Ship Construction, Stability and Engineering Watchkeeping
Core concepts
Understand hull structure: keel, frames, floors, stringers, bulkheads, double bottom, deep tanks, and the purpose of cofferdams. Know watertight subdivision, collision bulkhead position, and the effect of free surface on stability. For stability, know KG, GM, GZ curves, list and trim calculations, and the inclining experiment. Watchkeeping covers the engine room watch routine, log entries, handover, and responding to alarms and emergencies.
Traps
- Confusing GM (metacentric height) with GZ (righting lever); positive GM does not guarantee adequate GZ range.
- Forgetting that free surface reduces GM and is worst in partly filled tanks.
- Misplacing the collision bulkhead; it is forward of the forepeak, at a minimum distance from the forward perpendicular.
- Assuming a cofferdam is a cargo space; it is a void separating incompatible spaces.
- Overlooking that a watch handover must include machinery condition, ongoing work, and outstanding defects.
Worked example / rule of thumb
Rule of thumb: free surface correction = (second moment of area of free surface × density) / (displacement × 1.025). For a rectangular tank of breadth b and length l, the free surface moment is l·b³/12. Keep tanks either full or empty in heavy weather. During watchkeeping, log all parameters hourly and record any alarm and action taken.