Flow is important not only from resistance point of view, but also for many other problems such as flow induced vibrations. Imagine a strut projecting out of the ship for some purpose such as forming a bracket, for holding the shaft, if the bracket is not aligned in the direction of flow, there is bound to be eddy shedding or separation near the bracket- in the aft end of the bracket- this separated flow may cause flutter of the shaft bracket or vibration; similar thing can happen due to flow around a rudder behind a ship, or due to unevenness of flow there can be vibration of the aft body, which is not fully supported by buoyancy that is, the overhang of the aft portion.