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

Design wind speed (kt)

Required number in kt. Design or comfortable upwind apparent wind speed of the boat.

Reef factor (%)

Optional number in %. Percentage of the design wind speed at which the reef is taken.

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

Calculate the apparent wind speed at which to reef using reefWindKt = designWindKt * reefFactorPercent / 100. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

Reefing wind speed (kt)

Returned number in kt. Apparent wind speed threshold for reefing.

• 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.
• Reef factor must be at least 10.
• Reef factor must be no greater than 100.
• 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.

• Lee Bow Effect Calculator

  Project current onto the desired course as current speed times the cosine of the relative angle. This along-course proxy does not model the true lee-bow effect, which also depends on heading, boat speed, and apparent wind.

• Apparent Wind Angle Calculator

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

• Apparent Wind Speed Calculator

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