Mast tuning is one of those topics that gets treated as either a mystery requiring specialist intervention or a five-minute job that any sailor can eyeball. Neither is accurate. Correct tuning matters for two independent reasons: performance, and rig longevity. A poorly tuned rig — too little tension, wrong rake, no prebend — sails worse upwind, causes the mast to pump in a seaway (accelerating fatigue in the standing rigging), and can allow the mast to invert under load in certain conditions. A significantly over-tensioned rig can bend or crack the mast, overload the chainplates, and cause premature fatigue in the wire at the fittings. The correct tension is specific to your rig, your sails, and your sailing style, and it's not that hard to find if you work through it systematically.
Before You Touch a Turnbuckle: Athwartships Alignment
The first check is not about tension. It's about whether the mast is centered in the boat. A mast that's leaning five degrees to port before you've touched the shrouds is not a tuning problem — it's an alignment problem, and no amount of shroud adjustment will make a crooked rig perform correctly.
The measurement: take a steel tape measure — not a cloth tape, which stretches — and measure from each chainplate to the same point on the mast. Do this on both sides of the boat with the boat level. The two measurements should be within two centimeters of each other. If they're not, loosen the leeward shroud and tighten the windward shroud (turning the turnbuckle clockwise to shorten the shroud and pull the mast toward that side) until the measurements equalize. Do the cap shrouds first, then check the lowers. Don't move on to setting tension until the mast is centered.
On fractional rigs, also check mast position relative to the partner fore-and-aft: the mast should be sitting in the center of the partner opening, or wherever the mast builder specifies, before you set rake.
Rake
Rake is the fore-and-aft lean of the mast. A mast that rakes aft — leans toward the stern — shifts the center of effort of the sail plan aft relative to the center of lateral resistance of the hull, which increases weather helm. Some weather helm is desirable and safe; it gives the boat a tendency to round up toward the wind if you let go of the helm, which is what you want as the default response in a squall. No weather helm at all, or lee helm, is a handling problem.
Standard mast rake for a cruising sailboat is 1 to 2 degrees aft, which on a 15-meter mast translates to roughly 25 to 50 centimeters of aft lean at the masthead. The measurement method: hang a plumb bob or heavy weight from the main halyard to the deck. Measure the horizontal distance from the main halyard exit at the mast base to where the plumb line contacts the deck, then compare to the same measurement from the masthead halyard sheave. The difference is the rake amount.
Rake is set primarily by the forestay length relative to the backstay. To rake the mast aft, lengthen the forestay (unscrew the turnbuckle, adding slack) or shorten the backstay (tighten the backstay adjuster or turnbuckle). The two adjustments are linked — you can't change one without affecting the other, so make changes in small increments and measure between them. If your boat has a fixed backstay with no adjuster, rake is set at the forestay turnbuckle only, and that's the only adjustment available.
Prebend
Prebend is the deliberate introduction of forward curvature into the mast — bowing the mast slightly forward at mid-height by setting the aft lower shrouds tighter than the forward lowers. The purpose is to flatten the mainsail along the luff: a mainsail cut for a straight mast will be too full if the mast is straight and will set correctly when the mast is bent forward by the designed amount. Many production sails are cut for a specific prebend, and the sailmaker's specifications should be the starting point for this setting.
To measure prebend: hold a straight edge (or a taut line from the gooseneck fitting to the masthead, mimicking the luff of the sail) against the aft face of the mast and measure the maximum gap between the mast and the straight line. That gap — typically 50 to 100mm at the point of maximum curvature for a cruising mainsail, or as specified by the sailmaker — is the prebend. If your sails came with a tuning guide specifying prebend, use it. If not, start with light prebend (tighten aft lowers approximately 10% more than forward lowers) and assess sail shape upwind before increasing.
Setting Shroud Tension
Tension on the shrouds is the most consequential single adjustment in mast tuning, and it's where most sailors are flying blind. "Hand tight" is not a tension specification; it describes whatever the person doing the tightening happens to apply, which varies enormously. The tool that removes the guesswork is a rig tension gauge — a Rig-Rite TC-2 or equivalent — that converts wire deflection under a known load to a tension reading. Every offshore boat should have one, and every sailor who does their own tuning should know how to use it.
