Boundary layer

The boundary layer is the thin viscous region next to the hull where flow velocity rises from zero at the surface (the no-slip condition) to nearly free-stream at its outer edge. Its growth sets the frictional resistance, since wall shear integrated over the wetted surface is the friction drag. On a ship hull the layer is turbulent over almost its whole length, with Reynolds numbers of order 1e9 at full scale against about 1e7 at model scale, which is why model tests need turbulence stimulators and why the ITTC-1957 line plus a form factor are used to scale friction. Boundary-layer thickening toward the stern also feeds the wake the propeller works in.