The critical period
When a ship first takes the keel blocks, part of the weight is supported by the dock instead of by buoyancy. That upthrust at the keel reduces GM. The time from first touching the blocks until the ship is fully landed is the critical period. If GM becomes negative the ship can take a sudden list and damage the blocks or the hull.
How this calculator works
Virtual GM loss ≈ (P × KM) / Displacement. GM during docking ≈ Initial GM − loss. P is the upthrust already on the blocks. This is a simplified teaching formula. Shipyard docking calculations use the approved docking plan, trim, block positions and the stability booklet. Tanks should be pressed up or emptied as the docking plan requires, so free surface does not make GM worse.
Who should use it
Useful for officers preparing a docking condition. Always follow the shipyard docking master and the company’s docking procedure.