First of all, the fin limits the size of a sail.

Based on a fin size, the FSC (Fin to Sail Size Calculator) calculates a suitable sail size depending on elevation, outside temperature and relative humidity. Higher values for elevation, temperature or humidity result in larger sails, lower values in smaller sails. The effect of humidity is low. As a reference, an elevation of 0 m above sea level, an outside temperature of 15°C and a humidity of 55% are assumed.

The results are based on the assumption of a passive sailing technique on a half-wind course with a take-off speed of 4 m/s and a planing threshold of 5 m/s.

This would also categorise the target group of the FSC. It is the recreational wind surfer with planing respectively take-off experience.
 
With an active sailing style (e.g. by pumping), smaller material (sail, fin/foil) can be chosen, which would have the advantage of increasing the upper wind limit, which would put you on the safer side, especially in gusty wind conditions. 

Please always pay attention to this:

Windsurfing and windsurf foiling can be dangerous sports!

Due to the large number of sail types optimized for specific use, the FSC can only serve as a rough guide. The results are not recommendations. I designed the FSC for my own use. In fin mode the focus is on the upper wind range, in foil mode on the lower wind range. Whether these fit regardless of sail type or other hard-to-specify fin or foil characteristics is up to you.

The use of the results and information is at the user's own risk. Please note the  disclaimer.

The default settings of the light blue fields can be overwritten with your own values.

Some input fields have multiple entries, but should be explained by themselves.

Mode
The selection is made via the Front Wing Lever field
Fin  to Sail Size Calculator:  Front Wing lever arm = 0;
(Sail to sail calculator: Online )
Foil to Sail Size Calculator:  Front Wing lever arm > 1 < 60.

INPUT FIELDS

Front Wing Arm (cm)
If not known, the lever arm of the front wing can be easily determined. See tip 2 below. 

Fin / FW Span width  (cm)
In foil mode, the field for the fin length is filled with the span of the front wing.

Fin/FW (cm²) 
The fin area is preset with an estimated value in the corresponding mode and should be replaced by its real size (see also tip 1 below). But be careful, once overwritten, no matter in which mode, they are no longer available until a reset (Set button in the app - top right in the menu).  In foil mode, the area of the front wing should always be known. 

As a sail-sail calculator, see tip 3.

Elevation (m)
Altitude of the surfing spot above sea level.

Air Temperature (°C)
Outside temperature at the surf spot.

Humidity (%)
Relative humidity at the surf spot.

Wind Unit 
The wind unit is selected in the top left-hand field of the EXCEL table or in the correspondingly labelled field of the app, provided this option is enabled:

Wind speed:
0 = m/s,
2 = kts,
3 = km/h.
4 = mph.

Wind force:
1 = Bft.

Sailor (kg)
In fin mode, your own body weight only affects the calculation of the wind speed.

FOIL+ Board + Sail (kg)
In addition to your own body weight, it is very important to use a realistic value. Does not play a role in fin mode.


OUTPUT FIELDS

Aspect Ratio
Is the aspect ratio (λ - Lambda) of the fin or front wing, the ratio of the length/span to the average width. The higher the value, the slimmer the fin or front wing. (See also explanation of formula (4) in FSC_Physik.pdf, currently only available in German.)

Sail Size (m²)
The result of the calculated sail size.

Wind
How to evaluate wind speed in relation to sail size is outlined in the description.


Tips.

1. If you don't 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 increments and add up their lengths. The result is very close to the real fin surface.

2 The length of the lever arm can be measured along the fuselage from the centre of the mast to the centre of the front wing.

3. If you only want to know the dependence of a certain sail size on the altitude and/or the outside temperature - and very slightly on the humidity - you can activate this mode by setting Wing Lever Arm = 1 and Fin/FW Span = 1 and entering the corresponding sail size in m² in the Area field. 

