To do an effective job of mast tuning, it's important that you understand the principles involved. Let's start with some definitions and explanations. The term 'standing rigging' refers to fixed pieces of stainless steel wire or rod which support the mast. If they offer principally for and aft support, they are called 'stays' (backstay, forestay, etc.). If the support is principally transverse, they are called 'shrouds'.
The shroud which runs from the masthead to a chainplate on the deck near the rail is called the main or upper shroud. If it were to travel this route directly, then the angle of support would be so fine as to induce extreme large tensile forces in the shroud and equally large compressive forces in the mast. To increase this angle of support to the desirable 12 degree or greater, a spreader is inserted approximately half way. This spreader should be angled upwards approximately 5 degrees to bisect the angle formed by the shroud as it bends over the spreader tip. A horizontal spreader or worse still a spreader angled slightly down, is not only unsightly but unseamanlike and dangerous; the spreader may be forced to slip further down the shroud resulting in the loss of the spreader and thus collapse of the mast. The spreader becomes a compressive member and when properly loaded would tend to push the middle of the spar leeward. To avoid the resulting leeward bow, a lower shroud is installed running from the mast at the spreader to the deck near the upper shroud chainplate. Although the principal purpose of the lower shroud is to provide athwartship support, some for and aft support can also be achieved a times by adding two lower shrouds per side, one running forward and the other aft. Therefore, we can have a single spreader rig with single or double lowers. The addition of the spreader and lower shrouds means the mast is now supported at more places transversely than it is fore and aft; and the mast section itself need not be as strong transversely as it does fore and aft. Thus, almost all keel boat masts have a greater fore and aft dimension than transverse.
It is an obvious extension that the more spreaders and shrouds used transversely, and intermediate forestays and running backstays used longitudinally, the smaller the allowable mast section will be. This can be advantageous as it reduces the weight and windage of the mast, and thus the undesirable influence of the mast on the mainsail. The smaller the mast section, the better the flow over the main. However, to keep such a small section standing would require a complex maze of wires. The spar would be difficult to keep in tune and the running backstays and the intermediate forestays would make tacking difficult. Therefore, except in the case of very sophisticated racing craft with large experienced crews, the rigs have a design that is as simple as possible to reduce the degree of attention required. To reduce the drag of the spar and its detrimental influence on the main, triagular and diamond-shaped sections, and extruded aluminum airfoil spreaders are used.
Be sure all turnbuckles are equipped with toggles at their base to eliminate any bending load on the swage and turnbuckle threads. Also see that there is a toggle at both ends of the forestay. As the boat tacks and headsail loading varies from side to side, the forestay terminals experience a much higher fatigue loading.
Start tuning the spar by ensuring that the mast is in the center of the boat, perpendicular to the designed tranverse waterline. Boats often will not sit level at the dock due to the distribution of their accommodation plan and the internal weight or location of crew. So to make sure the mast is plumb transversely, slacken the lower shrouds fully by undoing their turnbuckles. If the spar is stepped through deck, remove the mast wedges as well. Take the main halyard and lead the shackle end to a point on the rail or chainlplate. Adjust the tension in the halyard so that the shackle just touches the rail or chainplate with a given tension, and then cleat the halyard. Take the halyard to the same location on the other side of the deck and with the same tension, the shackle should just touch the rail or chainplate. If not, let off one upper shroud's turnbuckle and take up on the other to bring the masthead closer to centreline until the halyard shackle touches both sides under the same tension. The particular part of the rail or deck you choose as your reference point is not important as long as it is the same point on each side. After the mast is centered transversely, tighten both upper shroud turnbuckles uniformly, one full turn on one side, then one full turn on the other. Repeat until the turnbuckles become difficult to turn. Pin the turnbuckles. Tighten up the lower shroud turnbuckle so that almost all of the slack is removed. That is, the shroud itself should be able to flop about 1" in each direction. Sight up the trailing edge of the spar to make sure that it is still straight.
