Residuary resistance (RR)

Residuary resistance R_R is what remains of a ship’s calm-water resistance after the flat-plate frictional part is subtracted: R_R = R_T - R_F. In Froude’s method the frictional resistance is estimated from a friction line (ITTC-1957) and the residue is treated as scaling at equal Froude number, since it is dominated by wave-making plus the viscous-pressure (form) drag. As a coefficient, C_R = C_T - C_F. Systematic series such as Series 60 and the Delft yacht series tabulate C_R against Froude number and hull parameters. The ITTC-1978 method refines this by replacing the bare friction subtraction with a form factor, so the residuary lump becomes the wave coefficient C_W.