Double Bottoms

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Double Bottoms - an extra space that isolates the cargo tanks from the water outside. These spaces allow for a "crumple zone" should the vessel run aground, and are often used as ballast tanks. There are several types of double bottoms:
-Open: the floors in this double bottom are open, with large holes cut in them in order to lessen their weight. They are built more as framework than a bulkhead
-Closed: these floors are completely sealed, and are often used to segregate one section of double bottom from another.
-Solid: these floors are semi-closed, and resemble bulkheads with manholes and lightening holes cut in them
-Floor: the vertical frames that comprise the structural support within a double bottom
-Lightening Holes: Cutouts in the floors that reduce the weight of the floor itself without affecting the structural intergrity of the floor (less metal = less weight)
-Air Holes: Allow for air to enter or leave each compartment as the tank is filled or emptied. Without them, vacuums or bubbles could form against the overheads between the floors.
-Limber holes: Allow for water to flow along the bottom between the floors.
-Angle Strut: Vertical support for the frame in an open bottom floor (like a truss in roofing)
-Inner bottom: inner section of shell plating found between the double bottom and the tank above. Also known as the "tank top"
-Outer Bottom: the bottom (exterior) shell plating in a double-bottom ship
-Ceiling: the wooden planking that protects the tank top from cargo loading
-Bottom longitudinals: running parallel to the keel of the ship, these keep the vessel's hull structure from folding over and help to reduce the free surface effect of liquids within the double bottoms
-Stiffeners: vertical structural members attached to the floors that add strength to the floors