Afterpeak
The Afterpeak is the compartment right aft in the stern of the ship, abaft the aftermost watertight bulkhead. Like the forepeak it is normally a ballast tank — the afterpeak tank — used to trim the ship by the stern, and it surrounds the stern tube through which the propeller shaft passes out of the hull.
Adjusting the afterpeak and forepeak together lets the crew set the fore-and-aft trim of the ship for best propeller immersion, steering and fuel efficiency. The afterpeak is also a space watched for leakage past the stern-tube seal.
Trim has a real effect on fuel consumption and on propeller and rudder performance, and the afterpeak tank is one of the main tools for setting it. Its position around the stern tube also makes it the place where stern-tube leakage first shows, so it is watched in engine-room rounds.
How do the forepeak and afterpeak control trim?
Filling the forepeak trims the ship by the head; filling the afterpeak trims it by the stern. Balancing the two sets the fore-and-aft trim for best performance.