Lug and square sails are four-sided sails set on yards. This calculator compares two selected attachment locations along one normalized yard. Its percentages are geometric positions only; they do not determine sail balance, aerodynamic force, rig suitability or a recommended sling point.
Lug and Square Sail Yard · Selected Attachment Ledger
Compare two selected attachment marks along a normalized yard without inferring sail balance or rig performance.
Equation Used
- The yard is represented as a straight normalized 0% to 100% reference.
- Both percentages are measured from the same selected forward end.
- Midspan is exactly 50% of the normalized yard length.
- The labels lug and square distinguish selected comparison marks only.
- Sail geometry, forces, trim, stability, rigging loads, recommended positions and safety are excluded.
Former aft-bias, re-tie and balance implications are narrowed to signed offsets, spans and separation between selected marks.
Normalized yard-position ledger
The forward yard end is 0%, the aft end is 100%, and midspan is 50%. The two selected marks are plotted directly on that reference.
What the calculator determines
Use it to compare two entered attachment positions, their signed offsets from midspan, the aft span from the selected lug mark, and the signed separation between marks.
Exact selected arithmetic
Square offset from midspan=ps−50; lug forward offset from midspan=50−pl; lug aft span=100−pl; signed attachment separation=ps−pl.
Selected-input example
For pl=33% and ps=50%, the selected lug mark is 17 percentage points forward of midspan, its aft span is 67%, and the marks are separated by 17 percentage points.
Geometry, not rig tuning
Actual sail behavior depends on sail cut and area, yard and mast geometry, halyard and parrel arrangement, sheet forces, trim, apparent wind, hull stability and operating conditions. No safe rigging or tuning recommendation is made.
Questions
Does an attachment at 50% prove a square sail is balanced?
No. It identifies yard midspan only.
Does a 33% mark define the correct lug position?
No. It is merely the selected input; the physical rig requires its own design and operating guidance.
References & Further Reading
- Wikipedia contributors. Lug sail. Wikipedia
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