Speed reduction is a practice way for the ship operator to increase efficiency and reduce Green House Gas (GHG) emission in the shipping industry. In this paper, using Kwon’s method to predict the added resistance caused by wave and wind for specific vessel, and a cubic function of speed and fuel rate to predict the total fuel consumption at varyig speed, encounter angles and various sea state during her voyage. This study aims to determine the optimum operational speed for minimizing fuel consumption using the energy-efficiency operational indicator (EEOI) concept. Using two anchor handling tug supply (AHTS) vessel as the research object, the comparison between the recorded and predicted EEOI values indicates the influence of hull fouling, propeller condition, and engine degradation. The results show that AHTS A had a 5.91% prediction error over 14 months, while maintenance during dry docking (AHTS B) reduced the error by 6.57%. A 0.5-knot speed reduction decreased the fuel consumption by 0.5%, but increased the sailing time by 4.88%. Thus, lower speeds improve the fuel efficiency, although longer voyage durations are required. Environmental conditions also have a significant impact on the fuel performance.
Syafiuddin et al. (2026) studied this question.