Ratio of underwater volume to volume of a prism with cross-section equal to the maximum hull section, length = LBP.
Regulatory detail & full definition
Cp = V / (Am · L), where Am is the maximum cross-section area and L is length between perpendiculars. High Cp (~0.65-0.75 on bulkers) implies a fuller hull with higher carrying capacity. Container ships and warships have lower Cp (~0.55-0.65) for finer entry.