A schooner divides its sail plan among two or more masts. This page illustrates a two-masted gaff arrangement, with the mainmast aft of the foremast. Use the calculator to explore an ideal forward sail-force component from apparent wind, area and assumed lift and drag coefficients.
Full Schooner Rig Interactive Calculator
Compare apparent wind speed, total sail area, wind angle and assumed coefficients. The tool estimates a forward aerodynamic force component, not boat speed, heel or a reefing limit.
Equation Used
The calculator applies the article driving-force equation for a fore-and-aft schooner sail plan. Apparent wind creates lift and drag on the sails; the forward component is estimated from sail area, apparent wind speed, lift coefficient, drag coefficient, and beta angle.
- Air density is fixed at 1.225 kg/m3 at sea level.
- Apparent wind speed is converted from knots to m/s.
- Lift and drag coefficients are treated as steady average values.
- Beta is the apparent wind arrival angle measured off the bow, from 25° to 90° in this tool; it is not the sail angle of attack.
Reading the sail plan
The illustrated foremast carries the forward lower sail and the mainmast carries the aft lower sail. A gaff supports the upper edge of a four-sided sail, while a boom supports its foot in this arrangement. Headsails are carried forward, and additional sails may occupy spaces between or above the lower sails.
Standing rigging supports the masts. Running rigging controls sails and movable spars. The sketch identifies components but does not specify spar dimensions, wire tension, block loads or a complete rigging arrangement.
The animation separates a side elevation of the rig from a plan view of wind direction. In the side view the bow points left; the mainmast is aft and taller than the foremast. Gaffs, booms, topsails and headsails remain attached to their supporting spars or stays. In the plan view the bow points upward and beta is measured from it. Blue marks indicate incoming apparent wind, green shows lift and the separate forward-force arrow, and gold shows drag. Arrow lengths are display scales. The sail shapes stay fixed while the entered total area changes the force calculation; they are not a measured sail plan or an aerodynamic simulation.
Schooners and historical sail plans
The schooner Bluenose is a documented example of a vessel designed for fishing and racing. The Canadian Museum of History preserves William James Roué’s collection, including a Bluenose sail plan and construction drawings. Those vessel-specific records are more useful for studying the actual layout than assigning universal dimensions to every schooner.
The calculator uses a total area and average coefficients rather than the plan of Bluenose or any other named ship. It cannot reproduce a particular vessel’s performance, crew requirement or sailing procedure.
Forward force from assumed lift and drag
Dynamic pressure is q = ½ρV². The calculator fixes ρ at 1.225 kg/m³ and converts apparent wind from knots using V = 0.514444 × wind_kn. This density is a reference assumption, not a measurement of the current weather.
The forward component is F_drive = qA[C_L sin(β) − C_D cos(β)]. The brackets apply to both coefficient terms. Beta is the apparent wind arrival angle off the bow. It differs from the angle between the sail and its local airflow. Lift and drag coefficients are entered assumptions held constant as beta changes.
The two Drive Force outputs are the same force in newtons and pounds-force. Dynamic Pressure is q in pascals. Pressure / 25 kn is (V_kn/25)²: the dynamic-pressure ratio to 25 kn at unchanged density. It is not a rig load, a reefing instruction or an allowable wind speed.
At 90°, the drag term has no forward component in this projection. Away from 90°, increasing drag reduces the modeled forward component. Changes in sail trim, interference between sails, hull resistance and stability are not solved.
Example: the default force comparison
At 16 kn apparent wind, A = 1,000 m², β = 45°, C_L = 1.30 and C_D = 0.15, dynamic pressure is 41.4975 Pa. The forward coefficient is 1.30 sin 45° − 0.15 cos 45° = 0.81317. Their product with area is 33,744.7 N, or about 7,586.1 lbf. Pressure / 25 kn is 0.4096, displayed as 0.41×.
For a separate hypothetical sail set of 850 m² at 90°, with C_L = 1.35, the calculated force is 11,904.6 N at 8 kn, 41,852.1 N at 15 kn and 116,255.9 N at 25 kn. With all other inputs unchanged, force scales with wind speed squared. These values establish no boat speed, heel angle or safe sail-carrying condition.
The selectable combinations produce positive forward forces from approximately 26.8 N to 350,135.4 N. Pressure / 25 kn ranges from 0.04× to 1.44×. These are mathematical input bounds, not operating limits for a vessel.
What a force estimate leaves out
Dividing sail area among several sails changes how the rig is handled, but handling time and crew needs depend on the actual vessel and equipment. A generic rig label cannot provide fixed crew numbers, service life, cost or pointing performance.
Forward drive is only part of the load. Heel and rig stresses require the other force components, their points of action and the vessel’s stability and structural data. Reefing and rigging settings must follow the actual vessel’s procedures; this calculation supplies neither.
Boat speed needs a balance between propulsion and resistance, with the corresponding apparent wind. Entering wind and area here does not solve that coupled problem.
Frequently asked questions
Is Wind Angle the sail trim angle?
No. Beta is the apparent wind arrival angle off the bow. The tool does not calculate sail angle of attack or choose sail trim.
Does the tool predict boat speed?
No. It estimates a forward aerodynamic force component. Hull resistance, leeway and the relationship between boat speed and apparent wind are not solved.
Does Pressure / 25 kn tell me when to reef?
No. It is only a dynamic-pressure ratio at fixed density. It contains no vessel stability, rig strength or sail-handling information.
Are the coefficients measured for a full schooner rig?
No. They are inputs for a simplified comparison. Appropriate values depend on the actual sail plan, trim and airflow.
Why does drag not reduce forward force at 90°?
The forward projection of drag is proportional to cosβ, which is zero at 90°. This does not mean drag or sideways load disappears.
Can the result set shroud tension or halyard size?
No. It does not calculate structural load paths, local sail loads or rigging requirements.
Does more modeled drive guarantee a faster or safer vessel?
No. Resistance, heel, stability and structural limits must be considered for the particular vessel.
Is the diagram a measured Bluenose sail plan?
No. It is a schematic arrangement. The linked museum collection provides vessel-specific historical drawings.
Sail-plan and force references
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