Mainsail

Mainsail Measurement Guide

Follow the 4-Layer protocol for a systematic, error-proof mainsail measurement.

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L1 — Geometry (Maximum Dimensions)

Goal: Determine the maximum sail size (P & E).

1. P (Maximum Luff Length):

   • Definition: Vertical distance from the bottom edge of the highest halyard sheave to the top surface of the boom.
   • How to measure: Pull the main halyard to its highest point, mark it, then measure from that mark down to the boom’s top surface.
   • In-mast furling: Measure the exit slot length instead.

2. E (Maximum Foot Length):

   • Definition: Horizontal distance from the inner edge of the aft black band on the boom to the aft face of the mast.
   • No black band? Measure to the outhaul sheave’s forward edge, or use the existing mainsail foot as a reference.

3. MTW (Mast Top Width):

   • Definition: Width of the mast at the top. Critical for square-top mainsails.

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L2 — Physical Constraints

Goal: Establish maximum roach boundaries.

1. Backstay:

   • Measure: Horizontal distance from the backstay pin center to the mast’s aft face.
   • Why it matters: Determines whether the mainsail can have a large roach or square top. If the backstay is close, roach must be limited.
   • Note: If the boat has running backstays, note this clearly.

2. Mast Bend (Pre-bend):

   • Measure: At rest, measure how far the mast center deflects from a straight line.
   • Why it matters: Pre-bend determines the luff curve of the mainsail. Mismatch = wrinkles along the luff.

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L3 — Hardware Interfaces

Goal: Confirm all hardware connections.

1. Luff System:

   • Type (check one):
     • Bolt rope → Groove diameter? (e.g., 9mm, 11mm)
     • Slides → Model? (provide photo, measure waist width)
     • Cars → Brand? (e.g., Battslide, Rutgerson) → Track width?
   • Quantity: Total number of slides/cars.

2. Tack (Lower Forward Corner):

   • Cutback: Distance from tack ring center to the mast’s aft face.
   • Offset: Distance from tack ring center to the boom’s top surface.
   • Connection: Pin or shackle? Diameter?

3. Clew (Lower Aft Corner):

   • Connection: Slider / Velcro strap / Ring?
   • Outhaul: Direction of pull relative to boom.

4. Head (Top Corner):

   • Headboard width: Typically 150mm — confirm or measure to actual.
   • Pin diameter: Measure the halyard shackle pin.

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L4 — Protection & Accessories

Goal: Identify reinforcement positions.

1. Spreaders:

   • Position: Height of each spreader level, measured from the boom’s top surface.
   • Patch needed? High-performance sails and any sail with roach near the spreaders should have patches.

2. Lifelines:

   • Height: Lifeline height above deck, to ensure the mainsail foot clears without chafing.

3. Reefing:

   • Type: Single-line / Slab reefing?
   • Reef height: If specific heights are required (e.g., to clear a dodger or bimini), note them.

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For the measurement philosophy behind this protocol, see 01 — Measurement Philosophy.