For the amphibious story, the most important consequence of Lend-Lease may have been that whatever the British asked for fed into U.S. naval programs, because the U.S. Navy became responsible for filling the orders. Thus the beaching tank ship became the LST, more than a thousand of which were built.
In 1941 the British did not imagine that such numbers would ever be available, and they feared that the LST was an inefficient solution to their problem. Because the whole ship had to be able to beach, and because it also had to be stable enough in the open sea, much of the ship’s tonnage was devoted to ballast tanks – full at sea for stability, empty when beaching to reduce draft. Not too much was left over for tanks. There had to be a better way to move heavy weights over long distances and then to deliver them to a beach.
The Danube idea was simpler. The ship would flood down until the lighter could float off. The British called the ship a tank transport, and they asked the U.S. Navy to build them some under Lend-Lease.
A British officer remembered a commercial project that could solve the problem. About 1928 the Danube River commission patented a ship-lighter combination, what would now probably be called a flo-flo (float-on, float-off) ship. The commission’s problem was moving bulk cargo such as grain in barges downriver through locks and then transshipping to freighters capable of reaching distant ports. No ocean-going ship could pass through the locks, but no shallow-draft barge could operate in the open ocean. That was not far from the tank delivery problem as understood in 1940-41. No efficient beachable lighter could cross the open sea, but no efficient ship could beach.
What if a satisfactory ship could transport the lighter? The lighter had to be big enough to carry a fair number of tanks, because the tanks had to be delivered rapidly to the beach. The lighters the U.S. Navy planned to carry on board its transports could carry, at best, only one tank at a time. The British tried various means of launching a large lighter from a conventional ship, such as massive gantry cranes, but all were clearly unsatisfactory.
The Danube idea was simpler. The ship would flood down until the lighter could float off. The British called the ship a tank transport, and they asked the U.S. Navy to build them some under Lend-Lease. Gibbs & Cox became responsible for the design. It was essentially a powered floating dry dock (the floodable part – the dock – is normally called a well deck). Because the ship did not have to beach, it could be efficient as a ship, which meant reasonably fast and seaworthy. This was called an LSD, a Landing Ship, Dock. In its wartime form it carried pre-loaded beachable craft (LCTs). Additional vehicles could be carried on a deck built above the docking well. Once the United States entered the war, the U.S. Navy took over most of the LSDs conceived for the Royal Navy. They proved very successful.
When the war ended, the U.S. Navy had three alternative ways to move equipment onto a beach. It had hundreds of attack transports and attack cargo ships with davits, cranes, and cargo nets. They represented the technology of the past: a conventional ship, with considerable capacity, but with a limited ability to move its cargo quickly onto a beach.
Its rather awkward solution to the beaching problem was to use a conventional bow and to extend a very long ramp through its upper part, supported by an arch overhead.
A second way was the LST, a relatively slow and inefficient ship that could unload directly onto a beach.
The LSD was the third alternative.
Now the amphibious problem began to change. The ships unloading offshore had been vulnerable enough during World War II, but the postwar threat worsened with the advent of nuclear weapons and also anti-ship guided missiles (which the Germans had used during the war; the Japanese Kamikazes were also considered guided missiles). The Marines argued that the best solution to the nuclear threat was to get troops and their supporting vehicles onto the beach more quickly, because once they were close to enemy troops an enemy would be much less likely to use nuclear weapons against them.
There was also a subtler change. The U.S. Navy and the U.S. Marines built an amphibious force to seize defended places, such as the islands in the Pacific. That is why landings were always associated with massive bombardments. In most places, however, an enemy would be hard-put to fortify his entire coast. The British had always thought in terms of landing where an enemy was not in place, in which case the enemy would have to move toward the landing place before trying to expel the invaders.
The United States adopted much this idea after 1945. Ultimately that was inescapable, because the postwar Navy lacked the firepower used during World War II landings (the United States retained heavily gunned warships as long as it could, but nearly all of them had to be discarded by the 1970s). For example, when the Marines and the U.S. Army landed at Inchon in 1950, the place was not already occupied in strength by the North Koreans.* This new idea had two implications. One was that the landing had to be possible over the widest possible sweep of enemy shore, so that an enemy could not be sure of where it would occur. The second was that the enemy would use the only weapon that could cover a whole coast – air and long-range missile attack. The latter encouraged efforts to make landings as quick as possible.
The missile threat made it more urgent than ever to unload the transports. The U.S. Navy could not simply scrap the bulk of its amphibious force – the davit transports – but it built very few new ones.
The other new threat was the fast submarine, heralded by the wartime German Type XXI. The U.S. response, by about 1947, was to push up the speed of the future amphibious force from about 11 to 20 knots. That was not too difficult for a more or less conventional ship like an LSD, but it was very difficult for the beaching LST. Beaching seemed to require a bluff bow accommodating a ramp. The technical problem was so bad that in the early 1950s the U.S. Navy seriously considered building an LST that would steam toward its objective with the ramp in its stern, then turn around to beach. This ship would have had its propellers under its bow, since they could not be anywhere near the ramp. Eventually the U.S. Navy did build a 20-knot LST, the Newport-class. Its rather awkward solution to the beaching problem was to use a conventional bow and to extend a very long ramp through its upper part, supported by an arch overhead.