Shaft Generator
A Shaft Generator (SG) is an electrical generator driven from the main propulsion shaft or engine, so that it produces the ship electrical power using the main engine while the ship is under way. Because the large, efficient main engine burns cheaper fuel than the smaller auxiliary generators, running on the shaft generator at sea usually saves fuel and running hours.
Modern shaft-generator systems use power electronics to keep the electrical frequency constant even as engine and shaft speed vary, and some can also run in reverse as a power take-in to give the propeller an electric boost. It is a common energy-efficiency feature on merchant ships.
At sea, generating electricity from the main engine instead of running separate auxiliary diesels cuts fuel cost, emissions and machinery running hours — a straightforward efficiency win. That is why shaft generators are a standard part of the energy-saving package on modern ships.
Why does a shaft generator save fuel?
The main engine is larger and more efficient and often burns cheaper heavy fuel; producing electricity from it at sea is usually cheaper than running smaller auxiliary generators, and it reduces their running hours.