LIGHT AIR HEADSTAY SAG & JIB TRIM

What do we look for in headstay sag on the J/70 to optimize jib trim for a variety of conditions? Here are some thoughts from One Design expert Tim Healy

One of the things I have really come to enjoy over the years is having the opportunity to crew for many talented sailors, and watch and learn (or re-learn) how they approach their setup and race preparation. When Skip Dieball (reigning Etchells World Champion) got his new J/70 and invited me to sail with him, his wife, and his brother at the Bayview One Design Regatta (BOD), I jumped at the chance. Skip knows a thing or two about making sailboats go fast and it was fun to interact with him and get his input and feedback on the J/70.

The conditions at the regatta were light air and occasional moderate chop, and rig tension, headstay sag, and jib trim were especially important factors in setting up the boat. The synergy between these three variables was a topic that we discussed quite a bit: basically, what do we look for in headstay sag on the J/70 to optimize jib trim for a variety of conditions? After the dock talk and some great discussions with Skip, I gathered all my thoughts on how one affects the other and how to tune and trim for maximum performance with a focus on light air. I hope some of these notes can help you understand your J/70 setup better and help you go faster.

My six bullet points:

• In light to moderate wind, having adequate headstay sag reduces the need for weather sheeting the jib. (At this regatta, pulling the clew to weather about 1 inch was about our maximum weather sheeting.)
• The shrouds should be tensioned so that we are targeting 4-6 inches of headstay sag at the mid-stripe of the jib and 1/2 to 3/4 inch of mid mast side sag. The basic idea is that headstay sag adds depth to the jib (and this is an important point to remember).
• The mast side sag is an indicator that the mast is free to move/flex with little restriction of the lowers. This will allow the mast to be more flexible and dynamic in light air and increase headstay sag.
• It helps to know that the middle of the jib luff does fall off (“sags”) to leeward which changes your angle of attack to the wind.
• Conversely the mid-leech “rotates” to weather (hence reducing the need for windward sheeting referred to in bullet point #1)
• The net result of these variables is the ability to point higher.
  So why is this so important and how do we get there?

Light Air & Flat Water

In light air and flat water it is important to create the power in the sail plan, that the J/70 wants, while keeping flow across the sails. In lighter conditions and flat water the jib needs depth for added power and sagging the headstay is the easiest and most direct method to create depth in the jib.

When the headstay sags, it not only sags to leeward but also sags aft, towards the jib tracks, which puts the luff closer to the leech, thereby adding depth into the jib. What we find is at the middle section of the jib, when it is sagging 4-6″, the luff of the jib is actually falling off to leeward slightly and the leech, by default, “rotates” to windward just a bit. This creates a scenario where the middle of the jib luff has a better angle of attack to the wind and, because of this better angle of attack, you will be able to point slightly higher without luffing or losing flow over the front of the jib.

In this scenario, when the mid -leech “rotates” to windward, the effect reduces the need to windward sheet as aggressively because the leech has, effectively, moved inboard already because of the sag.

When you have this all setup correctly it is still important to make sure the jib leech tell tales are flowing 90%-100% of the time so that the airflow stays attached in the “slot”, and the sailplan does not have any “breaks” or stalls” in the airflow. That said, it is OK (and actually encouraged) to test to make certain you are at maximum trim by sheeting in till you see the jib leech telltales stall slightly, then ease out till they are full streaming. This is your jib sheet trim range which, in the case of the J/70 jib in light air and flat water, may be as little as 1” of sheet.

It is important to stay on top of your rig tensions in these conditions and find the settings that work best for your driving style. In general rig tensions that are loose enough to create 6″ of sag are good for light air and flat water. As you can see in the tuning guide, the “base” setting is 18/7 for 10 knots so when the breeze is quite light (0-6 knots) I will loosen 2-3 turns on the uppers and between 2-3 1/2 turns on the lowers. This gets my uppers closer to 14-16 and the lowers are very loose (they do not register on the gauge). The mast side sag at the spreaders should be 1/2 to 3/4 inch when the lowers are loose enough. This side sag is also a signal that the mast is free to bend forward as well as off to the side. This forward bend is good in light air to create a flatter main that can be sheeted tighter without stalling the leech. This will also help pointing ability.

Light Air & Choppy Water

When chop is introduced into the picture, a loose rig is still good, but you may find that the rig will pump too much in the heavy chop. This is normal, but in order to temporarily stabilize the rig, pull on enough backstay to take the deflection out of the backstay flicker at the top of the mast and put a small amount of tension to pull back on the mast. This should minimize the pumping of the rig and headstay and keep a more consistent sail shape though the chop. As soon as the patch of chop is over, release the backstay to get slightly more sag back in the headstay.

Final Thoughts

The J/70 is one of the few boats where weather sheeting is effective, but it has to be done with an eye on the headstay sag. With a saggy headstay, you will tend to use less weather sheet. With a tight headstay, in general, you can use more weather sheet. Essentially the angle of attack and leech position changes with the headstay sag, so you adjust your weather sheet accordingly.

These are tips that can make a big difference in light air speed. Feel free to contact Tim Healy or any member of the North Sails One Design Team if you have other questions on how to make your J/70 boat GO BEYOND.