If a body submerged in an ideal (viscous fluid) as the fluid flows around it, there is a pressure distribution normal to the body. In the forward section of the hull there is a component of pressure resisting motion, and in the aft section of the body there is a component of pressure assisting motion. In an ideal fluid these pressure forces are equal and the body experiences no resistance.
However, water is not an ideal fluid, and therefore some differences in the flow around the body exist. In the forward portion of the hull pressure forces act normal to the surface, however, in the aft portion of the hull the boundary layer reduces the forward component of pressure. This reduction in the forward acting component results in a net resistance force that is called: