Method to control at least a first and a second parallel hybrid driveline (101, 102; 310, 320, 330) arranged to drive a marine vessel, where each driveline comprises a first propulsion unit (111, 112; 311, 321, 331) in the form of an internal combustion engine operatively connected with a second propulsion unit (121, 122; 312, 322, 332) in the form of an electric motor to drive a propeller shaft (107, 108; 313,323, 333) and produce a thrust force, and where at least one control unit (316, 326, 336; 317, 327, 337; 340) is arranged to control each first and second propulsion unit in all the parallel hybrid drivelines; performing the steps of: - receiving a request indicative of a vessel speed; - determining a rotational speed for each first propulsion unit (111, 112; 311, 321, 331) for achieving the requested vessel speed, based on the received request; characterized by: - determining efficiency points (P 1, P 2; E 1, E 2) for each of the first and the second propulsion units from efficiency maps for each propulsion unit, based on the determined rotational speeds; - individually adjusting the rotational speed (n 1, n 2) of the first propulsion unit (111, 112; 311, 321, 331) in each driveline towards the determined efficiency point to improve the efficiency of this first propulsion unit while maintaining the requested vessel speed, and - simultaneously adjusting the load from the corresponding second propulsion unit (121, 122; 312, 322, 332) in each driveline by reducing or increasing the load from the second propulsion unit in response to the adjustment of the rotational speed of the corresponding first propulsion unit to improve the efficiency of each driveline and the complete driveline installation; wherein the individual drivelines are controlled by exchanging data required for controlling the propulsion units between a central control unit and the drivelines or between individual control units for each driveline so that the combined rotational speed from all first propulsion units is sufficient for maintaining the requested vessel speed, wherein the method further comprises adjusting the rotational speed of at least one first propulsion unit (111, 112; 311, 321, 331) and allowing it to be operated at a different rotational speed than at least one other first propulsion unit. - receiving a request indicative of a vessel speed; - determining a rotational speed for each first propulsion unit (111, 112; 311, 321, 331) for achieving the requested vessel speed, based on the received request; characterized by: - determining efficiency points (P 1, P 2; E 1, E 2) for each of the first and the second propulsion units from efficiency maps for each propulsion unit, based on the determined rotational speeds; - individually adjusting the rotational speed (n 1, n 2) of the first propulsion unit (111, 112; 311, 321, 331) in each driveline towards the determined efficiency point to improve the efficiency of this first propulsion unit while maintaining the requested vessel speed, and - simultaneously adjusting the load from the corresponding second propulsion unit (121, 122; 312, 322, 332) in each driveline by reducing or increasing the load from the second propulsion unit in response to the adjustment of the rotational speed of the corresponding first propulsion unit to improve the efficiency of each driveline and the complete driveline installation;