A sailboat’s rig is a dynamic thing, even though it appears to be fixed in place with stays, shrouds, turnbuckles and pins. Wire stretches, decks shift, sails change shape and halyards stretch. To make sure your rig is safely set up and well tuned for the best possible performance, it is important to tune the rig every six months or more often if you are actively racing.
First, you need to set up the mast so it is straight from side to side. Use the cap shrouds to adjust the sideways position and measure this with the main halyard; attach a measuring tape to the halyard and measure to the base of the two cap shrouds. Once straight, tighten the intermediates and lowers, while sighting up the mast at the mainsail track to make sure it remains straight.
For a standard mast, you can build in a little pre-bend in the mast to maintain good mainsail shape and to keep the spar under constant compression. Tighten the forward lowers slightly so the middle of the mast is secure; then, tighten the backstay and headstay so you have a nice, even curve in the mast with the top or truck no more than six inches or so aft of the hounds. If you have a hydraulic backstay, this is the shape you want when the backstay is fully cranked down.
All boat designs are different; some have natural lee helm, some have neutral helm and some have weather helm. You can tune out lee and weather helm by either raking the mast aft a bit (correct for lee helm) or raking it slight forward (for weather helm). Ideally, when the mast is set up correctly and tuned precisely, and when the sails are trimmed to their best shape for sailing up wind, you want about two degrees of weather helm so you can “feel” the boat.