Concrete Floats?

Have you ever heard of a concrete ship? The idea of a concrete ship may contradict your initial reaction – concrete doesn’t float. In fact, concrete can float. Objects can float in almost any medium under one of two techniques. The first, perhaps simplest, technique is: an object of lesser density will float in a medium of heavier density. Consider an ice cube in liquid water. Ice has a density slightly lighter than water. Another example is wood. The density of wood is much less than water; hence, the old practice of using wood for duck decoys. The second technique is to displace a sufficient volume of medium with an object such that the weight of displaced medium exceeds the weight of the object to be floated. The former technique is simply a characteristic of the material property, density. The latter technique is founded in Archimedes Principle. 

Archimedes Principle introduces the concept of a buoyant force. The buoyant force is an upward force exerted by a medium on an object, which is equal to the weight of medium displaced by the object. A medium can be most any liquid or gas; consider ships on the ocean or a balloon in the air. For the duration of this article, we will continue with the premise that our medium is water. So, restating Archimedes Principle: a buoyant force on an object is equal to the weight of water displaced by the object. The trick to making the object float is to displace enough water to exceed the weight of the object. 

In days of yore, ships were fabricated with wood. Think of the USS Constitution. Compared to concrete, there was little problem floating a wooden ship. What may be another light bulb moment is that modern-day warships are fabricated from steel. With your same initial rationale applied at the top of the article, steel doesn’t float … but, somehow it does! The application of the Archimedes Principle to hull design has aided engineers and naval architects to create warships of robust materials. Again, the trick is to make the hull of the ship displace a volume of water whose weight exceeds the weight of the ship.

With the outbreak of World War I, steel became a scare material. The United States government sought to explore the use of concrete as a material for shipbuilding and authorized the construction of 24 concrete ships. Twelve concrete ships were under construction by the end of the War. With servicemen still overseas at the end of the war, at least one concrete ship, the S.S. Atlantus, voyaged across the Atlantic Ocean to bring servicemen home from France. She made at least two trips to France and later hauled coal to New England, before being scrapped after her first, and only, year of service. The ships were not intended to last forever; they were intended to serve an immediate wartime need. Within a decade, the government would sell the concrete ships to private industry. 

Perhaps the most famous of the concrete ships is the S.S. Atlantus. Those on the east coast of the United States, may be familiar with the S.S. Atlantus. The odds of knowing about the concrete ship are greater if you are in the Mid-Atlantic United States, and even greater if you are in New Jersey. The Atlantus was sold for use as a wharf for a ferry line between Cape May, New Jersey, and Cape Henlopen, Delaware. The Atlantus and two sister ships were to be beached in a Y-shaped configuration, with the two sister ships receiving the ferry and the Atlantus anchoring them to shore. Unfortunately, while moored off the coast of Cape May in the summer of 1926, a storm broke the Atlantus free from her mooring. The scavenged hull drifted, only to strike a sandbar and run aground 150 feet off the shore. Numerous attempts were made to rescue the Atlantus. However, none were successful. Today, she is an unrecognizable monument to ingenuity, visited by thousands each year at Sunset Beach in Cape May, New Jersey.

The study of floating concrete continues to this day. Every year, for at least the last three decades, civil engineering students across the United States research, design, fabricate, and race concrete canoes. While on a vastly smaller scale than the concrete ships of World War I, the competition is fierce. Students have attempted to solve the problem of floatation in numerous ways. The goal is to keep the gunwales of the vessel above water with as many as four paddlers aboard … and cross the finish line first. Some have sought to develop a concrete mix that has a density less than water. Most others endeavor to build a hull so stout, that its shape has sufficient strength to support the paddlers and the rigors of racing, while still being able to lift it from the lab to the open water. 

The ingenuity that inspired concrete ships more than a century ago lives on. As engineers, we aspire to solve problems in ways not yet conceived. Sometimes a solution lies in the unimaginable … floating concrete. 
 
Sources: 
ASCE Concrete Canoe Competition. (n.d.). ASCE. Retrieved April 28, 2022, from https://www.asce.org/communities/student-members/conferences/asce-concrete-canoe-competition

Magazine, C. M. (2020, May 7). Beneath the Surface: The story of the SS Atlantus. Cape May Magazine. https://www.capemaymag.com/beneath-the-surface-the-story-of-the-ss-atlantus/

Rob Bender, R. B. (n.d.). Concrete Ships. Concrete Ships.Org. Retrieved April 28, 2022, from http://www.concreteships.org/

About the Author

Edward M. Deegan, Jr., M.S., P.E. is EDT's Mid-Atlantic District Engineering Manager and a Consulting Engineer in our Mid-Atlantic Office. Mr. Deegan provides specialized consultations in the areas of structures and the built environment, including cause of damage, scope of damage, and value of loss. You may contact him for your forensic engineering needs at emdeegan@edtengineers.com or (856) 662-0070.

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