True Wind Calculator
On a moving boat the wind you feel is the apparent wind, which is the true wind mixed with the boat's own speed. Enter what you measure and this works out the true wind.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
The boat's speed works against the wind you feel, so the true wind is a vector sum. With the apparent wind speed, the boat speed and the angle between the apparent wind and the bow, the law of cosines gives the true wind speed.
The true wind angle is measured off the bow in the same way. It is usually larger than the apparent angle, because the boat's motion pulls the apparent wind forward.
Worked example
A boat sailing at 6 kn with apparent wind of 14 kn at 40 degrees off the bow has a true wind of 10.2 kn, coming from about 62 degrees off the bow.
Questions
What is apparent wind?
It is the wind you feel on a moving boat. It combines the true wind with the wind created by the boat's own motion.
Why is the true wind angle different?
Boat speed pulls the apparent wind forward, so the true wind comes from farther aft than it seems to.
Does this include leeway or current?
No. It works from the speeds and angle you enter. Leeway, current and waves change what a sailor feels, so take readings in steady conditions for a clean sum.
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