Tuning a Rig with Swept Aft Spreaders
In my article on Controlling Headstay Sag I focused mainly on conventional rigs with in-line spreaders. On those rigs your adjustments are done mostly with the backstay, runners, and checkstays. Tuning a rig with swept aft spreaders and shrouds is quite different because in these rigs a large component of both headstay sag and mast bend are controlled through shroud tension. These rigs are most often found on boats with jibs that don't overlap the rig.
To start you need to understand what the individual shrouds do, and what effect changing their tension will have. Your rig will have an upper shroud, also called the V1 or cap shroud, which runs from chainplate at the deck up to the hounds or point where the headstay attaches to the mast. It will have a lower shroud, called the D1, which goes from the chainplate to the base of the first spreader. If you have multiple spreaders you will also have diagonal shrouds that runs from the tip of the spreader below to the base of the spreader above. With a two spreader rig this would be called a D2, with a three spreader rig you will also have a D3 and so on. These diagonals may pass over the end of the lower spreader and continue to the deck where they are adjustable or you may have "discontinuous" rigging where each of the diagonals terminates at the end of a spreader where it will be adjustable.
The V1 shrouds center the top of the mast in the boat but since they also pull aft they tighten the headstay and induce pre-bend in the mast from the compression load. The D1 shrouds support the sidewise load in the middle of the mast and since they are pulling aft on the middle they will also control how much pre-bend you have. If you have have a two or more spreader rig the upper diagonals, D2's, D3's, etc. will control the sidewise bend in each section where they are attached.
Tuning one of these rigs takes some time because your ultimate goal is to reach a point where you have the correct amount of pre-bend, at the same time that the mast stays straight sideways, at the same time you have the correct amount of headstay sag. Start by setting the mast with the desired amount mast rake. This is set only by the length of the headstay; make it longer and you have more rake, shorten it and you will have less. The boat designer, if available, will have an amount he designed for the rig when he developed the rig and hull package. If you are sailing in a one-design class there will probably be tuning guides developed by the top sailors in the class that will give you a rake measurement. In the absence of either one of those try starting out with about 1 degree of rake. That is an amount that is often used by designers and should certainly be close enough for a good starting point. 1 degree equates to about 4" of rake for every 20 feet of mast.
Next you will need to know how much pre-bend you should have in the rig. The spar makers should be able to give you a measurement for the amount of pre-bend they designed the mast to have, or again if you are sailing in a good one-design class there will be tuning guides with the amount of pre-bend that has been proven to work in the class.
With this information at hand start with a basic "dock tune". With all the diagonal shrouds slightly loose start tightening the V1's, measuring as you go to keep the top of the mast centered in the boat. Tighten them to the point that you have about 25% more pre-bend in the mast than your designed spec. Next tighten the D1's enough to pull the pre-bend back to the desired amount while sighting up the mast to see that it is straight side to side. If you have other diagonals go ahead and tighten them just enough to take the slack out, again sighting the mast as you go to keep it straight side to side.
The final tune has to be done while sailing. You should do the initial tuning in light to medium air with just the main up and then as you dial that in work up to more wind. Start by sailing hard on the wind and sighting up the mast, it helps to make notes as you go. First look at the leeward V1, if it is very loose you will need to tighten both V1's equally to keep the top of the mast centered. Take up turns on the leeward V1 enough to take about 1/2 of the slack out of it. Now tack over and tighten the other V1 the same amount. Now look at your pre-bend, since you have tightened the V1's the mast will have a little more pre-bend so tighten the D1's enough to pull the bend back to the desired amount. do this by tightening up the leeward D1 a turn or two at a time, tacking over and adjusting the D1 on the other side. You have to keep sighting up the mast on each tack to be sure your are keeping the middle of the mast centered. Once the D1's are set move up to any other diagonals you have, adjusting the leeward side a turn at a time, tacking over and adjusting the other side as you go. If you have discontinuous rigging this has to be done by a crew member that is aloft so he can adjust the turnbuckles that are at the ends of each spreader.
Once your are satisfied that the mast has about the right amount of pre-bend and that it is staying straight side to side, you can start on you final tuning that involves fitting the rig to you sails. Your mainsail should have been built to match the amount of pre-bend the rig builder specified but you can change the amount of pre-bend slightly to make it fit better if needed. Do not reduce the amount of pre-bend to less than the specified amount! Next hoist the jib and sail up wind with it, tacking back and forth a few times while checking the mast to see that it is still straight. At the same time look at the headstay sag and how full the jib is. If you have too much headstay sag when you have enough backstay tension on to make the main look good you will need to tighten up the rig. Tighten up the leeward V1 by two or three turns and tighten up the leeward D1 by half that amount. tack over and do the same on the other side while looking the mast to see if the ratio you used between the V1's and D1's was about right to keep the mast straight side to side. You may have to adjust the D1's a little more or a little less and then re-adjust any other upper diagonals you have.
This is a slow process and needs to be done in different wind conditions to really get it right. You will find that to really do it right you will need to adjust the shroud tensions differently for different wind conditions. This need to be done by tuning in the different conditions while sailing with the jib you will use in those conditions. Each jib is cut for different amounts of headstay sag so the overall rig tension has to be changed to match that sag. On boats with multiple jibs you shouldn't have to adjust the rig very much because the jibs should have been designed to work in a narrower range and cut to match the headstay sag you get. On boats that sail with only one or two jibs you will need to adjust the rig a lot more to make the jib work over a wider range. Make sure you record the exact settings you start with and how much you change it for different conditions. This is often done by having a "base" setting and then keeping track of how many turns on or off you put on each turnbuckle as conditions change. This works but it is very easy to lose count of exactly what you have done. A much better way is to get a dial caliper that costs around $25.00 and measure the distance between the studs inside the turnbuckle. Record these numbers in your rig notebook for the different settings. That way you always have a number you can come back to no matter what happens, if you get confused about how many turn you went in which direction, or even if you pull the rig out. Getting back to your desired setting is just a matter of tightening each turnbuckle to the desired measurement.
This whole process is slow and usually an ongoing process. Each time you race you should record the conditions, the rig settings, and your results. If the results weren't what they should have been make small changes in the rig to match the mast bend and headstay sag to what you needed to do better. Write it down each time and you will develop your own set of settings that work for you boat and your sails.