Why does my mainsail keep luffing or losing drive, and how can I fix it?
On my 32-foot cruising yacht, the mainsail keeps lifting near the mast when sailing upwind, and sometimes the back edge flutters even with the mainsheet pulled in. We also lose speed after tacking. What should I check and adjust first?
Answer
Your mainsail usually luffs because you are pointing too close to the wind or the boom is too far out; it can also lose drive when sheeted too tightly and stalled. Check your heading and mainsheet first, then halyard tension, twist and battens, because flutter near the mast needs a different remedy from flutter along the trailing edge.
Find the trouble spot
Hold a steady close-hauled or close-reaching course and have someone watch the sail while you steer. Keep everyone clear of the boom and mainsheet tackle during adjustments.
- The front edge lifts or bubbles: bear away a few degrees without changing the sheet. If the sail fills and the boat accelerates, you were pinching too close to the wind.
- Much of the sail flaps: if your course is sensible, bring the mainsheet in gradually until the flapping stops.
- The sail looks full but the boat feels lifeless: ease the mainsheet slightly. An over-sheeted main can stall without obvious flapping.
- Only the trailing edge vibrates: inspect the leech line and battens. Hauling harder on the mainsheet may simply stall the sail.
After a tack, bear away slightly and ease the main enough to let the boat accelerate, then sheet in progressively. Trying to point high straight away leaves the keel and rudder working poorly at low speed.
Check hoist and sail shape
Check that the main is fully hoisted and the halyard has not slipped through its clutch. Small wrinkles running aft from the mast commonly indicate insufficient luff tension. Too little tension allows excessive fullness, particularly higher up, making the boat harder to steer and the leech harder to control.
With a conventional slab-reefing main, ease the mainsheet and kicker before adjusting the halyard. Check that the topping lift and unused reefing lines are not holding the boom or sail in the wrong position. As the breeze builds, apply enough halyard tension to remove persistent wrinkles. Light airs generally need less tension.
If the sail will not rise properly, check the slides, mast track for dirt and halyard routing before applying more winch force. There is no universal halyard load: sailcloth, sail size and the luff system determine it.
Use the outhaul to flatten the lower sail as the wind increases. Leave some depth for light winds and waves. A main working behind a large overlapping genoa generally benefits from a flatter lower section than one paired with an approximately 105% jib.