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Ask SAIL: Tuning the Rig

Rig tuning is a subject worthy of many pages. Rather than trying to answer your specific questions, I will offer some general principles to guide you.
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Philip Donegan, Ballina, Ireland asks:

I have a 39ft Jeanneau 409 with a 7/8ths rig and two sets of swept-back spreaders with no running backstays. There is no bottlescrew adjustment on the forestay, but there is one on the backstay. There is quite a bend in the forestay when sailing in 10 knots true wind.

I’ve read that with this rig you should tension the cap shrouds at the dock by bending the mast and then tightening the shrouds very hard. Do you agree? How hard should I tighten them and how do I measure this? I’ve read that this “sets” the forestay tension, but I don’t understand how the forestay tension is not subsequently affected by the mast bending while sailing. Also, I don’t understand how tightening the cap shrouds tensions the forestay, as doing that pushes the mast forward at the shrouds. What impact does such tensioning have on the shape of the mainsail? Should I tension the lowers just enough that the leeward one is not slack?

Win Fowler REPLIES

Rig tuning is a subject worthy of many pages. Rather than trying to answer your specific questions, I will offer some general principles to guide you. First, keep your mast centered and straight athwartships. Second, the more control you have of fore and aft bend the better, within the range intended by the mainsail designer. Of course, the less fore and aft bend there is, the stiffer the mast will be and thus better able to carry the compression load of the cap shrouds and headstay. Modern sails are usually designed for small amounts of mast prebend. If you have standard sails, Jeanneau should be able to give you specific prebend guidance. Otherwise, consult your sailmaker.

As you suggest, cap, intermediate and lower shrouds should all be tight enough that the leeward set is not slack when the rig is fully loaded. Sometimes limiting headstay sag requires more than minimum tension on the cap shrouds, but as you tension the caps, you will increase the mast bend, so you’ll need to tension the intermediates and lowers to compensate. Just how tight is tight enough depends a great deal on your boat. At some point you start bending the boat instead of increasing headstay tension. Generally cruising headsails should sag 0.5 percent to 1 percent of their luff length, but that is not always achievable.

Proper alignment of the mast from the maststep through the deck to the hounds is also essential to achieve proper prebend. Because you have no control over your headstay length, you may have to adjust the step or partner wedges to get everything in alignment.

Although you have a fractional rig, much backstay (and mainsail leech) tension is transmitted to the headstay. One of the best performance upgrades you can make is to add a backstay adjuster. A bottlescrew (or turnbuckle, as we also call it) is difficult to adjust under way. Fortunately, your boat has a split backstay. I believe Jeanneau offers a mechanical adjuster as an option, but installing your own is an easy retrofit. Tensioning the backstay will tension and straighten the headstay, flattening the jib, while at the same time bending the mast and flattening the mainsail. Conversely, easing the backstay will power up both sails. 

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