Naval mine ship, HMHS "Britannic", which was the sister ship of the RMS "Titanic." During World War II, the U-boat fleet, which dominated much of the battle of the Atlantic, was small at the beginning of the war and much of the early action by German forces involved mining convoy routes and ports around Britain. German submarines also operated in the Mediterranean Sea, in the Caribbean Sea, and along the U.S. coast. Initially, contact mines (requiring a ship to physically strike a mine to detonate it) were employed, usually tethered at the end of a cable just below the surface of the
Sister ship American sister ships and , and the German Hindenburg class airship's "Hindenburg" and "Graf Zeppelin II". The generally accepted commercial distinctions of a "sister ship" are the following: The critical overriding criteria are the same hull design. For example, the popular TESS-57 standard design built by Tsunishi Shipbuilding are built in Japan, China, and the Philippines. All the ships of this design are classed as "sister ships". The International Maritime Organization defined "sister ship" in IMO resolution MSC/Circ.1158 in 2006. Criteria included these: Sister ship A sister ship is a ship of the same class or of virtually identical design