Barb, The U.S. could not afford to build actual drydock facilities at every island outpost overran or acquired during its island hopping campaigns in the Pacific. The speed of the advance of the Allied forces and the length of the logistical supply lines also made this undesirable. The U.S. Navy built various forms of floating drydocks which contained ballast tanks that could be flooded, such as the ARD's. This of course caused the repair vessel or dock to partially submerge which would allow the vessel to be repaired to enter the dock or in the case of the ARD's the hull, which contained keel rails and blocking timbers to steady her vertically. Once fully blocked and secured, the ballast tanks pumped out the sea-water and the vessel was actually lifted out of the water to allow repair below the waterline and did not subject the vessel to rough seas while the project was ongoing. Rick