Ship drive system having at least a first and a second electric drive shaft (11, 12), each of which is provided for driving a propulsion unit, in particular a waterjet, wherein each of the electric drive shafts (11, 12) comprises - at least one speed-variable generator driven by an internal combustion engine (3, 6) for generating a motor voltage having variable amplitude and variable frequency, and - at least one speed-variable drive motor supplied with said motor voltage and coupled to a propulsion unit, having a first operating state and a second operating state, wherein in the first operating state the first and the second drive shaft (11, 12) are separated electrically from one another, wherein in the second operating state the first and the second drive shaft (11, 12) are coupled electrically to one another, wherein the first and the second drive shaft (11, 12) can be switched from the first operating state into the second operating state, wherein in the second operating state the first and the second drive shaft (11, 12) are coupled electrically to each other in such a way that a transfer of energy can take place from the at least one generator of one drive shaft (11, 12) to the at least one drive motor of the other drive shaft (11, 12), having a third electrical drive shaft for driving a further propulsion unit, in particular a waterjet, wherein the third drive shaft comprises - at least one speed-variable generator driven by an internal combustion engine for generating a motor voltage having variable amplitude and variable frequency, and - at least one speed-variable drive motor supplied with said motor voltage and coupled to the further propulsion unit, with a first further operating state and a second further operating state, wherein in the first further operating state the first and the third drive shaft (11, 13) are separated electrically from one another, wherein in the second further operating state the first and the third drive shaft (11, 13) are coupled electrically to one another, wherein the first and the third drive shaft (11, 13) can be switched from the first further operating state into the second further operating state, wherein in the second further operating state the first and the third drive shaft (11, 13) are coupled electrically to each other in such a way that a transfer of energy can take place from the at least one generator of one drive shaft (11, 13) to the at least one drive motor of the other drive shaft (11, 13), characterised by an onboard power network converter, supplied with a first onboard power network converter supplied with the motor voltage having variable amplitude and variable frequency of the second drive shaft which converts this voltage into a voltage having constant amplitude and constant frequency for a first onboard power subnetwork and with a second electrical onboard power network converter supplied with the motor voltage having variable amplitude and variable frequency of the third drive shaft which converts this voltage into a voltage having constant amplitude and constant frequency for a second onboard power subnetwork, wherein the two onboard power subnetworks can be coupled to one another, wherein with the drive shafts (11, 12, 13) in each case a generator and a drive motor are connected electrically to one another by way of a bus bar. - at least one speed-variable generator driven by an internal combustion engine (3, 6) for generating a motor voltage having variable amplitude and variable frequency, and - at least one speed-variable drive motor supplied with said motor voltage and coupled to a propulsion unit, having a first operating state and a second operating state, - at least one speed-variable generator driven by an internal combustion engine for generating a motor voltage having variable amplitude and variable frequency, and - at least one speed-variable drive motor supplied with said motor voltage and coupled to the further propulsion unit,