Formula 4S Powerboat World Championship
The Formula-4s Powerboat World Championship is an international inshore powerboat racing competition for powerboats organised by the Union Internationale Motonautique (UIM), hence it is often referred to as F4s. The "S" letter is an abbreviation of four-stroke, which denotes that four-stroke boat engines are used in this class.
Source: Wikipedia — Formula 4S Powerboat World Championship (CC BY-SA 4.0)