Exam Question

(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.
At the critical moment , the centre of gravity G rises faster than the metacentre M , so G catches up with M, and the vessel stability is compromised. So, GM needs to be positive until the critical moment.