A method for monitoring an efficiency of a vessel during waterway navigation, the method comprising: - initially determining a reference efficiency term (EEOl r ); - obtaining an efficiency indication (SEl i ) corresponding to a time interval (Δt i ) including: - acquiring (148-152) an indication of a fuel consumption (FC i ) by the vessel in the time interval; - acquiring (140-144) an indication of a distance (D i ) travelled by the vessel in the time interval; - calculating a current efficiency term (EEOI i ) based on the indications of the fuel consumption (FC i ) and the distance (D i ) acquired for the time interval (Δt i ); - acquiring an indication (164-168) of fluctuation of a rudder angle (RA) during at least the time interval; - comparing the indication of fluctuation of the rudder angle (RA) against a first threshold value ; - maintaining the reference efficiency term (EEOI r ), if the indication of fluctuation of the rudder angle (RA) equals or exceeds the first threshold value; - overwriting the reference efficiency term with the current efficiency term (EEOI i ), if the indication of fluctuation of the rudder angle (RA) remains below the first threshold value , and - calculating the efficiency indication (SEI i ) including determining a ratio between the reference efficiency term (EEOI r ) and the current efficiency term (EEOI i ); wherein the method further comprises: - providing the efficiency indication to a display device . - initially determining a reference efficiency term (EEOl r ); - obtaining an efficiency indication (SEl i ) corresponding to a time interval (Δt i ) including: - acquiring (148-152) an indication of a fuel consumption (FC i ) by the vessel in the time interval; - acquiring (140-144) an indication of a distance (D i ) travelled by the vessel in the time interval; - calculating a current efficiency term (EEOI i ) based on the indications of the fuel consumption (FC i ) and the distance (D i ) acquired for the time interval (Δt i ); - acquiring an indication (164-168) of fluctuation of a rudder angle (RA) during at least the time interval; - comparing the indication of fluctuation of the rudder angle (RA) against a first threshold value ; - maintaining the reference efficiency term (EEOI r ), if the indication of fluctuation of the rudder angle (RA) equals or exceeds the first threshold value; - overwriting the reference efficiency term with the current efficiency term (EEOI i ), if the indication of fluctuation of the rudder angle (RA) remains below the first threshold value , and - calculating the efficiency indication (SEI i ) including determining a ratio between the reference efficiency term (EEOI r ) and the current efficiency term (EEOI i ); wherein the method further comprises: - acquiring (148-152) an indication of a fuel consumption (FC i ) by the vessel in the time interval; - acquiring (140-144) an indication of a distance (D i ) travelled by the vessel in the time interval; - calculating a current efficiency term (EEOI i ) based on the indications of the fuel consumption (FC i ) and the distance (D i ) acquired for the time interval (Δt i ); - acquiring an indication (164-168) of fluctuation of a rudder angle (RA) during at least the time interval; - comparing the indication of fluctuation of the rudder angle (RA) against a first threshold value ; - maintaining the reference efficiency term (EEOI r ), if the indication of fluctuation of the rudder angle (RA) equals or exceeds the first threshold value; - overwriting the reference efficiency term with the current efficiency term (EEOI i ), if the indication of fluctuation of the rudder angle (RA) remains below the first threshold value , and - calculating the efficiency indication (SEI i ) including determining a ratio between the reference efficiency term (EEOI r ) and the current efficiency term (EEOI i ); - providing the efficiency indication to a display device . <EOS>