A floating drydock is a type of ship repair facility that has been used since at least the early 19th century. It was adopted for widespread use by the US Navy in the early 20th century as a critical element in the Service Force that was being conceived to provide support for the fleet in an expected war with Japan. A floating drydock is generally a trough-shaped construction larger than a ship which incorporates ballast tanks under its floor and in its sidewalls. When a ship needs repair the ballast tanks are flooded, lowering the structure into the water to a depth at which its floor is deeper underwater than the bottom of the ship to be repaired. The ship is then maneuvered into a position between the sidewalls and over the floor of the dock, at which point the tanks are pumped out again, lifting the floor of the structure back out of the water, with the ship perched on a series of supports carefully positioned to match the contour of its underside. This leaves the vessel perched securely out of the water so that work on its underside can be undertaken.
Concept (U. S. Navy) All ships, but particularly naval vessels in continuous use during wartime, need periodic maintenance (such as cleaning of barnacles and seaweed off of hulls, and repainting) and repairs (of battle damage, or of damage resulting from collisions, groundings, or other types of accident) below the waterline. Such work often requires dry docking. Since before World War I American plans for a Pacific war had called for a movement across the central Pacific by amphibious forces, who would capture strategically placed territories and establish advance bases there. Ultimately, when this string of bases reached points close enough to Japanese home territory, bombing aircraft would be sent to the farthest forward of the bases and used to prosecute war against the Japanese home islands, a task that would require significant logistical support in and of itself. This was an unprecedented type of warfare, because of the lengths of supply lines dictated by the distances involved, and the quantities of supplies that would be necessary to support such a massive offensive so far from home. Ships involved in the advance would require maintenance and repairs. It was recognized at an early date that, to minimize the need to send ships in need of such servicing all the way back to North America, it would make sense to equip these newly established advance bases with the most comprehensive repair facilities that could be set up in faraway locations in short order. The provision of dry docks for these faraway bases was going to be necessary, and in 1920 the Navy began to develop a number of types of floating dry dock, of various sizes, that would be suitable for towing across long distances. Construction of such drydocks was begun on a large scale in 1940.
At the start of World War II, the US Navy had only three steel floating dry docks:
By the end of the war the Navy had either built or acquired from private owners a total of 155 docks of varying types and sizes. Between 1 October 1944 and 17 October 1945, 7,000 ships were repaired in these docks.
General Characteristics The newly built floating dry docks had lift capacities ranging between 400 and 100,000 tons. While most floating drydocks were made of steel, considerable numbers were made of concrete or timber instead. Timber floating dry docks had lift capacities of anywhere from 400 to 20,000 tons. They were not meant to be towed across the open ocean and were not given a US Navy class designation.Building the Navy's Bases in World War II, History of the Bureau of Yards and Docks and the Civil Engineer Corps 1940-1946 Chapter IX, Floating Drydocks
Docks come in different sizes to accommodate ships of different sizes; while most, including the earliest examples, are built as a single unit, the largest, designed to accommodate the largest ships, are too large to be moved across open ocean as a single unit, and are therefore built in sections which can be transported individually and assembled at their work site.
Most floating drydocks had no propulsion system of their own, and therefore had to be towed to their destinations. Most incorporated power stations, ballast pumps, repair shops, and machine shops, and many had cranes. Most incorporated quarters, mess halls, and other accommodations for their own crews, to make them as self-sustaining as possible, and thereby minimize the need for support vessels or additional facilities ashore. Some also had accommodations for crews of the ship under repair, although most of the time the crew of the damaged ship remained on board their own ship while repairs were done.
Most auxiliary floating drydocks were armed with 40 mm and/or 20 mm anti-aircraft guns for defense. Japanese pilots sometimes mistook empty auxiliary floating drydocks for aircraft carriers."Sea Going Navy Yard Follows The Fleet", November 1945, Popular Science
After World War II some auxiliary floating dry docks were sold for private use and others were scrapped.
List of Types and Docks
Myriad designations were applied to the different types of drydock built under this program. Post-war, to rationalize the system of designations, most drydocks were redesignated as Auxiliary Floating Drydocks, with Big (AFDBs), Medium (AFDMs), and Light (or Little; AFDLs) subsets. A few other designations (two Medium Auxiliary Repair Docks [ARDMs], of post-war origin, in active service, and several Auxiliary Repair Docks [ARDs] and Auxiliary Repair Docks, Concrete [ARDCs], sold into private or foreign service and possibly available for U. S. Navy use) are also still in use, for docks that do not quite meet the parameters of any of the AFD categories.
Auxiliary Floating Docks, Big (AFDBs) (Previously Advanced Base Sectional Docks (ABSDs))
The largest floating drydocks built for the Navy during World War II were two classes initially categorized as Advanced Base Sectional Docks (ABSDs). These docks were built in multiple sections for towing to the locations where they would be used; the sections were then assembled, side by side, to produce a single structure capable of accommodating ships nearly 1,000 feet long. Post-war the ABSDs were redesignated as Auxiliary Floating Dry Docks, Big (AFDBs).
