On the basis of simple conformal mappings, we present a method to describe a two-dimensional potential flow around a rotating obstacle of arbitrary section. As a result, a simplified expression of the matrix of added mass and added moment of inertia is obtained. We also underline the need to determine this perturbation potential all over the fluid domain. Indeed, this is of practical interest in order to express the hydrodynamic moment in incompressible viscous flow. (c) 2005 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves.