Form factor (1+k)

The form factor $(1+k)$ scales the flat-plate frictional coefficient up to the actual viscous resistance of the three-dimensional hull, so viscous resistance becomes $(1+k) C_F$. It captures pressure drag from the hull’s curvature and is found from low-speed model tests by Prohaska’s method, plotting $C_T / C_F$ against $Fn^4$ and extrapolating to zero speed. Fuller, blunter hulls carry higher $k$ (0.2 to 0.4); fine forms sit near 0.1.