Keels are the backbone of any ship. Each ship is designed so that all of the stress
within the hull is directed toward the keel, and thus, this structure has to be absolutely sturdy. There are several different
types of keels depending on the necessities of the vessel. For more, consult Eyers' "Ship Construction."
Flat Plate Keel:
The simplest type of keel, this structure is actually integrated completely into the internal framework of
the vessel. The stresses are centered on a continuous centre girder that runs longitudinally down the centerline of the ship.
Are simple because they can be built completely within the internal structure and allow for the complete double bottom to
be used as tank space.
A little more complex, the bar keel is the only keel that protrudes (sticks out) beneath the bottom of the
vessel. This type of keel was common in older wooden vessels (Viking ships, galleons, etc.) but has seen little use in the
steel era as it unneccessarily increases the ship's draft without increasing it's cargo capacity.
Common in tankers as they allow access to the double bottoms even with cargo in all of the tanks. Duct keels also
act as their name implies, as ducts, allowing cables and piping to be run throughout the ship and between the tanks without
actually having to run through the tanks, which may cause contamination.