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

Wind speed (kt)

Required number in kt. True wind speed.

Efficiency ratio

Optional number. Fraction of wind speed the boat is expected to achieve; zero point five approximates a cruising monohull.

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

Compute the polar target boat speed by multiplying true wind speed by an efficiency ratio. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

Target boat speed (kt)

Returned number in kt. Expected upwind-to-downwind average target speed.

• 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.
• Efficiency ratio must be at least 0.1.
• Efficiency ratio must be no greater than 2.
• 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.

• Downwind VMG Calculator

  Compute downwind velocity made good as target boat speed times the absolute cosine of the true wind angle.

• Upwind VMG Calculator

  Compute upwind velocity made good as target boat speed times the cosine of the true wind angle.

• Apparent Wind Angle Calculator

  Compute the apparent wind angle from true wind speed, boat speed, and true wind angle.