During the peak of World War I and World War II, global steel supplies were heavily rationed for tanks, artillery, and conventional weaponry. To bypass this bottleneck, the United States Emergency Fleet Corporation commissioned the construction of ferro-concrete ships. Engineers leveraged Archimedes' principle, demonstrating that as long as a concrete vessel displaced a mass of water equal to its own mass, it would float just as effectively as a steel ship.
The primary challenge was reducing the deadweight of traditional concrete. Designers utilized lightweight aggregates such as expanded shale and clay, which significantly reduced hull density while maintaining structural integrity. The SS Atlantus, launched in 1919, served as a prime prototype of this era.
Measuring 250 feet in length, her hull was constructed from a dense matrix of concrete and steel rebar. Although fireproof and impervious to rot, concrete ships faced severe practical drawbacks: their heavy hulls required significantly more fuel to operate, and their thick walls reduced cargo capacity compared to steel vessels of equivalent dimensions. Following World War I, many were decommissioned, but the steel scarcity of World War II prompted a second wave of construction.
McCloskey and Company in Tampa, Florida, built twenty-four concrete cargo ships known as the McCloskey fleet. Rather than serving as primary combat logistics vessels, many were ultimately deployed as floating supply depots, blockships, or breakwaters. Today, the sunken remains of the SS Atlantus are still visible off the coast of Cape May, New Jersey, serving as a striking monument to wartime material resourcefulness.