The target tension for cap shrouds at rest (boat sitting in flat water at the dock with no sail load) is 15 to 20 percent of the wire's breaking load. For 7/16-inch (11mm) 1x19 stainless, which is common on 38 to 42-foot cruising boats with a breaking load of roughly 5,500 to 6,000 kg, that translates to 825 to 1,200 kg of tension — a range that is precise enough to use as a target but allows variation between different boats' rig geometry. For a 40-foot boat with slightly smaller standing rigging, 300 to 450 kg on the cap shrouds is a reasonable starting point. Check the mast manufacturer's tuning guide for your specific spar, which will give you a tighter specification.
Lower shrouds should be tensioned to approximately 60 to 70 percent of the cap tension. They're doing different work — limiting aft and fore-and-aft mast deflection rather than carrying the primary athwartships load — and overtightening them causes mast column issues. The aft lowers can be set slightly tighter than the forward lowers to induce the prebend described above.
Set both cap shrouds in equal increments. Tighten the port cap shroud two full turns, then the starboard cap shroud two full turns, then measure alignment again. Alternate sides rather than fully tensioning one side before starting the other; this way you catch alignment drift before it becomes significant. Use the tape measure check between each round of adjustments.
The Sailing Verification
Dock tuning gets you in the right neighborhood; the sailing check confirms you're there. The test condition is upwind sailing in 12 to 18 knots of true wind, close-hauled with sails properly trimmed. Look at the leeward cap shroud. In correct tune, the leeward shroud should be just barely slack — not taut, not flopping and swinging. The wire should have a very slight catenary (droop) in it but should not be visibly oscillating or waving.
If the leeward cap shroud is visibly loose — swinging, flopping, able to push sideways with one finger under load — the cap shrouds need more tension. Go back to the dock, add two to four turns to each cap shroud equally, and test again. If the leeward cap shroud is drum-tight in 15 knots of breeze, you're carrying more tension than necessary for the sea state, which increases fatigue loading on the fittings and wire without adding meaningful rig stability. Neither extreme is acceptable for offshore work.
Mast Builders' Guides
Selden, Sparcraft, Hall Spars, and most other production mast manufacturers publish specific tuning guides for their sections, often downloadable from their websites. These documents specify shroud tension by wire size, mast section, and boat length, and include prebend specifications for the section and advice on spreader angle. If you have a Selden mast, use the Selden guide. The generic approach described here gets you most of the way to a correctly tuned rig; the mast-specific guide takes you the rest of the way and may include cautions about that particular extrusion that are worth knowing.
Fractional Rigs and Jumper Struts
A fractional rig — where the forestay attaches at a point below the masthead, typically 7/8 or 3/4 of the mast height — uses the backstay tension to control mast bend rather than relying primarily on spreader angle and shroud tension. The jumper struts (the angled struts at the upper mast on fractional rigs) control the interaction between backstay tension and the mast's bend response: the strut angle and tension determine how the mast reacts when backstay is applied.
The advice here is simple and important: do not adjust jumper strut tension without a tuning guide from the mast builder or a qualified rigger who knows your specific rig. The jumper strut geometry is designed for a specific combination of rig geometry, spreader angle, and sail plan. Changing it without understanding the interaction between the variables can result in a mast that inverts — bends aft at the top — when backstay is applied, which can cause structural failure. If you have a fractional rig and you're unsure whether your jumper strut tension is correct, get a rigger involved before sailing offshore.
Record Everything
Tuning is not a one-time event. Shrouds stretch slightly in their first few passages, turnbuckle threads relax slightly after hard use, and the boat's rigging finds a new equilibrium through a season. Record your turnbuckle positions (count the threads showing below the locking nut and write it down), your gauge readings at each cap and lower shroud, and the date you set the rig. Check tension after the first offshore passage and again at the mid-season point. If your tension readings have dropped significantly from the initial setting, find out why — either the shrouds are stretching (normal for new wire, first season only), the turnbuckles are backing off (check locking nuts and replace locking mechanisms if needed), or something in the rig geometry has changed that needs investigation.