Sternpost
The Sternpost is the principal structural member at the after end of the hull, rising from the after end of the keel. It forms the backbone of the stern and, on many ships, carries the fittings for the rudder and, on single-screw vessels, the opening or boss for the propeller shaft.
In older and single-screw ships the sternpost is a substantial casting or forging. Modern ships often use a fabricated stern-frame structure that performs the same role — supporting the rudder and propeller and closing the after end of the hull.
The sternpost anchors the two things that drive and steer the ship — the propeller and the rudder — and closes the after end of the hull against the loads of the sea and the thrust of the propeller. A defect here affects steering and propulsion, so it is a heavily built and closely surveyed part of the structure.
What does the sternpost support?
The rudder fittings and, on single-screw ships, the propeller shaft boss — making it the structural anchor for both steering and propulsion at the after end of the hull.