Short answer
Without a gauge, set rig tension by counting turnbuckle turns equally on each side and judging the result under sail: in working breeze the leeward cap shroud should go slack-but-not-flopping, and the mast should stay in column. Record turn counts so the setting is repeatable — but a gauge remains the only way to hit a specified figure.
Counting turns is a relative method, not an absolute one. It keeps both sides matched and makes a setting repeatable, which solves most owner problems, but it cannot tell you whether you have reached the percentage of breaking load a manufacturer specifies.
The under-sail check is the practical test. In working breeze on each tack, the leeward cap shroud should be noticeably slack without swinging freely, and the mast should stay straight in column. Excessive leeward slack suggests under-tension; a mast that will not straighten suggests the lowers need work.
Over-tensioning causes real damage. Excess standing-rigging tension loads chainplates, the hull structure and the mast step continuously, not just when sailing. More tension is not a safety margin.
Where a manufacturer specifies a figure, that figure wins over feel. Borrow or hire a gauge, or have a rigger set the baseline once and record it so you can return to it. See how to tune a masthead rig.
Key facts
- Relative method
- Equal turnbuckle turns each side, counted and recorded
- Under-sail check
- Leeward cap shroud slack but not flopping; mast in column
- Over-tension risk
- Continuous load on chainplates, hull structure and mast step
- Gauge required for
- Hitting a manufacturer-specified percentage of breaking load
Full background: read the guide