Wetted surface area (S)

The wetted surface area S is the submerged area of the hull skin in contact with water at a given displacement and trim, the reference area for every resistance coefficient: R = C 0.5 rho V^2 S. It is read off the hull geometry by integrating girth along the length, or estimated with formulas such as Mumford or Denny when only main dimensions are known. Because frictional resistance is the dominant component at normal merchant speeds and is directly proportional to S, minimizing wetted surface for a given displacement is a basic resistance lever, traded against the longer waterline that lowers wave-making. Appendages add their own wetted area.