Thread by @nick_kapur

Iceberg ship! During World War II, Great Britain came surprisingly close to building a gargantuan aircraft carrier with a hull made primarily out of ice. 1/

The proposed HMS Habbukkuk would have been truly enormous, carrying *several hundred* aircraft, and with a deck long enough to launch full-sized bombers. 2/

The hull was going to be made of "pykrete" - a material composed of 86% ice and 14% sawdust. The sawdust of the Canadian spruce was found to make the strongest pykrete. 3/

Pykrete was the eponymous brainchild of eccentric British inventor Geoffrey Pyke. In tests it was found to be 5-8 times stronger than ice, was resistant to damage when struck by bullets or artillery shells, and melted at only a fraction of the rate of normal ice. 4/

In these ways, the physical properties of pykrete were more like those of concrete than ice, except that unlike concrete, pykrete floats on water! 5/

The various claims about pykrete were tested by the Mythbusters guys in season 7, episode 2 in 2009, and they were astonished by how strong pykrete was. They even made a motorboat hull entirely out of a pykrete-like material and drove it 25 miles per hour. 6/

The reason to use pykrete for Habbukkuk was it was significantly cheaper to produce than steel (given the technology at the time) and Habbukkuk was going to be HUGE. At 1,200 meters long and displacing 2.2 million tons, it would have been 2x as long as a modern supercarrier. 7/

But why build such a giant ship in the first place? Well, for most of the war, Britain simply had no answer for German U-boats, which ran wild in the Atlantic and destroyed large numbers of Allied transport ships. 8/

Most of the ships lost to U-Boats were in the "Mid-Atlantic Gap" (aka "The Black Pit") in the center of the Atlantic and out of reach of land-based air cover. Habbukkuk was intended to camp out in the middle of the "gap" and hunt Nazi submarines. 9/

However, the larger, twin-engined aircraft needed for long-range anti-sub patrols could not be launched from the decks of even he largest existing aircraft carriers, nor could such carriers carry enough fuel for longer patrols. Hence Habbukkuk's immense size. 10/

Plus, regular aircraft carriers were extremely vulnerable to submarine attack themselves, and would have to be protected. But Habbukkuk would have a hull of solid pykrete 40 ft (12 m) thick, which would have made it essentially immune to torpedoes. 11/

But even though pykrete melts extremely slowly, it does melt. How to deal with a slowly melting ship? This issue was to be solved by having a cooling system that would constantly keep the hull frozen. 12/

The Habbukkuk would also have bunkers filled with large amounts of extra sawdust, so it would have been able to repair its hull at will using sawdust and seawater. 13/

The Habbukkuk was to have been propelled by 28 propellers in externally mounted nacelles. Originally, it was to be steered by reversing the propellers on one side, although in the final plan the admiralty demanded the addition of a 100-foot-high rudder as well. 14/

These 28 propellers would have allowed the Habbukkuk to reach a top speed of... 6 knots. Speed was apparently not all that important for a ship felt to be nigh invulnerable and more akin to a man-made island than a ship. 15/

How close did Habbukkuk come to being built? As late as 1944, the ship was still considered crucial to victory. Prime Minister Winston Churchill and Chief of Combined Operations Lord Mountbatten were staunch supporters and urged its construction. 16/

As the Battle of the Atlantic continued to go badly, research and planning accelerated throughout 1942 and 1943. Canadian researchers even built a large scale model of the Habbukkuk on Patricia Lake, which was deemed so successful that the project was ordered to continue. 17/

However, three developments ultimately doomed the project: 1) Portugal granted the allies use of airbases in the Azores, greatly reducing the Mid-Atlantic Gap, 2) B-24 Liberator bombers were modified with VRL ("very long range") fuel tanks land-based anti-sub patrols, and... 18/

3) The Americans built dozens of super-mini "escort carriers" for convoy escort. These chibi aircraft carriers were incredibly cheap to build and reasonably effective at protecting transports from subs (pictured: model of escort carrier USS Gambier Bay) 19/

Lastly, you may wonder why the ship was called "Habukkuk." This referred to Habukkuk 1:5 in the King James Bible:

"Behold ye among the heathen, and regard, and wonder marvelously: for I will work a work in your days which ye will not believe, though it be told you." 20/20