3.(a) Explain, with the aid of a sketch, what is meant by the term Reserve Buoyancy, stating why it is important.(4)
(b) Explain what is meant by Free Surface Effect, stating how this can be minimised.(6)
(a) Reserve Buoyancy (4)
Meaning:
- Reserve buoyancy is the watertight volume of a ship above the waterline (freeboard deck and superstructure spaces that are enclosed and watertight).
- It represents the vessel’s ability to remain afloat when additional weight or flooding occurs.
Importance:
- Provides safety margin against sinking in case of damage or heavy seas.
- Directly linked to freeboard and load line regulations under SOLAS/Load Line Convention.
- Greater reserve buoyancy = better survivability in emergencies.
(Sketch: side profile of a ship with waterline marked, shaded area above waterline up to deck edge labelled as “Reserve Buoyancy”).
(b) Free Surface Effect (FSE) (6)
Meaning:
- Occurs when a partly filled tank or compartment allows liquid to move freely as the ship heels.
- This movement shifts the centre of gravity transversely, reducing the vessel’s metacentric height (GM) and hence stability.
Dangers:
- Can cause excessive rolling, reduced righting lever, and in extreme cases, capsizing.
- Particularly dangerous in wide, slack tanks with large surface areas.
Minimisation methods:
- Keep tanks either pressed full or pumped empty (avoid slack condition).
- Fit longitudinal and transverse baffles in tanks to restrict liquid movement.
- Proper planning of ballast and fuel transfers to reduce slack tanks.
(Sketch: cross-section of a ship showing liquid in a slack tank shifting to the low side when the vessel heels, arrows showing shift of centre of gravity).