To obtain the new shape from modifying an existing geometric shape is a common process of design encountered in automobile, aircraft, and shipbuilding industries. In particular, designing a new ship from a well-made existing ship, called hull form variation or variation, for short, has been a crucial process used in every design department of shipyards for prompt and efficient initial hull form creation. This process, however, is not only complicated but also unintuitive and thus requires an expert’s skill and experience. An approach to performing the variation with the given geometric constraints of area and centroid is proposed. To modify an existing hull shape, a basic boundary curve of the shape is selected as a design variable. A parametric piecewise polynomial curve that satisfies new geometric requirements is generated and superimposed on the top of the selected boundary curve to yield the desired curve. The main process of the variation is performed in a linearized fashion that preserves the original shape as much as possible; thus, a new form is efficiently and promptly obtained. The proposed concept can be readily extended to similar modification processes of an existing geometric shape by adopting different geometric requirements.