Ship Construction, Stability and Engine Department Structures
This domain covers hull structure, the stability calculations an engineer must understand during tank transfers, and the structural elements specific to the machinery space.
Structure
Transverse framing is common in the midship region, combined with longitudinal strength members: keel, centre and side girders, stringers, and longitudinal stiffeners on deck and bottom. The double bottom protects against grounding and holds fuel, ballast and fresh water. Other essentials are the collision bulkhead forward, cofferdams between fuel and ballast, and deep tanks. In the engine room, know the tank top, engine seating and thrust block, sea chests with gratings, shaft tunnel and stern tube.
Stability
GM = KM - KG. Positive GM is stable; negative GM means the ship will loll. Free surface effect reduces effective GM: FSC = i x density / displacement.
Worked example
KM = 8.10 m and KG = 7.55 m, so solid GM = 0.55 m. A partly filled tank contributes a free surface correction of 0.12 m. Effective GM = 0.43 m. It is still positive, so the ship is stable, but the righting lever and roll period change and the vessel feels stiffer or tender in a seaway.
Traps
Confusing list with loll: loll is corrected by lowering G, for example by filling low centre tanks, not by shifting weight across. Adding weight high raises KG. Ignoring free surface after every tank transfer.