A properly-centered rig is necessary before sailboat masts can be adjusted. Improperly-centered rigs will cause masts to lean on one side and result in asymmetrical jib-lead positions from tack to tack.
Getting your rig centered in the boat is a prerequisite to any mast-tuning project. Skip this job and you’re asking for chronic speed problems. Your mast will lean to one side, causing different helm and the need for asymmetrical jib-lead positions from tack to tack. And if the mast is “blocked” out of column at the partners, you may never get it to sit straight sideways in the boat.
- Sight the mast Before stepping the mast, lay it on saw horses with the sail track facing up. Sight the length of the track for any permanent side bend (a common problem in masts that are trailered). If there’s bend, turn the mast on its side and bend it back. This is much easier than it sounds, as most masts are relatively soft sideways.
Next, sight the spreaders to see if they have the same fore-and-aft angle to the mast. If the spreaders have noticeably different angles, modify or adjust the spreader brackets to correct the asymmetry.
- Center mast in partners Step the mast, but do not insert the mast blocks in the deck partners. The mast must float freely from side to side in the partners. Adjust the primary shrouds, using only hand tension, until the space between the sides of the mast and both sides of the partners is equal. Let any diagonal shrouds or backstay remain slack; leave the boom off the gooseneck. (Note: those with deck-stepped masts can ignore Steps 2 and 4.)
- Center rig over boat Measure back from the bow and make two equidistant marks abeam of the mast on the gunwale. Then take a metal tape measure, hoist it on the main halyard, and measure first to one mark and then to the other. The two measurements should be close – within 1/4 inch in a dinghy and 3/4 inch on a 40-footer. If the rig is centered within these limits, you can skip to Step 5.
- Adjust blocking or step Mostly likely, the tip of your mast will be off-center by an unacceptable amount. The cause? Either the deck or mast step is mounted off-centerline. To check your deck, measure from the partners out to both gunwale marks. Checking the mast step is a bit harder: level the boat on the trailer or poppets, then drop a plumb bob from the partners to the step.
To make up for any asymmetry, adjust the primary shrouds until the masthead tape measure says the rig is centered. This means that the mast will no longer lie in the center of the partners. Next, modify the mast blocks accordingly, shaving one side and building up the other. If the mast is laying hard against one side of the partners without blocking, you’ll have to unscrew and move the mast step or take a grinder to widen the partners. It’s usually easier to modify the partners.
This step is the most important in the rig-tuning process. Too many sailors try to center their rig without checking and modifying the mast blocks. As a result, the mast leans hard against one side of the blocks and is pushed out of column. The diagonal shrouds can be asymmetrically tensioned to compensate, but this compensation only works in one wind strength. In other conditions, the unwanted side bend reappears. Proper mast blocking avoids this frustration.
- Tighten the shrouds By now, the mast should be centered over the boat. Primary shrouds are still hand tight, everything else is slack, and the modified mast blocks have yet to be inserted. If you followed Step 1, the mast is straight, the spreaders are symmetrical, and there shouldn’t be any sideways bend as you sight up the sail track.
Get out the rig tools and equally tension both primary shrouds. Keep sighting up the sail track. You should be able to apply moderate tension before the mast begins to bend sideways from compression. If significant side bend appears with only a couple of turns of shroud tension, you’ve got a problem: the rig is unstable. If your rig is OK, skip to Step 7.
- Stabilize the rig If the mast is unstable, it will be difficult to finish the tuning process. At full tension in a breeze, an S-shaped side bend is likely. There are several possible causes for an unstable rig, and most are easy to fix.
First, without unstepping the mast, recheck the spreader angles. The most accurate method is to measure from the spreader tips to a common mark on the forestay. Each spreader should measure the same. With swept-back spreaders, a quicker method is to get off the boat, stand to one side, and sight through both shrouds. From this angle, spreader asymmetry is usually apparent.
If the spreader angles are OK, make sure that both spreaders are the same length. Then check to see if the main bulkhead and chainplates are symmetrical. All of these asymmetries can be corrected by modifying your spreader lengths or angles.
If there’s nothing wrong with your spreaders, look at the mast step. If the step isn’t level athwartships, or if the bottom of the mast itself isn’t level, then one side of the mast will be under more compression than the other. This causes the mast to bow sideways, away from the loaded side. Shave the mast butt so both sides bear evenly.
If none of these solutions work, you can assume the worst: a twisted mast. Aluminum masts are pulled as they are extruded out of a die, and some degree of permanent twist is inevitable. If that twist is excessive, no amount of fudging with the diagonals will tune the mast straight sideways through a range of wind conditions. Luckily, excessive twist is rare, but when it happens, the only solution is to buy a new mast.
- Final tuning Centering your rig can take some time, but it will make the final tuning of the rig go much quicker. After the rig is centered and straight, insert the partners blocks and apply full tension to both primary shrouds in equal numbers of turns. Then take the slack out of the diagonals by hand and go sailing. All that should remain is some minor diagonal fine-tuning to ensure your mast remains stable and straight sideways. And if you mark your turnbuckles at the end of the season, you’ll never have to center the rig again.