Your mast is held aloft by the Standing Rigging (forestay, backstay, upper shrouds, double lower shrouds). The term tuning refers to adjustment of the standing rigging so that the mast remains “in column” (not bent) when under load. This is accomplished by following the procedure outlined below:
At The Dock:
1. Adjust forestay and backstay so that the mast is straight up and down (perpendicular). Tie a bolt to a 6 to 7 foot long piece of twine to make a quick easy plumb bob, and tape the free end of the line to the front of the mast as high up as you can reach. This device will help you determine whether the mast is perpendicular or not. Be sure the boat is level while checking this.
2. Adjust upper shrouds so that mast is straight up and down athwartships, that is, from side to side as opposed to bow and stern. A useful method is to use the main halyard as measuring tape. Stretch the shackle end of the main halyard down to the starboard toe rail abeam of the mast-step and then cleat the tail. Then walk halyard around to the portside toe rail. If it doesn’t reach the toe rail, you should loosen the starboard turnbuckle and take up on the port one.
3. The upper shrouds should be firm but not bar tight. A 50 pound push should deflect the upper shroud about 1 1/2″ at shoulder height.
4. The lower shrouds (4 of them) should be adjusted so that they are looser than the upper shroud. While at dock, they should have no slack, but also have no pressure on them. The lower shrouds are correct at this stage when a push on any one of the lower shrouds deflects the middle of the mast the same amount in the direction of the shroud pushed. No shroud when pushed should deflect the mast more than any other shroud when pushed equally hard. If this can’t be achieved, the upper shrouds are too tight. Back off 1/2 turn at a time on the upper shroud turnbuckles until the tension on the lower shrouds is brought into balance. The tuning of the mast is completed while under sail, as discussed in the following section.