The very name elicits snickers of a badly contrived joke. Yet recent technological advances in submarine and torpedo technology has contrived to make concrete submarines, or c-sub for short, a viable and potentially deadly weapon system.
Concrete has been used in ship building for decades now, in barges, houseboats, and sailboats. During World War II, the US government commissioned the building of experimental concrete naval vessels. In the 1990s, the Russian navy initiated a serious design studies on c-subs.
The main advantage of concrete-hulled submarines is that they are much cheaper and easier to manufacture than steel-hulled vessels. Concrete, even advanced forms of it, are dirt-cheap compared to reinforced steel. Another plus is that they are somewhat more resistant to crushing depths--they theoretically can dive deeper than the 1800 feet depth limit of most steel-hulled submarines. Whether in practice a concrete-hulled vessel would actually be able to withstand greater depth pressure than a steel-hulled vessel remains to be seen.
Another significant factor in building c-subs is that concrete hulls are difficult for sonar systems to pick out from the natural rock formations and sandy material on the ocean floor, where they will most likely be deployed (see below.)
Of course, the main disadvantage here is that c-subs are not naturally buoyant. They are, in essence, large hollow rocks with motors attached. A c-sub would be equipped with powerful electric turbine pumps that would propel water downward, allowing it to ascend and maneuver at the surface.
These characteristic combined allow c-subs a unique method of engaging enemies. A c-sub would maneuver into a harbor or well-trafficked sealane, sink to the bottom, and lay stealthily in wait until an enemy ship passes overhead. It would then launch a vertical rocket-propelled torpedo. These torpedoes have been developed by the Russians in the 1990s, and are code-name Shkval--Russian for "squall." Shkval torpedoes can travel at over 230 miles per hour, over four times as fast as conventional torpedoes.
The Russian design for a c-sub called for a small vessel about one-tenth the size of a modern attack sub that could be crewed by no more than six men. Because of their much lower price tag compared to steel-hulled subs and their sneak-attack damage potential, c-subs are often seen by strategic analysts as the means by which smaller, poorer countries could successfully counter the more powerful navies of the more dominant nations. Squadrons of c-subs could wreck havoc with major ports and major shipping routes if deployed effectively.
A potential flaw with these vessels is that concrete will tend to wear down quickly chemically when exposed to the salt water of the ocean environment, especially if it has to undergo many significant changes in pressure. the submarines would have to be inspected at every opportunity to monitor potential wear problems on the hull. Also, different mixes and formula for concrete can be developed to better resist these changes, as can numerous protective polymer coatings.