There is provided a method of measuring coupling ratios in a marine vessel. The vessel includes: (a) a source of mechanical power; (b) a coupling system operatively coupled via a first input shaft to the source of power and operatively coupled via a second output shaft to one or more propellers of the vessel; and (c) a controller coupled to a user interface and also to the coupling system such that the user interface is operable via the controller to control a degree of power coupling occurring in operation through the coupling system. The first and second shafts are provided with first and second rotation rate sensors respectively coupled to the controller for generating first and second rotation rate signals indicative in operation of rotation rates of the first and second shafts respectively. The method involves measuring a ratio of the first and second signals when the coupling system is in a fully coupled state.