Class 2 (2nd Engineer) SC&S 📅 Sep 2023

Exam Question

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

(b) An oil tanker 160m long and 22m beam floats at a draught of 9m in seawater. Cw is 0.865. The midship section is in the form of a rectangle with 1.2m radius at the bilges. A midship tank 10.5m long has twin longitudinal bulkheads and contains oil of 1.4m3/t to a depth of 11.5m. The tank is holed to the sea for the whole of its transverse section. Find the new draught. (10)

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.

← Browse more questions

Prepare on Android — MeoMock on Google Play