Rig Tuning Guide

Your rigging specifications page tells you what wire goes where. This page covers how to actually tune the rig once it's installed. The guidance here is synthesized from years of discussion on the IC27A forum (opens in new tab), with particular thanks to Judy Blumhorst of Hyde Sails for her detailed contributions.

Before You Start

• Inspect all rigging hardware — turnbuckles, clevis pins, cotter pins, toggles — before applying any tension.
• Check chainplate bulkheads with an awl test (see Critical Inspections).
• Have a helper or use a bosun's chair for measuring at the masthead.

Step 1: Center the Mast

Hoist a metal tape measure on the main halyard to the masthead. Measure from the masthead down to each upper shroud chainplate at deck level. Adjust the upper shroud turnbuckles until both measurements are exactly equal. The mast should now be standing perpendicular to the deck athwartships.

Step 2: Dock Tune the Uppers

With the mast centered, tension the upper shrouds to approximately 20% of the wire's breaking strength. For the 5/32" 1×19 wire used on C27 uppers, this is roughly 500 lbs per side.

Look up the mast along the sail track — it should run perfectly straight from bottom to top with no S-curves.

Step 3: Set the Lowers

The forward and aft lower shrouds control fore-and-aft mast bend. Tension them to approximately 15% of breaking strength. The aft lowers should be slightly tighter than the forward lowers to induce a small amount of forward prebend in the mast.

Step 4: Tune Prebend

Prebend is a slight forward bow in the middle of the mast. On a C27, you only need about one inch. Prebend ensures that when you tension the backstay going upwind, the mast bends forward (flattening the main) rather than aft (which would make the sail baggy and destabilize the rig).

To check prebend: sight up the aft face of the mast. The middle should bow slightly toward the bow. Adjust the aft lower shrouds to control the amount of prebend.

On a properly tuned rig, when you tension the backstay the aft lower shrouds tighten and the forward lowers loosen slightly. If the opposite happens, your mast may be inverting — re-check your prebend.

Step 5: On-the-Water Fine Tuning

Dock tuning gets you in the ballpark. Final tuning happens under sail in moderate breeze (10–15 knots):

• Sail close-hauled on each tack and sight up the mast track.
• The leeward uppers should be just barely slack — not flopping, but not bar-tight either.
• If the mast falls off to leeward, tighten the uppers equally on both sides.
• If the mast hooks to windward at the top, ease the uppers slightly.
• Count turnbuckle turns so you can reproduce the tune later.

Measuring Tension Without a Loos Gauge

A Loos tension gauge is the standard tool, but they cost $150–200 and you may need two sizes. An alternative is the "folding rule method" which directly measures wire elongation:

1. Mark a 2-meter section of shroud with tape.
2. Measure the distance between marks with calipers before tensioning.
3. Tension the shroud and re-measure.
4. For 1×19 stainless wire, each 1mm of stretch over 2 meters equals approximately 5% of breaking strength.

Target Tensions

These are starting points. No two boats are the same — sail condition, local conditions, and how you use the boat all affect the ideal tune. The Selden mast tuning guide (available in the IC27A file library (opens in new tab)) is an excellent detailed reference.