Historical square-topsail schooners combined schooner fore-and-aft sails with square sails on the foremast. This calculator performs conventional sail-area/displacement and direct entered-value arithmetic only; it does not predict sailing performance or safe sail handling.
Topsail Schooner Interactive Calculator
Enter illustrative sail areas, displacement weight and apparent-wind angle. Outputs are comparison and bookkeeping quantities, not performance, stability, load, reefing or seaworthiness guidance.
Equation Used
- Entered displacement is treated as weight in pounds and divided by 64 lb/ft^3.
- Entered working and square-topsail areas are user-supplied bookkeeping categories.
- Long-ton conversion uses 2240 lb per long ton.
- Apparent-wind angle is clamped to 0-180 degrees before subtracting from 180 degrees.
- No hull form, stability, righting moment, sail force, aerodynamic interaction, course, true wind, boat speed, sea state, rig load, handling, rating or safety is modeled.
Historical Square-topsail Schooner Rig
The Smithsonian's catalogue of American ship models describes square-topsail schooners with square forecourse, topsail and sometimes topgallant sails on the foremast, together with schooner fore-and-aft sails. A 1913 sailing manual in the Library of Congress similarly identifies the type by square sails on the foremast.
Those sources support rig identification, not universal performance or operating numbers.
What This Calculator Does
The tool evaluates a conventional imperial sail-area/displacement comparison index from entered sail area and displacement. It converts pounds to long tons, adds two entered area categories and subtracts an entered apparent-wind angle from 180 degrees.
The angle does not change the area or SA/D result and is not an aerodynamic model.
Comparison and Bookkeeping Relations
T = Delta/2240
Asum = SA+ST
alpha = 180-AWA
The SA/D index is dimensionless only under consistent area and displacement-volume units. Its value depends on the exact sail-area and loading definitions used.
Worked Arithmetic Example
For entered SA=4500 ft² and displacement=260,000 lb, the conventional index is about 17.6 and displacement is about 116.1 long tons. Adding entered ST=850 ft² gives 5350 ft². An entered AWA of 135 degrees is 45 degrees from dead astern.
None of these values predicts speed, heel, stability, sail load or a safe sail plan.
Model Boundary
- Historical arrangement: square sails on the foremast combined with schooner fore-and-aft rig.
- Calculated: conventional SA/D index, unit conversion, direct area sum and angular subtraction.
- Not calculated: actual sail inventory or projected area, aerodynamic force, sail interaction, drive, drag, speed, point of sail, leeway, heel, righting moment, stability, rig loads, line loads, weather, waves, crew requirements, reefing, handling, ratings or seaworthiness.
Frequently Asked Questions
Does the SA/D index predict boat speed?
No. It is a comparison ratio and omits drag, hull form and conditions.
Which sails belong in SA?
The calculator does not choose a convention; document the entered definition and compare only consistent values.
Does the area sum represent projected downwind area?
No. It is only SA plus ST.
Does the angle offset calculate sail force or trim?
No.
Does an index over 20 mean the vessel is unsafe or overpowered?
No universal threshold is applied here.
Can this guide reefing or vessel operation?
No. Use vessel-specific plans, procedures, forecasts and qualified maritime guidance.
References
- Smithsonian Institution, Bulletin of the United States National Museum 219: square-topsail schooner models and documented sail arrangement.
- Library of Congress, Edwin W. Kendrick, Practical Sailing and Motor-boating (1913), figure 7: topsail schooner carries square sails on the foremast.
- Sponberg Yacht Design, The Design Ratios: SA/D definition, displacement-volume conversion and input-definition cautions.
- U.S. Naval Academy, Buoyancy and Static Stability: 64 lb/ft³ salt-water displacement conversion.
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