The flat surface flow resistance can be calculated with the detailed study of flat plate in mecaflux (tab drag-lift / profiles with drag) .
For large hulls or completely planing hulls, resistance is mainly composed of frictional resistance on the surface in contact with the fluid. calculating the frictional resistance of a flat plate can quickly evaluate this component of ship resistance to advancing.
The width of the boundary layer is given and the critical distance where the flow changes from laminar to turbulent. It is thus readily identify whether an appendage comes out of the boundary layer. If this is the case, add the drag of the outgoing part of the boundary layer (using the tab drag lift, geometric object, for example)