Mast Rake and Rigging Tension, March 1976.
Firstly, I will state the obvious and that is that the rigging should be tensioned such as to hold the mast to its correct degree of rake and remain straight when beating.
First of all, the question of mast rake; there can be no hard and fast rule on the degree of mast rake since it will to some extent need to be modified due to the cut of the sails and ones own preferance in boat handling. My own preference is to have neutral helm in about a force three and therefore obviously one would have slight lee helm in extremely light winds and the helm would change to weather helm about force four and increase in strength as the wind force increases. The point at which you arrange neutral helm would be a question of your own personal preference.
Speaking purely on a rule of thumb basis, set the mast rake such that the main halyard is six inches away from the mast at boom height when a heavy weight is suspended from the halyard (as though it were a plumb line). This would give a good starting point for alterations due to personal preference. This pre-supposes that the forestay and backstay are now dead tight, with the backstay tensioner fully tightened. The cap shrouds should be really quite tight, the lower shrouds being a little less tight than the cap shrouds. This, of course, would be ascertained when the boat is moored and obviously therefore with no wind pressure in the sails; it is of some advantage if there is just enough extra tension in the lower shrouds to give a gentle forward curve to the mid-section of the mast but there must be no bend whatsoever athwartships.
It must be borne in mind that until your Achilles has seen some hard sailing the rigging wire will stretch; this despite that it has been made from the least stretchy type of wire and that it is some 50% stronger than it is usual to find in boats of her displacement and sail area. Therefore it may well be necessary to go through the above procedure at least a second time to settle the wire before finally deciding the amount of rake to use.
Briefly, to increase normal weather helm the forestay bottlescrew should be loosened and the backstay bottlescrew tightened. To reduce normal weather helm loosen the backstay bottlescrew and tighten the forestay bottlescrew. Abnormal weather helm is a function of sail setting, mainsail reefing and foresail area reduction, and is dealt with under a separate section.
Some weather helm is desirable of course. It gives feel to the helm and therefore the ability to steer a steady course. In addition it reduces leeway by increasing the hydrodynamic lift to windward; though this benefit is lost by inducing drag if the angle of the rudder exceeds about 4° from the centreline of the yacht.
Any further tuning can only be done while the boat is underway and the object here should be that with the large genoa up in a reasonable breeze (let us say a force 4) the mast will be straight. One should have a crew member lying on his back on the foredeck in a position so as to look up the mast and then check which way the mast bends and then progressively tighten the two shrouds on the "hollow" side of the mast after coming up head to wind to make this possible. Repeat the procedure for the other side of the boat and then check and re-check several times to ensure that the mast remains straight after going about on opposite tacks.
NOTE: With current boats we are supplying a backstay tensioner, and this should be slackened when leaving the boat at moorings, tightening up very hard when readying the boat for sailing, and of course prior to adjusting the backstay and forestay. It's only use is to slacken off the tension of the rigging when on the moorings; I do not consider there to be any advantage to adjust it while actually sailing.