Comparison of ship energy efficiency methods for bow foil technology

As part of efforts to achieve a 40% reduction in shipping emissions by 2030, the International Maritime Organisation (IMO) has recently enacted the requirement for existing ships (> 400gt) to pass an Efficiency Existing Ship Index (EEXI). The EEXI allows for the retrofitting of innovative energy efficiency technologies, such as bow foils, to ensure ship emissions compliance. Bow foils act to reduce the overall delivered power of ships in waves through both a reduction in ship motions and the generation of additional thrust. This paper presents and compares several methods for the inclusion of bow foils in the IMOs mandatory EEXI calculation. The results are based on a series of free running model tests with and without a passive, spring-loaded bow foil. The EEXI was calculated with and without the bow foil from measuring; the speed loss in a representative sea state; the increase in delivered power in waves at a constant speed; and the additional thrust force provided by the bow foil. The differences between the results highlight the importance of establishing a consistent methodology for the calculation of EEXI when using bow foils as an innovative energy efficiency technology.