A power distribution system is described that does not rely on a fixed frequency and which therefore allows prime movers to run at different speeds in response to load demand, typically so that fuel consumption and/or harmful exhaust emissions is/are minimised. A marine power distribution and propulsion system can include an ac busbar adapted to carry a variable-frequency ac distribution voltage. A plurality of ac generators are connected to the busbar, each having an associated prime mover such as a diesel engine, turbine etc. A power management controller is adapted to vary the rotational speed of the prime movers with reference to the electrical load on the ac busbar such that the generators provide a variable frequency output during normal operation of the power distribution system. Such operation is to be contrasted with conventional distribution systems which have a fixed frequency.