Before Stepping the Mast
The headstay should be 25′ 10,” measured from the attachment point on the mast to the center of the pin at the bow. This is the best setting for maximum versatility. Be sure that the turnbuckle cannot spin freely or your headstay might change length while you are stepping the mast.
After Stepping the Mast
You need to center your mast. To property center the mast, measure 7′ aft from the jib tack shackle to each rail and mark the rail on each side. Using the jib halyard, measure down to each mark, and adjust the upper shrouds until the measurement is the same on each side, then tighten the uppers an equal number of turns.
You should use a Loos gauge to read your shroud tension. Tighten the uppers to 250 lbs. Tighten the lowers until they are just taut, and adjust them to move the center of the mast in line with the hounds and mast tip. Once you have the mast straight, you can tighten and loosen the lowers an equal. number of turns, depending on the wind strength and wave conditions. You should just remove the slack in the lowers, which is not quantifiable on the Loos gauge, since they are generally so loose that you cannot get a reading.
Fine Tuning the Rig
Now that you are set to go sailing, you are ready to fine-tune the rig. You will set up the rig to the wind conditions you have and will need to adjust it for windier or lighter breezes on other days.
The two primary adjustments are the mast blocks and the lower shrouds. First we’ll take a look at the effects that these adjustments have on the sails.
The mast blocks control the amount of sag in the headstay. Headstay sag is your primary power control, so having the mast blocked correctly at the deck is very important. It you feel like you are not going well, this is more than likely the problem.
By adding blocks behind the mast and pushing the mast forward, you are sagging the headstay and adding power to the jib. This power comes at the expense of some pointing ability, but in light air and chop, it is important to keep the boat moving fast. Forward speed will allow you to point the boat, and it will get your underwater foils working to create lift.
As the wind increases, it is necessary to add blocks in front of the mast, moving the mast back and tightening the headstay. This removes power from the jib, and results in better pointing ability. It would be simple to say that the mast should be in a certain position at the deck for every wind range, but the real determining factor is the sea condition. In light air and flat water, you may actually sail with a tighter headstay than in medium air and big chop.
The lower shrouds affect the rig in two important ways. First, they restrict bend in the middle of the mast and tighten the headstay because the chainplates on a Sonar are positioned aft of the partners. Second, they control the side bend of the mast. These two considerations are of primary concern when setting the Sonar mainsail.
Side bend is a power control for the mainsail in much the same way that headstay sag is a power control for the jib. The lowers should be adjusted so that, in light air, the mast is perfectly straight. This adds power to the front of the sail and makes the overall shape fuller, matching the shape of the jib with the headstay sagged.
As the wind increases, you should tighten the lowers to keep the mast straight until the boat becomes overpowered. In heavy air when the boat is overpowered, ease the lowers as much as four full turns to help de-power the middle of the mainsail. Tight lowers will restrict fore and aft mast bend and counteract the flattening effect of the backstay on the mainsail. Your crew weight will determine when and how much you have to de-power the mainsail.
Your main is cut to fit a mast that bends evenly along its entire length, Because of this, it is important to check your lower shroud tension anytime you adjust your mast blocks. If you add blocks behind the mast, pushing it forward to sag the headstay more, the lowers will become tighter.
As they become tighter, they will restrict bend and possibly invert the top of the mast. The mast is being held forward at the hounds by the headstay, aft at the spreaders by the lowers, and fixed at the butt. Thus, if you push the section of the mast between the spreaders and the butt forward, the section between the spreaders and the hounds will actually move aft, straightening the top.
This is why it is important to check lower shroud tension whenever you change your mast blocks. They may not need adjusting, but we have found that 1/2-I turn on the lowers approximately equals one 1/2″ mast block.
Keep in mind that it is illegal to adjust the shrouds while racing, so leave yourself plenty of time before the race to ‘get up your rig.‘