The primary function of a bottom drain plug consists of draining and releasing water.

The bilge drain plug is used to remove the water in the cabin. After the ship is docked for repair, painting or watertight test, when it is necessary to put down the water in the bilge, the bilge water can be released by turning on the bilge drain plug.

After the ship conducts double bottom tank hydrostatic test, the drain plug is used to drain the residue and water in the bilge. As the residual liquid is easy to gather at the stern when the ship is bow-tilted, the drain plug is usually set at the rear of the compartment near the transom wall and close to the centerline of the ship.

The bilge drain plugs are usually located at the rear of each watertight compartment, near the watertight ribs, and on both sides of the flat keel to avoid being blocked by the docking piers and not being able to be dismantled. The plugs are designed so that they can be discharged without having to be sent off completely, and can be closed quickly in an emergency to ensure the watertightness of the vessel.