Translated with the help of www.DeepL.com/Translator (free version)

1. limitation of liability for website content

Pursuant to Section 7 (1) of the German Telemedia Act (TMG), the person responsible for the website within the meaning of Section 5 TMG is responsible for his own information that he makes available for use in accordance with general legislation.

The person responsible for the aforementioned website accepts no liability for the topicality, correctness and completeness of the content provided on this website. This does not apply if the person responsible can be accused of intentional or grossly negligent behavior. The contents were created with the greatest possible care and to the best of our knowledge. Nevertheless, the accuracy of the content cannot be guaranteed, especially in the case of complex topics, so that the person responsible recommends that users contact the relevant authorities for important information or seek legal advice.

Contributions identified by name that are made publicly available on the website reflect the opinion of the respective author and not always the opinion of the website operator.

Insofar as paid or free and freely accessible content or services are made available on the website, these are non-binding invitatio ad offerendum, which merely call for the submission of an offer by the user and do not themselves constitute a binding offer.

According to §§ 8 ff. TMG, the website operator is not responsible for third-party content that he publishes for a user, provided that

he has no knowledge of the illegal act or the information and, in the case of claims for damages, no facts or circumstances are known to him from which the illegal act or the information becomes apparent, or

he has taken immediate action to remove the information or to block access to it as soon as he became aware of it.

Liability in this regard is thus only possible from the time of gaining knowledge of a concrete infringement.

If the website operator becomes aware of such illegal content, it will be removed immediately.

2. liability for outgoing links

The website operator links from his website to external websites. These so-called „hyperlinks“ lead the user directly to the external website. The setting of so-called „hyperlinks“ does not mean that the website operator adopts the content behind the link as its own. The website operator has no influence whatsoever on the information on the external website. Therefore, no liability can be assumed for the topicality, correctness and completeness of the contents of the external website.

The respective provider or operator of the linked website is solely responsible for the content.

However, the website operator assures that he was not aware of any legal violations at the time of setting the link and that he has checked the external website within the scope of what is reasonable.

As long as there are no concrete indications of a violation of the law, a permanent control of the linked website is not reasonable for the website operator.

If the website operator becomes aware of the illegality of the linked content, the corresponding link will be removed immediately.

3. copyrights

The content published on this website is subject to German copyright and ancillary copyright law.

As the author within the meaning of Section 7 of the German Copyright Act (UrhG), the website operator is entitled to the sole and exclusive exploitation rights pursuant to Sections 15 et seq. UrhG. Any duplication or use of objects such as images, diagrams, sounds or texts in other electronic or printed publications is not permitted without the author’s agreement and will be prosecuted.

Contents that were not created by the website operator are marked as such. The copyrights of third parties are respected.

Should you nevertheless become aware of a copyright infringement, you are requested to notify the website operator accordingly. The website operator will then remove such content immediately.

4. data protection

The website can generally be used without entering personal data such as name or e-mail address. No personal data will be stored.

Should the possibility of entering such data exist, the communication of this data is on a voluntary basis by the user. A transfer of this data to third parties without the express consent of the user is excluded.

However, complete protection of the transmitted data against access by unauthorized third parties is not possible, as there may be security gaps in the area of data transmission on the Internet. However, the website operator attempts to prevent the risk of unauthorized access by taking appropriate measures and, if it becomes aware of a security breach, to close it by taking appropriate measures.

The website operator does not assume any liability for damages caused by such security breaches.

When you visit the website, the website operator collects the data that is technically necessary to display the website to you and to ensure stability and security (IP address, date and time of request, browser, etc.). This collection of personal data does not constitute an evaluation of personal data.

The use of contact data published in the imprint by third parties for sending unsolicited advertisements, in particular spam e-mails, is prohibited and in case of non-compliance may be pursued with legal action.

(source: BUSE HERZ GRUNST Rechtsanwälte)

2April 01, 2025 The previous app is publicly accessible again. Its hidden sail-sail calculator is now available online on my weather page.