In Appendix G of the RRS-Windsurfing Competition Rules (2009>2012) you find the rules about Identification on Sails. From 2009 you are required to display your country code (3 letters) and sail number – ” … in black and applied back to back on an opaque white background”.
Furthernmore, ” they shall be in the central third of that part of the sail above the boom, clearly separated from any advertising”.
ISAF Regulation 20 -Advertising Code- contains the rules regarding the carrying of advertising. In the past Category C , as adopted by the Formula Class, maximised the rights of the sailor, but defined space for manufacturers logos and event advertising. Those rights are still in place- although Category C no longer exists- by adopting the Code without further amendments.
However, the Code now defines the size of the area available for event advertising on a sail (the organisers sponsor ) and its positioning, and also permits event advertising on a bib to be worn by competitors.
The PDF summarises all the relevant info and includes a graphic illustration for clarity.
The complete Rulebook and relevant Regulations can be found here : https://www.formulawindsurfing.org/cstuff.php?what=RRS
