What we presume is therefore, that at a given section if I oscillate, as if the flow is all in z y plane, which means that if I take a velocity vector of fluid, it is like that. In other
words, if I take a v here of fluid, v will have only a z component and a y component and x component is assumed to be 0, because it is very small. See if this is allowed, then what happen; the ship can be assumed to be composed of locally 2 D sections, each one is oscillating, each one is creating a wave in its own plane and therefore, I can find the four added masses for each sections two dimensional sections and add them over the length. So, if I were calling added masses of 2 D section as, say a 2D z at a certain x. If we integrate that over d x, I can get a 3 D z.