Ballast tank in the context of "Ballast"

An amphibious transport dock, also called a landing platform dock (LPD), is an amphibious warfare ship, a warship that embarks, transports, and lands elements of a landing force for expeditionary warfare missions. Several navies currently operate this kind of ship. The ships are generally designed to transport troops into a war zone by sea, primarily using landing craft, although invariably they also have the capability to operate transport helicopters.

Amphibious transport docks perform the mission of amphibious transports, amphibious cargo ships, and the older dock landing ships (LSD) by incorporating both a flight deck and a well deck that can be ballasted and deballasted to support landing craft or amphibious vehicles. The main difference between LSDs and LPDs is that while both have helicopter landing decks, the LPD also has hangar facilities for protection and maintenance. In the United States Navy, the newer class of LPD has succeeded the older classes of LSDs, and both the Navy and United States Marine Corps are looking to the LPD to be the basis of their new LX(R) program to replace their LSDs.

Ballast is weight placed low in ships to lower their centre of gravity, which increases stability (more technically, to provide a righting moment to resist any heeling moment on the hull). Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel filling with water and/or capsizing. If a sailing vessel needs to voyage without cargo, then ballast of little or no value will be loaded to keep the vessel upright. Some or all of this ballast will then be discarded when the cargo is loaded.

If a cargo vessel (such as a tanker, bulk carrier or container ship) wishes to travel empty or partially empty to collect cargo, it must travel "in ballast". This keeps the vessel in trim and keeps the propeller and rudder submerged. Typically, being "in ballast" will mean flooding ballast tanks with sea water. Serious problems may arise when ballast water is discharged, as water-borne organisms can create havoc when deposited in new environments.