Downwind sails are measured differently from upwind sails. The geometry is simpler (no mast groove, no shroud interference), but the hardware interface and deployment system are often the most critical factors.
Goal: Establish the maximum size envelope for the downwind sail.
| Dimension | Definition | How to Measure |
|---|
| SLU (Luff) | Luff length from tack to head | Measure from bowsprit tip (or tack fitting) to halyard sheave position on mast |
| SLE (Leech) | Leech length from clew to head | Derived from design — typically 95-100% of SLU |
| SF (Half Width) | Half the maximum width of the sail at mid-height | Design parameter — determines power vs. pointing ability |
| SMW (Mid Width) | Width at 50% height | Design parameter, confirmed from previous sail or class rules |
| Bowsprit tip to forestay | Distance from the tack point to the base of the forestay | Critical for determining the sail’s effective tack position |
| Dimension | Definition | How to Measure |
|---|
| SPL (Pole Length) | Distance from mast face to pole end | Measure retracted and extended positions |
| SLU = SLE | Luff and leech are equal on symmetric sails | Measure from pole end height to halyard sheave |
| SF (Half Width) | Half the foot width | Design parameter |
| SMW (Mid Width) | Width at 50% height | Design parameter |
- Halyard sheave position: Is it the masthead (standard) or a fractional position? Fractional rigs limit SLU.
- Bowsprit tip position: How far forward of the forestay? Longer bowsprit = longer SLU, but also more tack-point leverage.
- Class rules: Many racing classes limit maximum spinnaker area. Confirm if class rules apply.
Goal: Identify what limits the sail’s shape or deployment.
| Constraint | Effect | How to Check |
|---|
| Forestay | Asymmetric luff wraps around or near forestay — tight luff interferes | Observe: does the bowsprit project far enough forward to clear? |
| Shrouds | Wide Code Zero on a close reach can contact shrouds | Measure shroud-to-centerline distance at relevant heights |
| Spreaders | Wide downwind sails can press against spreader tips | Note spreader length and sweep angle |
| Lifelines | Clew position — will sheets chafe on lifelines? | Note lifeline height and stanchion positions |
| Constraint | Effect | Notes |
|---|
| Forestay sag | If the forestay sags leeward under load, the Code Zero luff may foul | Consider forestay tension capability |
| Furler compatibility | If upgrading from non-furling to furling Code Zero, the existing foil may not support bottom-up furling | Verify furler type before measuring |
Goal: Confirm all attachment points and equipment.
| Question | Why It Matters |
|---|
| What is at the bowsprit tip? (Ring, tang, bobstay pin?) | Determines tack fitting type |
| Is there a tack line system? (Purchase ratio?) | Affects tack ring specification |
| What is the tack height above the waterline? | Affects foot length — too low and the foot drags in waves |
| Question | Why It Matters |
|---|
| Dedicated spinnaker halyard or shared? | Determines head fitting — snap shackle type |
| Halyard exit position on mast? | Determines maximum SLU |
| Halyard snap shackle type and size? | Must match head fitting |
| Question | Why It Matters |
|---|
| Sheet lead position? (Blocks, snatch blocks, or fixed?) | Affects clew reinforcement |
| Number of sheet sets? (Single or double?) | Symmetric needs 2 sheets + 2 guys |
| Sheet blocks or fairleads available? | May need additional deck hardware |
| Question | Why It Matters |
|---|
| Furler brand and model? | Determines luff tape sizing and torsion rope spec |
| Drum height above bowsprit? | Affects tack height deduction |
| Groove diameter? | Must match — same as headsail furler error-prone point |
Goal: Identify deployment system and wear points.
| System | Data Needed |
|---|
| Snuffer/Sock | Ring inner diameter (must clear head fitting), sock length |
| Continuous line furling | Already covered in L3 furler section |
| Free-flying (no system) | Note: recommend at least a retrieval line |
| System | Data Needed |
|---|
| Retrieval line (ATN type) | Line length, attachment point on sail |
| Rope catch | Position on sail for mid-hoist capture |
| V-stripe | Panel marking for crew reference during takedown |
| Item | Data Needed |
|---|
| Spreader patches | Rarely needed on downwind sails (unless sailed very deep) |
| Leech/foot chafe | Note any lifeline or stanchion contact zones |
| UV exposure | Minimal for spinnakers (stored below when not in use); Code Zero furling UV strip if applicable |
| # | Item | Value | ✓ |
|---|
| 1 | SLU (Luff) | ___ m | ☐ |
| 2 | SLE (Leech) | ___ m | ☐ |
| 3 | SF (Half Width) | ___ m | ☐ |
| 4 | SMW (Mid Width) | ___ m | ☐ |
| 5 | Bowsprit type & length | ___ | ☐ |
| 6 | Tack fitting type | ___ | ☐ |
| 7 | Halyard type & shackle | ___ | ☐ |
| 8 | Furler (if applicable) | ___ | ☐ |
| 9 | Deployment system | ___ | ☐ |
| 10 | Class rule constraints | ___ | ☐ |
| # | Item | Value | ✓ |
|---|
| 1 | SPL (Pole Length) | ___ m | ☐ |
| 2 | SLU/SLE (Luff = Leech) | ___ m | ☐ |
| 3 | SF (Half Width) | ___ m | ☐ |
| 4 | SMW (Mid Width) | ___ m | ☐ |
| 5 | Halyard exit position | ___ | ☐ |
| 6 | Pole end fitting type | ___ | ☐ |
| 7 | Sheet/guy setup | ___ | ☐ |
| 8 | Deployment system | ___ | ☐ |
| 9 | Class rule constraints | ___ | ☐ |
For the measurement philosophy behind this protocol, see 01 — Measurement Philosophy.