Now, check your rake. Rake is the for and aft angle of the spar. C&C yachts are designed to carry from 6" of rake on the 24 foot up to about 10" on the 38 foot. Some sailmakers prefer no rake at all; that is a perfectly plumb spar, but from an appearance point of view, as well as helm balance point of view, begin using the measurements mentioned above.
Forward rake should be avoided. Again, use the main halyard to check the amount of rake. Wait for a reasonably calm day and hang a weight, such as a hammer, a wrench or even a bucket of water, from the main halyard at approximately gooseneck level. The fore and aft distance between the halyard and the mast at the gooseneck is the amount of rake. Ease off the forestay turnbuckle and tighten up on the backstay turnbuckle or vice versa until the desired rake is achieved. Now, pin the forestay turnbuckle and the backstay turnbuckle. Unless the rake has to be re-adjusted in the future to correct the helm balance, these will need no more adjusting. Any additional tensioning can be applied by the backstay adjuster.
Reinstall the mast wedges if the mast is stepped through deck and pin the other turnbuckles.
At this time, check that the outboard end of the spreaders are taped and padded to avoid wear and tear on the genoas when tacking.
You now are ready to go sailing to complete the tuning procedure.
Select a pleasant sunny day with a steady 8 to 12 knots of breeze. Put the boat on a starboard tack, close hauled. Sight up the luff groove of the spar. If the mast seems to fall off to leeward at the spreaders, luff up slightly and tighten the starboard lower shroud a couple of turns. Put the boat back on the wind and check the spar again. When the mast appears straight, put the boat about and do the same on the port side. Check the following carefully. First, if the upper shrouds are at optimum tension, when at about 15 to 20 degrees of heel, the leeward rigging should begin to look slack. This is quite natural and should never be tightened. Secondly, when close hauled under genoa and main, the forestay will appear quite sagged. Tensioning the backstay will reduce the amount of say, but the sag itself can never be eliminated. As a rule of thumb, the maximum static backstay pressure should not exceed one-quarter the backstay breaking strength.
If your boat is equipped with double lowers, the forward lower shrouds should be tightened marginally more than the aft lowers to encourage a bit of a forward bow to the mast. This forward bow is conteracted by the luff of the mainsail and the aft lowers. Aft bow should not be allowed. It destroys the sail shape and is countered only by the forward lower shrouds. If you find that the mast, whether or not you have double or single lowers, tends to bow aft rather than forward under backstay tension, the problem may then lie in your mast step. For example, if the mast is resting on its forward end, it may tend to bow aft. Therefore, to correct this situation, wedge up the after part of the heel to encourage a forward bow.
Chainplates may seat and the rigging may stretch to the extent that tuning from scratch will be necessary in a matter of weeks. However, after this initial working-in period, you will find that your boat tends to hold this tune for fairly long periods of time. After becoming used to the feel of the boat, you may wish to either increase or decrease the amount of 'weather helm' - that is, the amount of feel on the tiller. Any sailboat, when going upwind, should have a tendency to 'round up' slightly or head ino the wind if you let go of the helm. However, if you're constantly fighting the boat in order to hold her off the wind, you have too much weather helm. This can be alleviated by taking some rake out of the spar; i.e., raking the spar further forward, and thus moving the centre of the effort of the sailplan further forward. If you find when sailing upwind that the boat tends to fall off the wind and you are constantly having to push her to weather, then you probably have lee helm. This can be overcome by putting a bit more rake into the spar.
With constant tuning as the season progresses, your boat performance will improve. The boat will beel more comfortable to sail. You will find that tuning is a bit of an art; you will begin to notice subtle changes in the behaviour and response of your boat as you make subtle changes in tuning. The important thing to remember is to go about things in a slow and orderly fashion, and before you make any change, make sure it makes sense in your own mind.
REPRINTED FROM C&C OWNERS' MANUAL - JUNE 30, 1976