What statement is true concerning the keel arrangements of a double-bottomed ship?

Correct Answer: Option D — A ship with an "I-section keel" has a single continuous longitudinal girder positioned along the centerline and perpendicular to the flat plate keel, and a ship with a "duct keel" has two continuous longitudinal girders spaced apart and positioned on either side of the centerline and perpendicular to the flat plate keel.

Structural keel design defines the vessel's longitudinal strength. An I-section keel utilizes a single centerline girder for support, while a duct keel employs two longitudinal girders to create the required internal passage. These configurations are fundamental to the vessel's hull girder strength and internal utility.