We are happy to have Elliott Pattison Sailmakers back on board SA for 2010. Harry Pattison has written a piece on rig tune for sept back spreaders. Enjoy.
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. Continue.