Class 2 (2nd Engineer) SC&S 📅 Oct 2024

Exam Question

(a) Describe stability requirements for dry-docking (6)

(b) A ship of 8000 tonne displacement, 110 m long, floats in sea water of 1.04 t/m3 at draughts of 6 m forward and 6.3 m aft. The TPC is 16, LCB 0.6 m aft of midships, LCF 3 m aft of midships and MCT1 cm 65 tonne m. The vessel now moves into fresh water of 1.000 t/m3. Calculate the distance a mass of 50 tonne must be moved to bring the vessel to an even keel and determine the final draught.

Reference Answer

(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.

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