A marine hybrid power system comprising: an internal combustion engine; a gear box having an input connectable to the engine and an output shaft to drive a marine vessel; a first power drive and take off external of the gear box and connected to the gear box input to provide a releasable drive connection to and from the gear box input; at least one motor/generator (21,41) connectable to the first power drive and take off; and a battery to power or be charged by the or each motor generator (21,41): characterised in that: the gear box affords forward, neutral and reverse drives; a second power drive and take off external of the gear box is connected to the gear box output shaft to provide a releasable drive connection to and from the gear box output; said at least one motor/generator (21,41) is connectable to a respective one of the first and second power drive and take offs (10,30); a connecting clutch provides a releasable drive connection between the first and second power drive and take offs (10,30); and in use, by controlling said releasable drive connections, the gear box input may be selectively connected to the gear box output, bypassing the gear box, and the or each motor generator (21,41) may be selectively connected to the gear box input or output, bypassing the gear box. an internal combustion engine; a gear box having an input connectable to the engine and an output shaft to drive a marine vessel; a first power drive and take off external of the gear box and connected to the gear box input to provide a releasable drive connection to and from the gear box input; at least one motor/generator (21,41) connectable to the first power drive and take off; and a battery to power or be charged by the or each motor generator (21,41): characterised in that: the gear box affords forward, neutral and reverse drives; a second power drive and take off external of the gear box is connected to the gear box output shaft to provide a releasable drive connection to and from the gear box output; said at least one motor/generator (21,41) is connectable to a respective one of the first and second power drive and take offs (10,30); a connecting clutch provides a releasable drive connection between the first and second power drive and take offs (10,30); and in use, by controlling said releasable drive connections, the gear box input may be selectively connected to the gear box output, bypassing the gear box, and the or each motor generator (21,41) may be selectively connected to the gear box input or output, bypassing the gear box. the gear box affords forward, neutral and reverse drives; a second power drive and take off external of the gear box is connected to the gear box output shaft to provide a releasable drive connection to and from the gear box output; said at least one motor/generator (21,41) is connectable to a respective one of the first and second power drive and take offs (10,30); a connecting clutch provides a releasable drive connection between the first and second power drive and take offs (10,30); and in use, by controlling said releasable drive connections, the gear box input may be selectively connected to the gear box output, bypassing the gear box, and the or each motor generator (21,41) may be selectively connected to the gear box input or output, bypassing the gear box.