U.S. Maritime Commission design type C1-S-D1
Thirty six of these concrete-hull ships were built for the U.S. Maritime Commission. Originally designed to carry sugar, the U.S. Army converted many into floating warehouses and intentionally sank several to form beachheads following the Normandy invasion in France.
Concrete ships and barges consisted of concrete hull shells reinforced with steel bars, which was easier for shipyards to acquire than steel plate. Despite misgivings about building ships with concrete (a similar program was largely unsuccessful during World War I), in the face of both severe losses at sea and material shortages, the U.S. Maritime Commission launched a small barge shipbuilding program in 1941.
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Although the vessels were an engineering success, they were neither cost- nor time-effective. Each concrete barge was two-thirds the cost of a Liberty Ship, used almost half as much steel, and were not capable of self-propulsion. Furthermore, the self-propelled C1-S-D1 vessels were only able to travel at seven knots. By the time the concrete construction program began, Liberty Ships had already eclipsed the C1-S-D1.