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 connectable to the first power drive and take off ; and a battery to power or be charged by the or each motor generator : 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 is connectable to a respective one of the first and second power drive and take offs ; a connecting clutch provides a releasable drive connection between the first and second power drive and take offs ; 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 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 connectable to the first power drive and take off ; and a battery to power or be charged by the or each motor generator : 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 is connectable to a respective one of the first and second power drive and take offs ; a connecting clutch provides a releasable drive connection between the first and second power drive and take offs ; 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 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 is connectable to a respective one of the first and second power drive and take offs ; a connecting clutch provides a releasable drive connection between the first and second power drive and take offs ; 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 may be selectively connected to the gear box input or output , bypassing the gear box . <EOS>