Instability of stern flow in the coexistence of 2-dimensional separated eddies and bilge vortices are observed using the two 2m full model ships both of which have the same vertical framelines, but one with usual round bilge M 30 R, and the another with sharp edged bilge M 30 S. The three velocity components are measured in the rear of M 30 5 at four sections to study the diffusive process of bilge vortices, and evaluate the induce drag. Wake survey and resistance tests are performed and viscous resistance components are compared. (1) Instability or queer stable state seems to occur on stern flow with interaction between 2-dimensional separated eddies and bilge vortices. (2) Hydrodynamical properties of strong bilge vortices have been studied. Path of vortex center, vorticity distribution, slight decay of circulation, velocity profiles of vortex core, diffusion of vorticity and induced drag were analyzed. (3) There seems no breakdown in case of strong bilge vortices in a certain range. (4) Viscous resistance obtained from wake survey at 0. 5 L aft of AP gave a good agreement with that from resistance test.