Question

(a) Describe the stability requirements of a ship for dry-docking. (6)

(b) A ship 130m long displaces 14000 tonnes when floating at draughts of 7.5m forward and 8.10 m aft. GML 125m, TPC 18, LCF 3m aft of midships. Calculate the final draughts when a mass of 180 tonne lying 40m aft of midships is removed from the ship. (10)

Answer 1

Part (a)

For safe dry-docking, a ship must meet two key stability requirements:

Positive GM (Metacentric Height): The ship needs a positive GM. GM is the distance between the centre of gravity (G) and the metacentre (M). A positive GM indicates inherent stability; the ship will right itself if tilted. During dry-docking, the loss of buoyancy as the ship rests on the blocks reduces GM. Insufficient positive GM increases the risk of the ship heeling over or capsizing.

Trim by the stern: The vessel should be trimmed slightly by the stern (aft end lower than the bow) to ensure the aft end sits on the keel blocks first. This controlled settling minimises the risk of instability during the docking process. An even keel is generally preferred for the initial floating condition before the dry-docking procedure begins.

✓ Verified Model Answer (Text Solution)

For safe dry-docking, a ship must meet two key stability requirements: