From World War II at Sea, by Craig L. Symonds (Oxford U. Press, 2018), Kindle pp. 425-429:
Eight months earlier in North Africa, the Allies had relied on whatever vessels they could scrape together, including car ferries and passenger liners, for the Torch landings. The results had been sobering. As American Major General Lucian Truscott had noted, the landings were “a hit-or-miss affair that would have spelled disaster against a well-armed enemy intent upon resistance.” Chaotic as it was to land the soldiers, an even more serious problem had been the offloading of jeeps, trucks, and especially tanks. As the British had learned at Dieppe, landing tanks onto a hostile beach was extraordinarily difficult. Those experiences led British and American ship designers to create vessels to fulfill that function. The result was the emergence of an entire family of specialized amphibious ships, each of which was routinely identified by an acronym.
The largest and most important of them was the “landing ship, tank,” or LST. Large, slow, and ungainly, LSTs were designed specifically to solve the problem of landing large numbers of heavy tanks on an enemy beach. Previously, that task had been the duty of a much smaller vessel called a “landing craft, mechanized” (LCM) or Mike boat, often referred to as a “tank lighter.” While an LCM could carry one thirty-three-ton Sherman tank, it was self-evident that depositing tanks one at a time onto a defended beach was unlikely to overwhelm a determined enemy. By contrast, one LST could accommodate twenty Sherman tanks or thirty two-and-a-half-ton trucks (the famous “deuce and a half”) in its cavernous hold, plus another thirty to forty jeeps or artillery pieces on its weather deck. Moreover, despite their great size, the LSTs had a flat bottom (as one veteran noted, they were “shaped like a bathtub”) and could push right up onto the sand of the invasion beach. There they opened massive bow doors and deployed a short ramp, and the tanks and trucks could then drive out onto the beach. After discharging their cargoes, the LSTs closed their bow doors and retracted from the beach by using a powerful winch on the stern that hauled in on an anchor that had been dropped offshore. As Churchill himself noted, the LST “became the foundation of all our future amphibious operations.”
Before the war was over, the United States would build more than a thousand LSTs, but in April and May 1943, when the Allies assembled the plan for the invasion of Sicily, there were fewer than two hundred of them, and many of those were still undergoing sea trials. As a result, the invasion groups for Operation Husky sought to maximize each LST to its fullest capacity. During one pre-invasion exercise, Allied planners loaded one with 450 men, all of their equipment, and no fewer than ninety-four vehicles to see if it could still operate. It could.
Another new amphibious ship was a smaller tank carrier that the British called a “tank landing craft” (TLC) and the Americans a “landing craft, tank” (LCT) [see note below]. Half the length of an LST, and displacing only a third the tonnage, an LCT could carry up to five tanks or trucks in its open-air hold. These sturdy amphibs were especially useful for bringing tanks ashore during the first several waves, when it was too dangerous to expose the large, scarce, and expensive LSTs to shore-based artillery fire.
To carry the men ashore, the Allies would again rely heavily on the small landing boats, officially LCAs (British) or LCVPs (American), often (and herein) called Higgins boats. The newest versions had an armored drop-front bow so that the men did not have to climb out over the sides to get to the beach. Small, cheap, and almost literally disposable, the Higgins boats were ideal for the first several assault waves, though in order to build up troop numbers quickly during subsequent waves, the Allies also had a larger troop carrier called a “landing craft, infantry” (LCI), which their crews affectionately called an LC or “Elsie.” The most common type was an LCI(L), the second L standing for “large.” Significantly bigger than the Higgins boats, an LCI(L) could carry up to two hundred soldiers at a time. They did not carry any vehicles, as they had no bow doors. After pushing up onto the beach, they deployed two narrow ramps, one on either side of the bow, and the embarked soldiers charged down those ramps onto the beach. Armed with only four 20 mm guns and mostly unarmored, an LCI was all but helpless against hostile shore fire, but it was indispensable for bringing in large numbers of infantry.
NOTE: Officially any vessel displacing more than 200 tons was a ship while vessels displacing less than 200 tons were craft. This rule of thumb was not universally applied, however, since both LCTs and LCIs displaced more than 500 tons but were still called craft.