Japanese submarine I-168. I-174 was from the same class
Nobukiyo Nambu couldn’t believe his luck. There in the middle of his periscope was every submariner’s dream: ten Allied freighters intent on crossing his path. True, four Royal Australian Navy corvettes accompanied them, but from what he could tell, he hadn’t been spotted.
It was late afternoon on June 16, 1943, and Nambu, now captain of the I-174, was on patrol between Sydney and Brisbane. Visibility was good, and the convoy was still 10,000 yards away, when he noticed that one of the ships was having trouble keeping up. Like a weak animal struggling to stay with the herd, it begged to be culled. Nambu ordered “down periscope” and began his attack approach.
Once he’d finished command school, Nambu was put in charge of a training boat. Teaching the next generation of submariners was important. Still, he was itching to get back to war.1 He finally got his wish five months later, when he was named commanding officer of the I-174, a frontline fleet boat. Built in Sasebo and commissioned in 1938, the I-174 was an excellent boat for the future captain of the I-401 to learn his trade. A Kaidai-class submarine with four torpedo tubes in the bow and two in the stern, she was 345 feet long and carried 70 officers and crew. Her previous CO, Lt. Cdr. Toshio Kusaka, was one of the premier sub captains in the Sixth Fleet. Nambu would cross paths with him again when Kusaka was appointed commanding officer of the I-400. Until then, it was natural that he should follow in Kusaka’s footsteps.
The I-174 had departed Kure on May 5 after two months of training. Nambu flew a large paper carp from the sub’s periscope in honor of Boys Day. It was his first war patrol as CO, and Nambu wanted to inspire his crew. It was an appropriate symbol since the Japanese believed carp were strong enough to overcome any obstacle. He also hoped it would distract them from the nerve-wracking business of departure.
Though he loved his role as commanding officer, Nambu found leavings difficult. His mind inevitably turned toward his family, whom he often talked about to his crew. He never forgot the time his wife, Yukiko, had come to see him before the attack on Pearl Harbor. She had come to Kure with their one-year-old son, Masamichi. They’d stayed together at a ryokan near the Yokosuka naval base, and though he’d been happy to see her, it was difficult as well.
Yukiko was expecting their second child at the time. Unfortunately, Nambu could tell her nothing about how long he’d be gone, or when he’d return. After three years of marriage, Yukiko was stoic; like many officers’ wives, she didn’t reveal her feelings. Nevertheless, the visit had been fraught with an undercurrent of emotion since their second child would be born while Nambu was at sea.
Few words needed to be spoken, since Japanese culture had taught husband and wife how to read between the lines. Yukiko wanted to be strong for her husband. This meant not burdening him with tears or the many worries she must have had. Nambu understood all this, but the visit wasn’t easy.
When Nambu bade his pregnant wife farewell, she was framed in a doorway, their infant son cradled in her arms. He knew this might be the last time he saw his family, but it was his duty to fight for the emperor. As he walked away, it was difficult not to look back. Still, what would be gained by lingering? It would only make things harder. Once he stepped outside the door, he resolved that only the mission would occupy his mind. This kind of leave-taking was not so much a hardening of heart as a focusing of purpose, and Nambu made a habit of it as the war dragged on. So much depended on how he performed that nothing could deter him. He had a job to do, and he’d trained long and hard to do it. There was no turning back.
Nambu’s latest orders were to disrupt supply lines between the United States, Australia, and New Guinea. If he succeeded, the United States would have difficulty maintaining her perch in the South Pacific. For the past five months, Sixth Fleet subs had wreaked havoc off the Australian coast. Results were so encouraging that the I-174 had been ordered to join the fight. Nambu wasn’t just on a hunting expedition—he’d been given a mission of strategic importance.
Nambu relayed the convoy’s range, speed, and bearing to his executive officer, who began to plot a firing solution. The convoy was too far away to do anything, but at least they had time to set up. Of course, Nambu had every intention of eluding the corvette screen. Meanwhile, the sub’s forward torpedo room prepared a reception.
When Nambu took his next sighting, he could see that in addition to the ten freighters, there were three American LSTs, or Tank Landing Ships. He kept the excitement from his voice as he informed his officers of this development. LSTs were the backbone of the Allied Pacific war effort because they delivered huge amounts of troops, cargo, and vehicles directly onto a beach. He couldn’t have asked for a better target.
When Nambu had first arrived off the Australian coast, he’d been frustrated by a shortage of targets. This probably had something to do with the successful number of sub attacks preceding his arrival. A crew’s morale depends on their captain’s ability to sink ships, though, and this made Nambu worry.
