will give you that needed power in the infrequent lulls. It’s just amazing how well the boat sails through the water when both sails are working together. The trick is to be able to “change gears” with variations in the wind velocity and waves.

Finally, I think it’s important to touch on prebend, and its effects on both the jib sag and mainsail shape. There are a couple of reasons for inducing prebend into our Lightning mast. In moderate breeze, blocking the mast forward (putting blocks behind the mast) will give you more control and feel over your jib sag and effectively allow you to sail your jib independent of the mainsail. Prebending your mast also affects your mainsail, making the lower third section of the sail flatter.

When we are sailing in heavier winds (15+ knots) with prebend, it is tough to control the jib sag. That is, to straighten the headsay (jib luff) we have to pull on maxi­mum backstay. This situation is not necessarily good because with maximum backstay, our mainsail becomes too flat and depowered for the conditions. Therefore, to find a happy medium in these conditions, we must change our blocking configuration and change our prebend. Removing the blocks from behind the mast and blocking the mast aft is recommended. Now we are able to pull on the backstay and control both sails effectively, not just one of them.

I hope this article helps you to understand jib sag. It is too easy to set up your boat on land and get caught up with all the measurements which are drilled into our heads. Granted, mast rake, butt position, side to side measurements, etc. are important, but I think it is important to channel some extra effort towards jib sag. I guarantee it will be helpful to you.

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