In 1943 Canadians tried to built an aircraft carrier from a mixture of ice and wood pulp. Known as “pykrete” this material is as strong as concrete, does not melt as fast as ice and is cheap to produce. A “bergship” made of pykrete would be not only unsinkable but also indestructible, since most of the damage can be repaired with seawater. Well, as long as the temperature around the ship is below freezing…
Project Habakkuk was the plan to build an enormous aircraft carrier, actually more of a floating island than a ship. The bergship would be providing a convenient landing site for the military aircraft operating between Canada and Greenland. Since exact properties of pykrete were unknown at the time, a 1000 ton 18m by 9m model was built on a lake near Alberta, Canada. However, the real ship was never finished. As it turned out, the ice ship still needed steel for reinforcement and proper operation and the engines were a pain to attach. Besides, you can’t have the crew eat cold rations and live in parkas all the time.
In December 1943 the project was officially abandoned. It took 3 hot summers for the prototype to melt completely.
In 2009, the Mythbusters dudes have built a small boat out of pykrete. They confirmed that the material is nearly bullet-proof and very strong. It melts anyway….