Assumptions: Multiple spreaders (or shrouds...upper, intermediate and lower), adjustable backstay, continuous rigging (not discontinuous, that is, it starts and stops at spreaders; doesn't really change the procedures, just harder to do), and all rigging is loose (just put the mast back in for example).
1. Make sure no adjustable rigging is tight, including vang, mainsheet, babystay, and backstay (runners if you have them are off), and mast is centered in the collar.
2. The TOP of the mast must be placed in the center of the boat. Conventional wisdom says use the halyard and the shroud chainplates as reference points, but the halyard may stretch and the chainplates may not be symmetrical! So, take a tape measure and measure from the forestay chainplate back to the shrouds (where you'll check the mast centering); if not the same distance (withing a 1/4", then measure a fixed distance from the forestay to the toe rail adjacent to the shrouds and mark it; this will be your reference point. Now, buy a long steel tape measure (60-75') that you fully hoist to measure centering to chain plates or reference points) and adjust upper shrouds until it's in the middle.
3. Now, do rake, but the value depends on the boat, usually somewhere between 6-10", measured from the back of the mast at the gooseneck to the halyard hanging free with a 10 lb. (or so) weight on the end. It is vital while measuring rake that no one be on the boat and that either the sails, etc are off or are piled in the center over the keel. Rake is adjusted by the backstay and forestay turnbuckles., but make sure the mast partners (blocks in collar) are fixed in place. The issue of prebend is a tricky one, but if you start playing with it, you may mess up the trim of your mainsail that was cut with the pre-exiting prebend. Most C&C's need little if any prebend and with in line shrouds-spreaders-mast, the only way to change it is with the mast partners and maybe mast step position (e.g., ram forward in collar to induce some prebend). Remember that prebend is fixed; your babystay induces adjustable bend. My recommendation: don't screw with the prebend unless your mast is "inverted"..bows backwards when you're all done tuning which will cause it to "go out of column" and collapse!
4. OK, so you've got the top of the mast in the center and the angle (rake) about right; now start tightening and adjusting athwardships (side to side). Start at the top and tighten the two uppers the SAME amount (because it's in the center). I suggest that you take a Shapie pen and mark a big line on your turnbuckles to serve as the reference point for counting turns; have someone else with you count too. Tighten the uppers so that you get a very low low bass note...maybe 5-6 turns past hand tight (where you were when you were centering. KEEP NOTES on the # of turns for each shroud at this point on; you'll need to refer to them later).
5. Sight up the mast track...which way and how much is it bending athwartships from a straight line? OK, so tighen the intemediate a few turns less than you did the upper (4?) on one side, then tighten the other to get it straight in the upper 1/2 of the mast. If you have discontinuous rigging, someone has to go up the mast to do this at your turnbuckle at the lower spreader. OK, adjust back and forth, but keep the total # of turns on any turnbuckle less than that of the upper ones; this may mean that you have to loosen one to achieve a straight line.
6. Right, now do the same procedure with the lowers. If you have double lowers, so you have to adjust 2 the same on one side, although you can induce a little prebend with the front lower...wait on this till the end.
7. Now, all of this adjustment should end up with a straight (athwartships) mast. Now, you need to adjust the forestay tension and this depends on the amount of adjustment your backstay has. In a light air area, you want a pretty loose forestay, but a lot of adjustment in the backstay. Keep in mind that assuming the rake is correct (it better be!), then tighten the backstay and forestay roughly equivalent to tighten the forestay (if you only adjust one, you change the rake). I think the forestay should be pretty "floppy"..moves side to side when you grab it and shake it about a foot. Then when you tighten the backstay adjuster, all this flop goes away and it is pretty solid. If you don't have enough backstay adjustability, then tighten the forestay as above a bit more.
8. OK, everything should be statically correct, so let's see what the adjustments do. First have a helper tighten the backstay while you sight up the mast track. The only thing you should see is a little bend fore and aft, but no change athwarships; if it does move sideways a bit, tighten all shrouds one turn. The forestay should be TIGHT. Now ease the backstay to about 1/2-3/4 full and tighten the babystay; the middle of the mast should bow forward (again helper does this while you watch), but not sideways.
9. Go sailing in 10-12 kts...close reach (not beating, not beam) and few tacks with sightings before any adjustments. If you did the dock tune right, the adjustments will be minor. ASSUME that the top of the mast is fixed, but verify by seeing if on a close reach in 10-12kts if the uppers are loose; if they are, first tighten both of them equivalently. Now, the only thing that can change tack to tack are the intermediates and lowers, and these are what you adjust after tacking and sighting up the mast; only do 1/2 turn adjustments but make sure you don't tighten the intermediates or lowers more than the uppers (recall you kept notes on # of turns on each?). Now that you've got this right, try adjusting backstay, babystay, etc as before to observe their operation and make sure they don't affect your side to side tuning...shouldn't. ALSO, make sure that there is NO aft bend to the mast when you're done.
10. Now that you're done, go on a beat (close hauled) and see how loose the leeward rig is. For these boats in about 10-12, I'd say they can get soft, but NO flooping around. Tighten everything equivalently to fix if it is., but I'd err on the loose side for most areas. If it's going to be a windy race, then tighten everything one turn. Now, pin the rig. I use split rings rather than cotter pins are they're easy to remove even with out tools and facilitate adjusting. If you have a Loos gauge for rod, I'd now make the measurements and log them (facilitates retuning); ideally, the tension should be the same on each shroud port and starboard, but if the rig is straight, then don't change the tension just to make them the same.
Greg Cutter, Professor
Department of Ocean, Earth, and Atmospheric Sciences Old Dominion
University 4600 Elkhorn Ave.
Norfolk, VA 23529-0276 USA