Concrete is one of the oldest materials. Mankind learned to use it back in the days of the first civilizations. But the idea of using concrete as a shipbuilding material crept into the minds of engineers only in the 19th century. Since then, a number of "stone" ships have been continuously plowing rivers and seas. How do they even manage to stay on the water and why are they being built?
A ship built of reinforced concrete does not sink for the same reason that a steel ship does not sink. The buoyancy (lift) force acts on the ship. Its indicator, according to Archimedes' law, is always equal to the mass pushed out by the object of the liquid, multiplied by the force of free fall. The mass of the ejected liquid is determined by the volume of the body acting on this liquid and its density. To simplify it as much as possible, it works something like this: if the Archimedes force created by an object on the water is higher than the gravitational force, then the object will be pushed out or held on the surface. If lower, sink. Therefore, the material of the ship's side is of secondary importance in this matter.
As already noted, people thought of making concrete ships in the 19th century. The first concrete boat was created in 1855 by the French engineer Louis Lambo. Lambo's experience was seriously remembered during the First World War in 1915, when all the belligerent countries faced an acute shortage of steel and began to look for ersatz solutions. After the end of the First World War, concrete shipbuilding was almost completely forgotten in the West. During the Civil War in Russia, as well as during the Great Patriotic War, reinforced concrete ships were made in the domestic open spaces for the same reason: concrete is cheaper and more affordable than steel. Therefore, in difficult conditions, it becomes an ideal material for barges, launch boats, ferries, floating platforms, tugs and floating workshops.
After the end of World War II, reinforced concrete shipbuilding came to naught, although individual concrete ships are still produced in different countries today. Nowadays, landing stages and floating docks are most often made of concrete for the repair of other ships. Moreover, a certain number of concrete floating facilities from the times of the World War remain in service to this day. This is because concrete turned out to be not only cheap, but also a much more reliable substitute for metal. However, such ships have one big drawback: they have disgusting driving characteristics, which does not allow the use of reinforced concrete in the construction of military, commercial, passenger ships under normal conditions.
If you want to know even more interesting things, then you should read about Soviet motor ship "Zarya": the waterbus that was advanced for its time.