Created Nov. 29, 2018
Mast tuning is simple if you remember a couple of basic principles. If you understand these principles, you can tune just about any mast.
The first principle, and probably the most important, concerns tuning the mast athwart ships. the diagonal shrouds, lowers and intermediates, always pull the mast to weather at the spreader where they terminate. The spreaders, on the other hand, due to the compression from the wires going over their tips, push the mast to leeward. in order to tune a mast, you need to establish a dynamic balance between "pull" of the diagonal and the "push" of the spreader.
The second principle is that the length of the headstay controls the rake of the mast, i.e., the amount that the mast is aft of plumb in the boat. masts, in general, should always have at least a small amount of rake, they are usually designed for one to two degrees of rake. the feel of the helm is the ultimate test of the rake. Making a mast more vertical will help weather helm and more rake will help to correct lee helm. this is a bit of a simplification, but after all this is the "light" version of mast tuning.
The third principle is that most masts should have a slight "prebend" over their length with the headstay firm from a minimum of backstay load. prebend can be visualized best by stretching the main halyard down the aft face of the mast. the maximum distance that the back of the mast is in front of the halyard is the prebend (you should take into account any offset that the position of the main halyard sheave causes). prebend can be attained by tightening forward lowers, chocking the mast forward in the collar at the deck, moving the mast step aft (on a keel stepped mast), or lengthening the headstay. the amount of prebend varies from about 1" for a single spreader deck stepped mast to 6" for larger keel stepped spars.
The last principle concerns the amount of tension in the rigging. as a general rule, when the rig is fully loaded up (top end of the #1), the leeward shrouds should be beginning to appear to slacken. they can be deflected by hand, but not swinging loose. This will approach optimum general rig tension for most normal boats. individually the wire tensions should be higher in the lowers and uppers than in any of the intermediates.
The tuning sequence that has worked the best for us is to start by centering the spar in the boat athwartships with the uppers. we tighten the uppers slightly. next the lowers are adjusted so that the mast at the lower spreader is centered on the masthead. sighting up the sail track is the best way to determine this. if the mast has multiple sets of spreaders, then the intermediates are adjusted next starting at the upper spreader. when the mast looks to be in column from the deck to the tip, then rig tension can be applied (chock the mast sideways and fore and aft now if it is a keel stepped mast- make sure the step position is correct for the required prebend). We add additional tension by adding equal numbers of turns to each side of the turnbuckles in the same sequence that we first used. make sure that the turnbuckles are lubricated with heavy lubricant to prevent galling and damage to the threads. check to see if additional adjusting of the shrouds is necessary as you add tension to the rig. check the headstay to see if the rake of the mast is correct. check the prebend. tension the backstay and see if the mast remains straight under load. that should conclude the dock tune portion of the setup.
A few hints
1. If the tip of your mast seems to fall off, and your uppers are fairly tight, try loosening the intermediates.
2. Check the rake of a mast by tying a heavy object to the main halyard and measuring the offset from the back of the mast. subtract any sheave offset present.
3. Make sure to do the final tuning of the mast when sailing. make sure that the mast remains straight athwart ships. check that the mast bends forward in the center (the reason for prebend).
4. Check to make sure that the bottom of the mast is square athwart ships, and for a keel stepped mast that the mast is straight through the deck. if it is not, the mast will be forced into an s bend that is impossible to tune out. we usually tune a keel stepped mast with the deck chocks out and shim the mast sideways after the mast is straight athwart ships. mast steps and mast collars are rarely exactly on the centerline of the boat.
5. Use a steel tape run up the pole lift or main halyard to get the mast vertical in the boat.
6. Always pin and tape turnbuckles and cotter pins after tuning. be sure the cotter pins are taped so that the sharp ends are covered to protect people and sails.
Well, there it is, forty-five years of experience condensed into one and one-half pages. now you should be ready to tackle tuning any mast. in fact, I hear there are some openings for riggers for the next America's Cup.
Buzz Ballenger, Pres.Back