THE United States Pacific Fleet during World War II was the largest amassed in military history, and destroyers like the USS Cassin Young bore the brunt of its losses, accounting for 40 percent of the ships that sank. Vessels like the Cassin Young braved the Japanese ''Divine Wind,'' including the 1,500 kamikaze strikes that rained down on American ships off Okinawa and the half-million men that manned them The destroyers were known as the Tin Can Navy, the small boys, the watchdogs of the fleet. These were not the smooth-riding carriers that inspired awe with their size and amenities. Nor did they possess the firepower of the gun-bristling battleships. They were the fast, multipurpose offspring of the World War I four-piper destroyers. There was a certain esprit de corps among World War II destroyermen not found on other vessels. Perhaps it was the cramped quarters on a 376-foot ship, the pervasive smell of diesel fuel, or the knowledge that, as the men lay in their bunks stacked three high and a forearm apart, only three-eighths of an inch of hull separated them from the ocean. Maybe it was the feeling of pride they got from being the eyes and ears of the fleet - the most useful tool of the Navy, as Adm. Chester Nimitz described it. The Cassin Young, worthy of the admiral's praise, was also a destroyer worthy of her namesake, Cmdr. Cassin Young. Young's vessel, the USS Vestal, was tethered to the battleship USS Arizona during the attack on Pearl Harbor. Young was blown overboard by an explosion on the flagship. He clambered back aboard and was told by a crew member that they were abandoning ship. Young balked at the idea, took command of the Vestal, and piloted it to safety. He received a Congressional Medal of Honor for the deed and was promoted to captain. Today, a half-century later, the heroics of the men of the Cassin Young linger in memory as the ship's crew visit her in Boston's Charlestown Navy Yard. To the men that lived on her, she was a floating rite of passage that taught them the unselfish drive to serve and protect the family and friends they had left behind. The ship is a monument to their courage, to the long hours and day-to-day hardships they endured to protect America's fleet. Many were just teenagers when the Cassin Young was launched in San Pedro, Calif., on Sept. 12, 1943. When their ship was commissioned on Dec. 31, 1943, the boys set a course for manhood. ''We grew up in a hurry. This would have normally been your college,'' says Petty Officer First Class James Marrs from his home in La Mirada, Calif. Having developed a seaman's eye, Mr. Marrs was the ship's helmsman during special sea detail. ''You can go and get a degree in psychology and learn about peer pressure, and you won't know what the guy who came aboard a destroyer learned in two or three months,'' he says. ''If you were late for a watch, you weren't cheating the Navy, you were cheating one of your shipmates.'' Life aboard a World War II destroyer was no ''South Pacific'' musical. The Cassin Young spent 41 weeks out of 52 at sea in its busiest year, and 90 percent of that time was routine. The work days were 10 to 12 hours long. ''Most of the time it was very boring duty and sometimes very fatiguing duty, like when we were out raiding in the East Philippine Sea,'' says retired Lt. Eugene Sevensma. He was the ship's doctor and photographer. A seaman's eye may have been adequate for manning the destroyer's helm, but nothing short of an iron stomach was sufficient for withstanding the destroyer's motion. Destroyers could list 45 degrees off keel. As Mr. Sevensma describes it, the motion of a destroyer at sea is not just up and down, but more like a corkscrew churning through water. ''The East Philippine Sea is just rough all the time,'' Sevensma says. ''One night you'd steam toward Luzon at top speed, so we'd be getting, Pound! Bang! Bang! Bang! all night long, hitting those waves. Our [carrier] planes would be launched at dawn, and we'd immediately turn the task force around and go back toward the east to get away from the land-based planes. This just happened night after night after night. We just took this beating.'' The men often went six weeks without seeing land. Shore liberty usually consisted of a few hours of softball on a tiny atoll before getting under way again. When they weren't climbing trees for coconuts on the island of Mog Mog, the crew broke out chisels to repaint the rusty hull, played mail carrier between the other ships, fired star shells to illuminate the enemy, or picked downed pilots out of the drink and traded them for ice cream - a rare commodity aboard a destroyer. Their most important role, however, was defending the Third and Fifth Fleets from air, surface, and submarine attacks. Japanese planes first damaged the Cassin Young at dusk on Oct. 14, 1944, in the preamble to the Battle of Leyte Gulf, the largest naval battle in history. In that confrontation, US forces sank four Japanese carriers and gave the Allies the upper hand in the Pacific theater. ''The Japanese had airfields on the Philippines and they were constantly launching air attacks on the Third Feet,'' says former Petty Officer