Ratio of underwater volume to a rectangular block of length × beam × draft.
Regulatory detail & full definition
Cb = V / (L · B · T). VLCC and bulker hulls run Cb ~0.85; container ships ~0.65; ferries ~0.5; yachts and naval vessels lower still. Used in resistance estimation (Holtrop-Mennen, Guldhammer-Harvald) and as a quick proxy for hull fineness.