Reversible vector
Converting true wind to apparent and back with identical boat inputs recovers the original vector within numeric tolerance.
Sailing Apparent & True Wind Calculator
Created by: Sophia Bennett
Last updated:
Convert between apparent and true wind using explicit relative-from angles, boat course and speed, optional current, and east/north vectors.
Sailing Apparent & True Wind Calculator
BoatingConvert wind vectors with a signed relative-from convention and an explicit STW/SOG current basis.
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Starboard positive; port negative; direction wind comes from.
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What is a Sailing Apparent & True Wind Calculator?
This calculator resolves apparent and true wind through two-dimensional velocity vectors with an explicit signed relative-from convention.
US Sailing publishes the same underlying mathematical relationship; this implementation also exposes east/north components and optional current handling.
How the Sailing Apparent & True Wind Calculator Works
Relative-from wind is converted into an absolute air-movement vector.
Boat ground velocity is subtracted for true-to-apparent and added for apparent-to-true.
AW = TW − boat ground velocity
TW = AW + boat ground velocity
Input Guide and Conventions
Wind angle is direction from which the wind arrives relative to the bow: zero ahead, starboard positive, port negative.
Boat course and speed must share a clear ground or water basis. Selecting STW adds the entered current to estimate ground motion; selecting SOG does not.
Current set is the direction toward which water flows, the opposite naming convention from wind direction-from.
Use calibrated, time-matched observations. Mixing averaged boat speed with an instantaneous gust produces a vector that never existed.
Examples
How to Interpret the Results
Solved east and north components expose the vector calculation and make sign mistakes easier to detect.
Apparent wind usually shifts forward as boat speed increases, but the exact result depends on true-wind angle and vector basis.
Converting apparent to true and back should recover the starting vector when every input and convention remains identical.
Unexpected angles often come from port/starboard signs, direction-from versus direction-toward, degrees outside the expected range, or double-counted current.
Recommended Planning Workflow
Write down course, speed basis, wind angle convention, current source, averaging period, and sensor height before entering data.
Compare the calculation with onboard instruments as a diagnostic exercise, not as automatic proof that either source is correct.
For navigation and performance decisions, include leeway, heel, mast motion, calibration, waves, gusts, and current variation.
Common Applications
- • Instrument cross-check education
- • Wind triangle exercises
- • Current-basis comparisons
Planning Tips
Use a consistent STW/SOG basis.
Verify sensor calibration.
Do not use the result for collision avoidance.
Frequently Asked Questions
What angle convention is used?
Angles describe where wind comes from relative to the bow. Zero is ahead, positive is starboard, negative is port, and results normalize to −180° through +180°.
Why does boat speed change apparent wind?
Apparent air movement equals true air movement minus boat ground velocity under this model. Moving into the wind increases and shifts the apparent vector.
How is current handled?
When speed-through-water basis is selected, entered current is added to the boat-through-water vector to estimate ground velocity. With speed-over-ground basis, current is not added again.
Does this replace calibrated instruments?
No. Mast motion, upwash, heel, leeway, sensor offset, height gradient, gusts, damping, and calibration are excluded.
Sources and References
- World Sailing. Equipment Rules of Sailing 2025–2028, accessed July 16, 2026; https://www.sailing.org/document/equipment-rules-of-sailing-2025-2028/.
- Offshore Racing Congress. Measurements and International Measurement System resources, accessed July 16, 2026; https://orc.org/measurements.
- US Sailing. Speed Guide Explanation, apparent/true wind mathematical relations, accessed July 16, 2026; https://www.ussailing.org/wp-content/uploads/2018/01/Speed-Guide-Explanation.pdf.
- Royal Yachting Association. Speed Charts and sailing-wind resources, accessed July 16, 2026; https://www.rya.org.uk/racing/speed-charts/.
- Empirical yacht-ratio and vector conventions are documented in each calculator.
Model limitation: The vector excludes sensor errors, mast motion, heel, leeway, upwash, height gradient, gusts, waves, and complex current.