The larger of the two classes, of which three (ABSD-1 through ABSD-3) were built, were made up of ten individual sections, oval-shaped pontoons that measured 256 feet (78 m) in length (from the tip of one roughly semicircular end to the other) and 80 feet (24.4 m) in width. The pontoons were 28 feet (8.5 m) thick (deep), with plating roughly 5 1/2 inches (14 cm) thick forming the upper and lower surfaces. Each pontoon incorporated 12 flotation tanks within its structure, flanking a 36-foot (11-m) wide, 80-foot (24.3-m) long vacant space, which contained accommodations for enlisted personnel on an upper floor, and storage compartments beneath, running widthwise across its center. Each pontoon supported two sidewalls, 80 feet (24.3 m) long (spanning the width of the pontoon), 55 feet (16.7 m) high, and 20 feet (6.1 m) thick. The sidewalls were spaced 137 feet, 6 inches (42 m) apart (inner surface to inner surface), with their outer edges at roughly the points where the curves of the semicircular ends began. The sidewalls could be folded down, to lie across the center of the pontoon, and then the whole segment could be towed, with the rounded ends fore and aft, at a reasonable speed. Fittings for anti-aircraft guns were located on the outer sides of the walls, so that such weapons could be mounted on top while the section was being towed. The crew generally travelled aboard the dock sections.
Upon reaching the destination the walls were raised and the pontoons joined together, side by side, with the semicircular ends of each pontoon projecting out beyond the walls, adding to lateral stability. Assembled, the core of the structure was 827 feet (252 m) long (80 feet [24 m] for each pontoon, with 3-foot [1-m] gaps left between each unit and the one next to it). Sources report that a 50-foot (15-m) long section of some sort was appended to each end of the assembled structure, giving an overall 927-foot (283-m) length to the structure; no readily available source gives a clear idea of what these extensions amounted to, although photos suggest that they may have been shelf-like structures that did not extend down even to the water's surface, much less supply any buoyancy. Flooding of ballast tanks incorporated into the pontoons could lower the deck to a depth of some 49 feet (15 m) below surface, so that a ship to be worked on could be maneuvered into a position over the pontoons and between the walls, with the ballast tanks then being pumped out, raising the pontoons back to the surface and leaving the ship perched atop carefully arrayed blocks set up on the pontoons' upper surfaces.
The smaller class of ABSD, of which four (ABSD 4 through 7) were built, was structured similarly to the larger class, but incorporated only seven pontoons, each 240 feet (73 m) long and 100 feet (30.5 m) wide, with 49-foot (15-m) high, 18-foot (5.5-m) thick sidewalls spaced 120 feet (36.5 m) apart (internal surface to internal surface). The decks of these structures could be submerged to a depth of approximately 44 feet (13.4 m) below surface. Overall length of the assembled dock was approximately 820 feet (250 m).
AFDBs had crews of 600 to 1,000 men. They were equipped with fresh-water distilling plants and, perched on a rail atop one of the sidewalls, a 15-ton-capacity crane with an 85-foot (26-m) reach. To pump water from the ballast tanks, on each section there were two discharge pumps, each of which could pump per minute, powered by a steam plant. For electricity, there were two 350-kw diesel AC generators on each section, producing 3-phase 60-cycle power at 440 volts. Each section could store of fuel oil to supply the ships under repair. Crews lived in barracks ships, called APLs, that docked next to the AFDB, as didtwo or more support bargess.NavSource, Artisan (AFDB-1)southpacificwwiimuseum.com, ABSD-1 Floating Dry Dock on Espiritu Santo, A glimpse of WWII history
USS Artisan (ABSD-1) (A-J), built by Everett-Pacific and others (A-J), built by Mare Island Naval Shipyard in Vallejo, California (E, F, H & I in use)pacificwrecks, USS AFDB-2https://southpacificwwiimuseum.com/absd/ The USS ABSD-1 USS AFDB-3 (A-I), saw fighting action in Guam, and was sold to Croatia in 2000.NavSource, USS AFDB-3Photos of USS Samuel B. Roberts on blocks in AFDB-3 in 1988 (A-G), built by Mare Island Naval Ship Yard (NSY). Attacked by air on April 27, 1945. Partially sunk 1989 as a reef.Pacific Wrecks - AFDB-4 / ABSD-4 Auxiliary Floating Dry Dock 4NavSource, NavSource, ABSD-4 (A-G), built by Chicago Bridge in Morgan City, Louisiana. Scrapped in 1997.USS AFDB-5 (A-G), built by Mare Island NSY. Scrapped 1976.NavSource, USS AFDB-6 USS Los Alamos (AFDB-7) (A-G), built by Chicago Bridge. Sold to a private shipyard in 1995. Post WW2 AFDB-8 Machinist, built by Seebeckwerft in Germany. Sold to Guam in 1997. AFDB-9 (A-B), built by Sun Shipbuilding in Chester, Pennsylvania. Sold to private owners in Galveston in 1985.
Auxiliary Floating Docks, Medium (AFDM)(previously Yard Floating Docks (YFDs))