What is a shaft generator and why is it used?

A shaft generator's appeal is straightforward economics: the main engine is already running, already converting fuel at its most efficient point during a sea passage, so tapping some of that shaft power to generate electricity is more fuel-efficient than running a separate auxiliary diesel purely for the electrical load, and it spreads running hours away from the auxiliaries, cutting their maintenance burden over the ship's life.

Mechanically the generator is coupled to the propulsion train, either on an extension of the shaft, through a step-up gearbox tapping off the main gearbox, or via a power take-off, depending on the vessel's layout. The complication is frequency: a generator's output frequency depends on its rotational speed, but propeller shaft speed varies with the ship's speed and loading, slower in poor weather, faster at full sea speed, which would produce a wandering electrical frequency unsuitable for shipboard equipment if connected raw to the switchboard.

Two solutions are used. On ships with a controllable-pitch propeller, the engine and shaft can run at constant speed while thrust is varied through blade pitch alone, keeping the shaft generator's frequency naturally stable, a major reason CPP and shaft generators are so often paired. Where shaft speed does vary, power electronics rectify the variable output to DC and invert it back to a stable frequency, adding cost and some conversion loss but letting the generator work across a wide speed range, and modern converter-based systems increasingly let the same machine run in reverse as a motor, drawing power from the switchboard to assist propulsion or even provide it if the main engine is stopped.

Auxiliary generators remain essential throughout: during manoeuvring and in port, when shaft speed is unsuitable or the main engine is stopped, the auxiliaries carry the full electrical load, taking over automatically or being brought on in good time whenever the shaft generator or main engine trips. Getting the changeover right, without a blackout or a bus-tie fault, is a standard part of electrical operating procedure and a common subject of drills, since losing power unexpectedly at sea, particularly during a critical manoeuvre, is a serious event in its own right.