Short answer

Check mast straightness by sighting up the mast track from directly beneath it at the gooseneck: the track should run straight with no sideways bend. Athwartships centring is confirmed by running a halyard to the same point on each toerail. Check both at the dock and again under sail on each tack.

The mast track is the best straightness reference on the spar because it runs the full length and is machined straight. Stand at the gooseneck, look directly up the track, and any sideways deviation is immediately obvious.

Separate the two questions. Sideways bend means the lowers or intermediates need adjusting. Being off-centre means the cap shrouds are unequal. They look similar from a distance and have different fixes.

Fore-and-aft bend is not a fault by itself. Many rigs are designed to carry controlled fore-and-aft bend, and backstay adjusters exist to induce it. Sideways bend in a masthead rig is what you are hunting.

Check under sail as well as at the dock. A mast that is straight at rest and bends sideways on one tack has a tuning problem the dock cannot reveal. Use the rig anatomy diagram if you are unsure which wire controls which section.

Step by step

  1. Sight the track at rest

    Stand at the gooseneck and look directly up the mast track for sideways deviation.

  2. Check centring

    Run a main halyard to the same point on each toerail and compare the measurements.

  3. Diagnose the cause

    Sideways bend points to lowers and intermediates; off-centre points to unequal cap shrouds.

  4. Repeat under sail

    Sight the track on each tack in working breeze, where tuning errors actually appear.

  5. Adjust and re-check

    Change one thing at a time, in small equal increments, and re-sight after each.

Key facts

Best reference
Sighting up the mast track from the gooseneck
Sideways bend
Adjust lowers and intermediates
Off-centre masthead
Adjust cap shrouds
Not necessarily a fault
Controlled fore-and-aft bend on many rigs

Full background: read the guide