In order to protect carbon steel in sea water cathodically, it is necessary to polarize it less noble than-0.77 V (referred to saturated calomel electrode). In the case of cathodic protection of the ship, -0.77 V is usually applied as the protective potential. And in most the protective effect of this application has been decided with measuring the hull potential of stationary ship. But it is very doubtful from the view point of the polarization characteristics whether the potential of ship hull is kept in equal without in active state or in stationary state or not. So that, foundamental study was conducted on the relationship with the sea velocity and the natural electrode potential or protective current of mild steel. Furthermore the influence of the speed of ship (destroyer) on the hull potential was investigated under the auspices of the Japan Defence Agency.