Third Class James O'Hara, who manned the port after 40-mm guns. ''We were off battle stations and heading down to the mess hall when torpedo bombers came in at water level,'' Mr. O'Hara says. The twin engine Betty bombers came in so low off the starboard quarter that their props were kicking up spray. ''By the time we got to general quarters they were pretty close.'' O'Hara could see the plane's waist gunner through a canopy. When the machine guns were blinking at him, he felt it had become a one-on-one war. ''I shot him down.'' None of the dozen Japanese planes made it past the destroyers. Ten days later, the light carrier USS Princeton was hit with a 550-pound bomb that dropped through the flight deck to the ship's bakery. It exploded into the hangar, engulfing torpedo planes in flames while they were being armed and refueled. The Cassin Young, along with three other destroyers and two cruisers, was ordered to help fight the fires. The destroyer hoses were not doing the job, so the cruiser USS Birmingham was ordered to come up alongside instead. It appeared as if the baby flattop was going to be saved, when a tremendous explosion in the after torpedo magazine ripped off her stern and shoved the Birmingham sideways. A shower of fire and debris killed 108 on the Princeton, 229 on the Birmingham, and wounded more than 400. ''If we were where we were going to be, we would have been blown right out of the water,'' O'Hara recalls, his eyes turning a paler shade of blue. Fire control man Sumner Wheeler was on the destroyer's bridge. Mr. Wheeler remembers the post-disaster pandemonium in the water. ''We were nosing into sharks and nosing into empty cartridges. The executive officer got a rifle and fired at the sharks. Then we put a boat in the water and picked up quite a few people.'' Marrs, who was in the whaleboat that helped save more than 120 men in a half-hour, says, ''Picking up survivors was the best feeling I had in the Navy.'' Two months after seeing its first action in battle, the Cassin Young faced the typhoon of Dec. 18, 1944. The storm seemed to have every intention of claiming more American lives and ships than the Japanese had, and it nearly scuttled Admiral Halsey's Navy career. ''I was on watch when word came we were to stay at 25 knots,'' Wheeler says. ''All I could think of was Admiral Halsey, who was on the [USS] New Jersey, sitting in his cabin. The Jersey at 25 knots just goes straight ahead through the typhoon. At 25 knots the Cassin Young bounces up and down like a tin can.'' LATER, as the typhoon reached its full pitch, Wheeler lay lashed down in his bunk. The Cassin Young's bow went up, then came down. Boom! The whole ship shuddered, waking Wheeler. A wall of water smashed over the bridge where Lt. Paul Smucker called out, ''Right rudder 10 degrees!'' ''Right rudder 10 degrees,'' Lt. David Overholt repeated to the helmsman, and he turned the wheel. ''The waves were probably about 20 to 25 feet high,'' Marrs says, recalling that the destroyer was rolling beyond her limits. ''When you're out there and you go through that 24 to 48 hours, you get beat to death. You're on an iron ship and everything is landing hard. I remember coming out of that and feeling like I played two football games in one day.'' Marrs says that as the temperatures rose, men stripped down to keep their soaked clothes from chafing. ''You could look through the glass at one of the carriers or battleships, and even the officers were down to their skivvies.'' The Cassin Young stayed afloat, as it would through the next eight months of kamikaze attacks. After World War II, the ship served in the Korean War and sailed the world until it was finally decommissioned in 1960. Two years ago, for the 50th anniversary of the ship's commissioning, more than 100 people from all corners of the nation gathered to stomp across the steel deck that had been the subject of so many war stories. Mr. Smucker saw a ship's hull that was veined with welding and dimpled with use. Long gone were the paint jobs he had supervised as an ensign aboard the Cassin Young. ''It's a part of your life that you're glad you served, but you just prefer to be quiet,'' Smucker says. ''Even when we went back on the 50th, we just sort of looked at each other and shook hands.... It was not a time to jump up and down for joy. It was a time just to reminisce the fact that we're still walking around.'' Aboard the old tin can once again, Smucker could look at the silent 40-mm cannons and recall a time when he caught the gunners playing cards with their guns elevated at 60 degrees, waiting for kamikazes. ''You fellows can continue to play cards, but better lower the guns to 10 degrees,'' a young ensign Smucker told them. They cranked the guns down and resumed playing. Several days later, Japanese planes came in at 10 degrees and the gunnery crews shot down three, leaving a hunk of shrapnel on the deck. ''Best decision I made,'' Smucker now says. He gave the shrapnel to the Charlestown Navy Yard at the 50th reunion. ''If you had to go to war, there wasn't any question of the motivation,'' Smucker adds. ''Everybody felt strongly they were doing the best thing.''