From World War II at Sea, by Craig L. Symonds (Oxford U. Press, 2018), Kindle pp. 540-541:
The American buildup for the invasion of Saipan (code-named Operation Forager) occurred simultaneously with preparations for Overlord; measured by firepower, the Saipan invasion fleet was even larger than the one devoted to Normandy. Raymond Spruance commanded the overall invasion force that included Pete Mitscher’s powerful Task Force 58, which by now consisted of fifteen carriers, seven battleships, eleven cruisers, and eighty-six destroyers. It would provide cover for an invasion force that included fifty-six attack transports and eighty-four LSTs carrying 127,571 soldiers and Marines. The employment of eighty-four LSTs in the Pacific at a time when Eisenhower was scrambling for just one or two more for Normandy was powerful evidence that the Germany-first principle had been virtually abandoned.
The invasion of Saipan also required a much longer sealift than at Normandy. While the invasion forces for Neptune-Overlord had to leap fifty or a hundred miles across the English Channel, many of the transports and amphibious ships loaded up at Pearl Harbor, more than thirty-five hundred miles from the target beach. For Neptune-Overlord, the LSTs could, and did, shuttle reinforcements and supplies to the beaches in a near-constant rotation for weeks after the initial landings. For Saipan, by contrast, the men, the equipment, the supplies, and the ammunition all had to cross the broad Pacific in a single giant stride. Eisenhower had warned Marshall that a shortage of LSTs at Normandy could mean that his invasion force might be stranded on the beach for as long as three days without resupply. By design, the men who invaded Saipan would be stranded there for three months before significant reinforcements or supplies could reach them, though of course the Japanese, too, would have to fight the battle with what they had on hand, since Saipan would be virtually cut off from support.
Like the men who invaded Normandy, the would-be invaders of Saipan first had to load the landing ships and landing craft; it was hard work, and dangerous, too. On May 17, as work parties were off-loading 4.2-inch mortar ammunition from LST-353 in Pearl Harbor, one of the mortar rounds detonated. The explosion ignited nearby barrels of gasoline, and the entire ship went up in a thunderous fireball, setting off a number of explosions on nearby ships. A witness recalled that “whole jeeps, parts of ships, guns, equipment, shrapnel, fragments of metal, all rained down on the waters of West Loch.” Before it was over, 168 men were dead, and six LSTs and three LCTs had been completely destroyed. It was just nineteen days after the loss of three LSTs off Slapton Sands in the English Channel. To replace the lost vessels, eight LSTs were transferred from MacArthur’s command. No doubt Ike wished it had been that easy for him.
The Saipan invasion force departed Pearl Harbor during the last three days of May. While en route, the tedium was broken by a not altogether unexpected announcement: “Now hear this. The invasion of France has started. Supreme Headquarters announced that the landings to date have been successful. That is all.” The news provoked loud and sustained cheering, and no doubt boosted the morale of those who were about to conduct their own D-Day.