Use this sailing calculation for planning or education when the vessel, units, reference directions, environmental conditions, and assumptions are explicitly known.

Displacement (t)

Required number in t. Displacement of the hull in tonnes.

Wetted coefficient

Optional number. Wetted surface scaling coefficient between 1 and 4.

The cited overview of Sailing supplies background for the terminology and domain context used by this tool.^[1]

Estimate wetted surface from displacement using wettedSurfaceAreaM2 = displacementTonnes^(2/3) * wettedCoefficient with a default coefficient of 2.5. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

Wetted surface area (m²)

Returned number in m². Estimated area of the hull wetted surface.

• Vessel design and loading, sea state, wind, current, tide datum, weather, chart accuracy, equipment, crew, and safety margins can make real conditions differ from a simplified result.
• Displacement must be at least 0.
• Wetted coefficient must be at least 1.
• Wetted coefficient must be no greater than 4.
• Use finite inputs in the displayed units, preserve source measurements and assumptions, and independently verify consequential decisions.

Cross-check current charts, tide and weather services, instrument data, vessel manuals, and applicable navigation rules, and retain a conservative alternative plan.

• Frictional Resistance

  Compute frictional hull resistance using frictionalResistanceNewtons = 0.5 * 1025 * speedMs^2 * wettedAreaM2 * frictionCoefficient with a default coefficient of 0.004.

• Air Draft With Heel

  Estimate effective air draft while heeled using a simplified rotation about the lower rail. Actual clearance also depends on hull geometry, trim, waves, and the heel pivot.

• Gennaker Tack Load

  Estimate the tack line load of a gennaker using tackLoadNewtons = 0.06 * windSpeedMs^2 * sailAreaM2 with a documented gennaker load coefficient.