Rigging is properly tuned when two conditions are met: The mast is centered on the boat from side to side and in column, which means that it is perfectly straight and vertical with the correct rake and pre-bend then applied. Also, the shrouds have correct tension that is nearly equal on both sides. Although more of an art than a science, rug tuning is not difficult, merely tedious. It is essential for good—or even merely adequate—sailing performance because it affects sail shape and how loads are distributed under sail.
To tune the rig, start by marking two reference points along the toe rail, on either side of the mast, that are precisely equal in distance from the bow. The base of the mast should be equally distant from both reference points, within a should be centered in the partners that secure it around the opening in the deck.
Measure next from the top of the mast to each reference point, either by using the main halyard or, preferably, by hoisting a tape measure aloft together with a length of stout twine made fast to the halyard shackle (in case the tape breaks). Adjust the upper or cap shrouds until these dimensions are equal within a quarter inch. Lube all turnbuckles with engine oil (not WD-40) before adjusting them to avoid galling that can damage their threads.
When the base and the top of the mast are both in position, sight up the mainsail track on the aft face of the mast to verify that it is straight, or in column. Remove any bend by adjusting tension in the intermediate and lower shrouds. If it becomes necessary to readjust the cap shrouds, ensure that the top of the mast remains equally distant from both reference points.
Once the mast is centered and in column, tension the backstay to apply the proper amount of pre- bend that stabilizes the middle of the mast. Some rigs, such as the Hunter B&R rig with its distinctive aft-swept spreaders and X-crossing shrouds, use the shrouds to apply pre-bend with the backstay fully eased. Pre-bend can range from an inch or two, in a keel stepped mast, to six or eight inches or more in a deck-stepped B&R rig. Measure it by tensioning the main halyard with its shackle made fast at the base of the mast and looking upward for its point of maximum distance from the mast, allowing for any masthead sheave offset. The mast must bow edge in tension and its aft edge in compression.
Rake, or the angle that the mast tilts toward the stern, is set by the length of the forestay and often is not easily adjustable. Measure rake, with the backstay tensioned, by tying a weight to the main halyard and using it as a plumb bob, allowing for any masthead sheave offset. This assumes that the boat is trimmed perfectly level, however, and there is no easy way to verify that. You can detect changes in mast rake indirectly by its effect on helm balance: Adding rake increases weather helm.
Finish the dockside tuning by tightening all shrouds to about the correct tension in port and starboard pairs, applying the same number of turns to matching turnbuckles on either side. The tension is about right when the shroud feels taut and under slight strain. This is about as tight as you can easily get the turnbuckle with the hand leverage of a standard wrench or screwdriver. Upper shrouds need somewhat more tension than lowers because they are longer and therefore stretch more. Intermediate shrouds have the least tension. Finish by checking, once again, that the mast is still in column and with its top equally distant from both reference marks.
Perform final adjustment of shroud tension while sailing upwind in flat water. With the boat heeling 15 degrees, the leeward shrouds should just begin to feel slack on each tack. Intermediates should feel looser than lowers. Uppers should stay fairly taut. As before, make adjustments by applying an equal number of
turns to the matching port and starboard turnbuckles. Apply no more than one or two turns at a time and then tack to test the tension. Tacking an over-tensioned rig can damage the hull. Lastly, insert cotter pins into the turnbuckles, bend their tips in opposite directions, snip off the excess to avoid snagging a sail or an ankle, and tape the turnbuckles only enough to cover the cotter pin ends while still allowing water to drain off.