The first nuclear submarine, Nautilus, was launched by the U.S. Navy in 1955. Submerged Valves closed Submarines dive and surface by filling and emptying their ballast tanks. Freighters Trade has been a major force behind ship building and development. This is because ships are the most cost effective way to move cargo over long distances. Containers go from factory to ship to factory with out being opened.
Oil tankers are by far the largest ships afloat.
These giants often need a flotilla of tugboats to maneuver them in port. Load lines or Plimsoll lines, show how much can be loaded onto a ship safely. Sea routes are determined by where cargo is found. These large ferries allow vehicles to drive on and off. Bulk Carriers have large cargo holds, suitable for coal and grain.