An Experimemtal Study on Scale Effect on Direction of Flow behind Ship Hulls Moving Obliquely

Abstract

This paper deals with scale effect on direction of a flow behind a ship's hull in oblique flows. This problem is related with scaled effect on rudder forces during maneuvering motions. The flow directions were determined experimentally, employing three geosim tanker models of 2m, 4m and 7m in length. The measured results were expressed by the coefficient of flow-straightening effect of a ship body, which was defined as the ratio of a flow direction angle to a drift angle. In addition, on the basis of the assumption to decompose the free stream velocity into lengthwise and crosswise components, calculation for the scale effect on flow-straightening effect was examined. In this calculation method, downwash due to vortex distribution correspnding to hydrodynamic forces on the ship's hull was taken into consideration. Through the study, the authors found that the coefficient becomes larger as ship model length increases. But on the contrary, calculated results indicate that the coefficient decreases with increment of ship model length. Therefore, the problem concerning the prediction method for the scale effect on the characteristics of the flow direction has been left unsolved.