Toward the end of the First World War, the United States commisioned the construction of twelve experimental concrete ships. None of the ships were finished until after the end of the war. Most were eventually sunk as breakwaters or recreational piers.
During the Second World War, another 24 concrete ships were commisioned by the US Maritime Commision. These ships were built by McCloskey and Company in Tampa, Florida. Due to improvements in construction material technology, these ships were stronger and more efficient than their WWI predecessors.
Also during WWII, several fleets of unpowered concrete barges were commisioned.
The world's largest floating breakwater resides on the Powell River in Canda. It is comprised of ten concrete ships: seven WWII McCloskey ships, two barges and the last ship of the WWI Emergency Fleet.
Another nine of the WWII McCloskey ships were sunk to form a breakwater in the Chesepeake Bay at Kiptopeke, Virginia.