A brigantine combines square sails on the forward mast with a fore-and-aft mainsail on the aft mast. This illustration follows the modern two-masted arrangement in the Scottish Fisheries Museum rigging guide. Historical terminology and topsail arrangements vary. Explore the rig and compare sail area with displacement using the calculator below.
Brigantine Sail Area-to-Displacement Interactive Calculator
Enter sail area and displacement to calculate SA/D, displaced volume and area per tonne. The 16–18 comparison band is an example, not a recommended brigantine target.
Equation Used
- Entered sail area must use a consistent sail-plan convention when comparing vessels.
- Metric tonnes are converted to pounds, using a seawater density of 64 lb/ft³.
- SA/D is dimensionless; it does not determine vessel speed, stability or required crew.
- The 16–18 band is an example only.
- Drawing geometry, sail proportions and gentle motion are illustrative, not a surveyed vessel or performance simulation.
Headsails connect ahead of the foremast, square yards carry the forward sails and a gaff supports the aft mainsail. Sail area and displacement determine SA/D; the example comparison band does not rate speed or handling.
Recognizing the rig
The forward mast carries square sails attached to yards. The aft mast carries a gaff mainsail, with a boom along its lower edge and a gaff supporting the upper edge. Headsails connect to stays leading forward toward the bowsprit. The museum reference also shows a fore-and-aft topsail above the gaff.
Those sail and spar attachments move together as the illustrated vessel gently rolls. The bow is at the left and the mainsail is aft on the right. Billowing canvas is illustrative; the drawing does not calculate wind loads, pointing angle or boat speed.
Comparing sail area and displacement
The calculator compares the amount of sail carried with the volume of water displaced. It is useful for checking units and comparing consistent sail-area definitions. It cannot by itself decide whether a rig is suitable for a hull.
A vessel’s actual behavior also depends on hull form, stability, sail shape, rig geometry, loading and conditions. Precise crew counts, speed guarantees and rigging settings are not implied by this diagram.
Sail-area/displacement ratio
Convert square metres to square feet using 10.7639104167 ft²/m². Convert metric tonnes to pounds using 2204.62262185 lb/t. Divide pounds of displacement by 64 lb/ft³ to obtain displaced volume in cubic feet.
SA/D = sail area(ft²) / [displaced volume(ft³)]^(2/3). The denominator has units of area, so the ratio is dimensionless. The area-per-tonne result is simply entered sail area divided by entered mass displacement.
The fourth result measures distance from the example 16–18 band. It is zero inside the band, 16 minus SA/D below it, or SA/D minus 18 above it. This distance compares SA/D with the example band; it is not a rating of the vessel.
Worked comparison
For 520 m² and 180 metric tonnes, the entered sail area is 5597.23 ft² and displaced volume 6200.50 ft³. The SA/D ratio is approximately 16.58, with 2.89 m² of sail per tonne. It lies inside the illustrative 16–18 band.
Doubling sail area at unchanged displacement doubles SA/D. Doubling displacement with unchanged sail area multiplies SA/D by 2^(−2/3), about 0.63. These relationships do not predict a corresponding change in speed.
What the drawing does not determine
Two numbers cannot establish mast positions, sail shapes, draught or stability. The rig illustration therefore keeps a coherent arrangement while reporting the calculated ratio and units separately. It is not a scale drawing of a vessel with the entered displacement.
Rig terminology varies across periods and sources. This page uses the modern fore-square/main-fore-and-aft arrangement in the cited museum guide; some historical brigantines carried square topsails aft. The calculator itself is not restricted to one rig definition.
Brigantine calculator questions
Which end is the bow?
The left end, with the projecting bowsprit and headsails ahead of the square-rigged foremast. The gaff mainsail sits aft on the right.
Is 16–18 the correct SA/D for every brigantine?
No. It is an example comparison band, not a recommended design target or stability criterion.
Why does displacement not change the drawn waterline?
Draught depends on hull geometry, which is not supplied. Showing a calculated waterline from displacement alone would be misleading.
Does the animation show actual wind performance?
No. It explains sail placement and attachments. Motion is illustrative, with no wind, force or speed solution.
References
- Scottish Fisheries Museum / Shipshape Heritage Training Partnership: Introduction to Sail and Rigging Types — brigantine photograph and definition on PDF page 22; sail and spar terminology.
- PHRF Southern California: calculated performance parameters — published sail-area/displacement formula and importance of the chosen sail-area definition.
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