The flat-plate friction data cannot be applied indiscriminately to the curved surfaces of ships. The wetted area of the ship is calculated by averaging the girth at a series of stations equally spaced along the length and multiplying by the wetted length. For calculating the friction resistance of a ship at any given speed, it is usually assumed that the friction value for each unit of wetted-surface area is equal to that for a friction plane having the same length as the ship and towed at ship speed. From the known towing forces and wetted area of the plank or plane there are derived a set of friction values per unit surface area of the plane, in terms of the towing speed. By using very thin sections, sharply pointed at the ends, wave making and eddy making are eliminated. The friction resistance of a ship can be computed from a knowledge of its wetted area and a friction value per unit area derived from the towing of flat planks or friction planes of various lengths at various speeds. The added drag due to separation behind the square stern of a skiff, immersed deeply by passengers sitting in the stern, is very real to the rower in that skiff.
The confused and eddying mass of water being dragged along in the separation zone behind the square transom stern of a motorboat is clearly visible at low and moderate speeds. Resistance is generated by the forward acceleration of water that would otherwise flow aft and be left behind. In the region known as the separation zone, water is dragged in from astern to fill the gap that would be left because the flow does not close in from the sides. Separation is caused by the lack of sufficient pressure in the water in a given region to force this water laterally inward and to make it flow closely along all parts of the ship, especially in the tapering or blunt after portion.
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