The fix, in short
Get the mast straight first. With the boat level at the dock, sight up the back of the mast to check it's straight side to side. Center it by adjusting the cap shrouds evenly.
Use a tension gauge. A rig tension gauge set for your wire size turns 'feels tight' into a number. Follow the gauge maker's instructions for reading it.
Use the mast maker's numbers. Selden and other spar makers publish tuning guides by rig type. Fractional and swept-spreader rigs often need different tension from masthead rigs.
Tune under sail. Sail upwind in moderate wind and look at the leeward shrouds. A little slack is common, but shrouds that flop mean the rig needs more tension. Adjust in small steps and tack to even both sides.
Lock everything. Secure every turnbuckle with cotter pins or locknuts and tape over sharp ends.
Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.
Equipment involved
- Rigging: Shrouds, stays, turnbuckles, chainplates and mast
Related guides
- How do I inspect standing rigging for cracks and wear?
- Why is my roller furler hard to furl or jammed?
- All ten public guides
Sources
Background reading we checked while writing this answer. The wording above is our own.
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