Twin centre girders with space between them. In­creases longitudinal strength and allows bilge and ballast piping to lie in the space and be easily accessible.

Longitudinal passage within the double bottom, usually on the centreline, extending from the collision bulkhead to the engine room, through which ballast, bilge, fuel and hydraulic piping may be condu [..]

Box structure running from the forward engine room bulkhead to the collision bulkhead and are utilized to carry pipelines and cables