Concrete ships are built primarily with ferrocement hulls, reinforced with steel bars. This contrasts against more traditional materials, such as pure steel or wood. The advantage of ferrocement construction is that materials are cheap and readily available, while the disadvantages are that construction labor costs are high, as are operating costs. During the late 19th century, there were concrete river barges in Europe, and during both World War I and World War II, steel shortages led the US military to order the construction of small fleets of ocean-going concrete ships, the largest of which was the SS Selma. United States Maritime Administration (MARAD) designation for concrete ships-barges was Type B ship. Few concrete ships were completed in time to see wartime service during World War I, but during 1944 and 1945, concrete ships and barges were used to support U.S. and British invasions in Europe and the Pacific. Since the late 1930s, there have also been ferrocement pleasure boats.