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Update
July 9, 2020. If the fin area is not available, the +/- buttons can now be used to change the sail size in 0.1 steps.
February 16, 2020. Wind speed error correction.
February 13, 2020. Correlation between app and print. Description.
February 12, 2020. Wind speed- optional. (BETA) 
January 21, 2020. Altitude and weather data can be accessed together.

Description.

The calculator is a mobile dashboard of an EXCEL sheet, which can be downloaded as a native app or run via an external cloud service.

The sail size is mainly limited by the choice of the fin, or more precisely by its area, whereby surface elevation, outside temperature and relative humidity are taken into account. The maximum sail size that a fin can withstand during gliding is calculated. Whether this sail size fits you is at your own discretion. The Fin to Sail Size Calculator can only serve as a non-binding guide. The calculated sail size refers to the given fin, not to its surfer!

λ (Lambda) is the aspect ratio. The higher the value, the slimmer the fin. Formula can reach values up to 8. (See also explanation of equation (5) in FSC_Physik.pdf, currently only available in German.)

The PRINT function creates an EXCEL overview as a PDF file. (The pop-up blocker has to be deactivated.) Up to three fins can be entered.

No values are assumed outside the specified limits.
Fin Length: 1 – 100 cm;
Fin Area: 14 – 1000 cm ²;
Sail Size: 0 – <13 m ²;
Air Temperature: -5 – 50 °C, default 15°C;
Relative Humidity: 0 – 100%, default 0%; 
Altitude: 0 – 1950 m, default 0 m. 
Optional (experimental):
Surfer: 0 - 250 kg. Default 0 kg - with this setting the wind speed fields are hidden.
Wind units: 0 = m/s (default), 1 = Bft, 2 = kts, 3 = km/h, 4 = mph.

Higher values for altitude, temperature or humidity tend to increase the sail size, lower values tend to decrease it. As comparison, the sail size for the default settings is always displayed.
The wind speed is an upper limit!

Notes. The GPS determines the altitude, the height above sea level. On the ground level it corresponds to the elevation. 
The FSC offers the possibility to retrieve site-related data. A click on the weather icon 🌦 opens the page. Closing it will return you to the FSC. The altitude is the result of a built-in GPS. If it is not or only poorly available, the elevation is queried in a device adequate resolution via elevation.api.io.
The raw weather data are requested from openweathermap as metric, values of other units are rounded. 

Tip. If you do not know the area of your fin, you can draw its outline on a sheet of paper, draw parallel lines starting 0.5 cm below the fin head in 1 cm steps and add up their lengths. The result comes very close to the real fin area. If this is too much work for you, you can specify a preferred sail size instead. This is marked red in the output file to indicate that no real fin area has been used. But be careful, the slimmer the fin, the more this emergency solution deviates from reality.

Terms of use. The acquired data for altitude/elevation and weather, may only be used for the Finn to Sail Size Calculator.

Disclaimer. Windsurfing can be a dangerous sport. I am not responsible for any damage or injury resulting from the use of this website and/or the Fin to Sail Size Calculator.