In order to reduce Greenhouse gas (GHG) emission from shipping sector, it is required to built and promote low GHG emission ships. On a project for 10 mode at sea in Japan, a calculation method on decrease of ship speed in actual seas, which is available in design stage, has been developed. For verification of the calculation method, full-scale onboard measurement has been conducted for a RoRo cargo ship and an oil tanker. As a result, it is confirmed that the calculation method is sufficiently reliable and can be accurately applied to both fine and blunt ships.
In order to estimate ship performance in actual seas, external forces acting on ships have to be calculated with accuracy. It is well-known that added resistance due to waves and winds cause decrease of ship speed. As a ship sails with a drift motion due to waves and winds and controls rudder to keep its course, added resistance due to drift motion and steering should be considered additionally. Because added resistance is predominant component in the external forces, it should be estimated accurately. Considering the index is evaluated in design stage, external forces are required to be estimated based on ship specification. However, it is difficult to estimate added resistance due to waves theoretically. For large ships, length of encountering waves is relatively short in relation to wave spectrum. In short waves, as ship motion is quite small and a ship is in diffraction condition, added resistance due to wave reflection at bow is predominant component. It is difficult to estimate the component accurately with theoretical method because it is not easy to take the effect of hull form above water line into consideration. Fujii and Takahashi proposed empirical formula to this problem (1975), and Takahashi corrected practically (1988).
