Although it was tried in the American freighter Savannah and in the German ship Otto Hahn, nuclear power is not used by merchant ships as it is too expensive to install, the nuclear scientists that would be needed to operate the nuclear reactor on board are very scarce and very highly-paid, and there is the constant danger of maritime perils or acts of terrorism to which merchant ships are more vulnerable than warships. Therefore, several warships (including giant American aircraft carriers, cruisers and submarines, as well as a few Russian ice-breakers) have nuclear-powered machinery. Boiler water is heated by nuclear energy and the high-pressure steam turns turbines which either drive the propeller shafts directly, or generate electricity to drive electric motors that turn the propeller shaft(s) . That electricity is also used to power a range of other machinery and equipment on board. The American nuclear-powered aircraft carrier Theodore Roosevelt (above), the Russian nuclear-powered battle-cruiser Peter the Great (below) and some nuclear-powered submarines have visited South African ports in recent years. Photographs : Brian Ingpen (above) and Andrew Ingpen (below).