Maksims Osobenkovs/Shutterstock Whether something can float in water seems pretty straightforward at first blush. Light things float, and heavy things don't — wood, ducks, and witches float, while rocks, metal, and Russian boss yachts sink. We all know it's not quite that simple, though. Sure, if a material is less dense than water, it'll float, but boats are made out of metal more often than not in modern times, and they float just fine, even while being laden with all kinds of cargo. There's no special property of metal that enables it to float while in boat shape. That's just Archimedes' Principle doing its job. Turns out, you can make boats out of basically anything if you're a good enough engineer. In times of scarcity, engineers have had to turn to some real unexpected materials to make seafaring possible, and one of the wackier desperation picks is concrete. Making functional boats out of concrete uses the same principles as making a boat out of metal, but it's even more challenging. There's a reason you don't see concrete cruise ships, and, unsurprisingly, none of the ones that have existed make the list of the fastest boats ever made. Concrete floats, if you're willing to work for it Christopher Furlong/Getty Images Boats leverage Archimedes' Principle to stay afloat, which means that the laws of buoyancy are what allow all boats — whether they're tankers stranded in the Strait of Hormuz or canoes floating lazily on small ponds — to traverse the world's waterways without sinking. What Archimedes' Principle says, in effect, is that an object placed in a fluid (in this case water, though gases — including the atmosphere — are fluids too) is acted upon by an upward force, called the buoyant force, in relation to the volume of the object that's submerged. Submerging an object this way displaces an equal volume of the fluid, and when the buoyant force and weight of the object are in equilibrium, the object floats. That means if you can make a boat with a big enough volume and low enough weight, you can make that boat out of just about anything. That's exactly what engineers in the United States sought to do when steel shortages occurred in both World War I and World War II. N.K. Fougner led the country's first study into concrete ships, and after it was determined that concrete ships reinforced with steel beams could indeed float, President Woodrow Wilson ordered the construction of two dozen such ships. None of the ships were completed by the time WWI ended, but eventually, half of the approved ships were finished. As if D-Day wasn't stressful enough already Layne Kennedy/Getty Images In 1943, amid steel shortages caused by WWII, the U.S. government returned to the idea of concrete ships and was able to pump out about one per month. A couple of them even saw combat, with the SS David O. Saylor and SS Vitruvius joining the D-Day fleet. As part of the invasion of Normandy, these two ships weren't used as troop transports or landing ships. Instead, they were loaded with dynamite and scheduled to be scuttled as blockships, along with several other ships to create breakwaters that made the approach easier for the infantry landing craft. The rest of the concrete ships made by the U.S. government during WWII were used mainly as transports, and they didn't last long after the war ended. Nine were sunk off the coast of Virginia in 1948 to create breakwaters for a ferry, and 10 are still part of a floating breakwater in Canada to this day. The U.S. hasn't commissioned any concrete ships since, and it doesn't seem likely that the government will make more anytime soon. While not extremely practical and certainly not as well-suited to seafaring as metal ships, concrete ships do still survive in some small niches today, though perhaps only because their quirks make them interesting to experiment with and learn from. The American Society of Civil Engineers holds a concrete canoe competition every year to challenge student engineers to build the best concrete canoe they can — and then race them, of course. So if you need your fill of concrete boating, you know where to look.