Question

State why for dry dock she must have a positive initial GM, be upright, and trimmed slightly, usually by the stern.

When a ship enters a dry dock she must have a positive initial GM, be upright, and trimmed slightly, usually by the stern.

On entering the dry-dock the ship is lined up with her centreline vertically over the centreline of the keel blocks and the shores are placed loosely in position. The dock gates are then closed and pumping out commences.

The rate of pumping is reduced as the ship’s stern post nears the blocks.

When the stern lands on the blocks the shores are hardened up commencing from aft and gradually working forward so that all of the shores will be hardened up in position by the time the ship takes the blocks overall.

The rate of pumping is then increased to quickly empty the dock.

As the water level falls in the dry dock there is no effect on the ship’s stability so long as the ship is completely waterborne, but after the stern lands on the blocks the draft aft will decrease and the trim will change by the head.

This will continue until the ship takes the blocks overall throughout her length, when the draft will then decrease uniformly for-ward and aft. The interval of time between the stern post landing on the blocks and the ship taking the blocks overall is referred to as the critical period. During this period part of the weight of the ship is being borne by the blocks, and this creates an upthrust at the stern which increases as the water level falls in the drydock.

The upthrust causes a virtual loss in metacentric height and it is essential that positive effective metacentric height be maintained throughout the critical period, or the ship will heel over and perhaps slip off the blocks with disastrous results.