After the marine disaster of “Nakhodka” in 1997 at the Japan Sea, we started in 1998 a five-year research project to develop an “Optimum Towing Support System” (OTSS). OTSS is a simulation system based on the data of external forces on the sea conditions and hydrodynamic forces acting on a hull, rudder and propeller. By OTSS we are able to obtain appropriate guidelines to avoid a secondary accident and to minimize marine pollution. In the simulation, the accurate prediction of hydrodynamic forces acting particularly on a hull would be most important. Using a ten-segmented model ship, we measured the distribution of lengthwise lateral force acting on a trimmed hull in order to investigate characteristics of the hydrodynamic forces. In this paper we report the result of the investigation and propose a theoretical prediction method based on the slender body theory for the hydrodynamic forces acting on a trimmed hull.