Just when he couldn’t take it any longer, they encountered the Edward Chambers, a U.S. Army transport.  In his eagerness to score a kill, Nambu mistook the ship for a freighter and pursued it aggressively. Even though he couldn’t get closer than 20,000 feet, he refused to let the opportunity slip by. Finally, when he’d run out of patience, he ordered the I-174 to “battle surface.” Moments later the sub’s gun crew began lobbing shells toward the army transport. 
Nambu was certain at least one shell had struck the vessel. What he didn’t count on though was a “freighter” firing back. Twelve rounds from the Edward Chambers’s three-inch gun forced the I-174 to submerge. It had been foolish to make a surface attack in broad daylight. One well-placed shot into the sail, and the thin-skinned sub would have sunk. Nambu learned something important that day. He might have raised crew morale, but unless he set impatience aside, he wasn’t going to last long enough to sink many ships.
Things went better after that. Nambu fired four torpedoes at a commercial vessel off Sydney, two of which he believed struck home.  Now they were facing their largest convoy yet, GP 55 out of Sydney. Composed of 13 ships and five Australian corvettes, the convoy was due in Brisbane on June 15. It was Nambu’s job to make sure it didn’t arrive.
Two of the corvettes, the Kalgoorlie and the Warrnambook, patrolled the same side of the convoy as the I-174. The Bundaberg and Cootamundra patrolled the port side, while the Deloraine protected the straggler. Originally built as minesweepers, the Bathurst-class corvettes were too slow for convoy duty. Given the pressure that Sixth Fleet subs were exerting on supply lines, every available warship had been pressed into service.
As the setting sun outlined the convoy’s silhouette, the I-174 easily slipped inside the corvette screen. The convoy’s weak link was the Portmar, a U.S. Army transport struggling to catch up. Nambu patiently waited for the transport to close the distance with another LST. When the two ships overlapped, he fired his first torpedo.
Two minutes later the stern of LST-469 erupted in an explosion, killing 26 men and destroying her steering gear. Nambu thought it a fatal blow  and turned toward the Portmar with a second salvo. The ship’s lookouts spotted his torpedoes and called for evasive action, but it was too late. A torpedo slammed into her number-one hold, igniting the ship’s volatile cargo of gasoline and ammunition. Moments after the torpedo detonated, the Portmar was engulfed by explosions. Seven minutes later, the transport was gone.
Cries of joy reverberated throughout the I-174. Nambu had shown daring in sinking two ships. Unfortunately, the euphoria didn’t last. The Warrnambook and Kalgoorlie immediately began searching for the sub as the Deloraine stayed behind to pick up survivors.
Nambu dove to 250 feet, the sub’s safety limit, and was creeping out of the area when his sound operator reported approaching screws. Though the I-174’s crew did their best to remain silent, the Warrnambook found them anyway.
A depth-charge attack is terrifying for anyone who’s experienced it. When a depth charge detonates close to a sub, the concussion rocks the boat like a hobby horse. Gear and food stores rocket every which way, and it’s not unusual for paint chips to peel from the inside of a hull. Sometimes water or hydraulic lines burst, sending high-pressure jets spewing everywhere, and if they weren’t sealed quickly enough, electrical equipment shorted out and caught fire. It was especially dangerous when a sub’s cooling system was ruptured. Freon gas filled the interior, which could be fatal, and because Japanese hulls weren’t welded, sea pressure popped rivets like rifle shots. Woe unto the crewman who stood in one’s way.
The Warrnambook laid a pattern of depth charges perfectly mirroring the I-174’s escape route. Within moments they exploded with all the fury of hell. Nambu’s crew could do little except watch their gauges dance. Many removed their sandals or took to their bunks to avoid making noise while imagining the next explosion a direct hit. As the sub’s interior temperature soared past 100 degrees, it became difficult to breathe, and the crew sweated profusely.
Water greatly amplifies sound, which means a depth charge explosion can deafen a crew if it is close enough. And nothing is more sickening than watching the hull flex inward every time a depth charge detonates. Many submariners swore that a detonation could convulse the hull as much as six inches. It wasn’t the kind of story you wanted to personally verify.
Though Hollywood loves depicting a sub being sunk by a single, well-placed depth charge, the truth was most subs were sunk by cumulative damage sustained over a prolonged attack. Another Hollywood fallacy shows a depth charge sending huge geysers of seawater cascading into the sky. Unless a sub was near the surface, this was the least effective means of attack because it meant the depth charge’s concussive pressure was vented into the air rather than against the sub’s hull. Nevertheless, depth-charge explosions were a nerve-wracking experience and submariners dreaded them.
The Warrnambool and Kalgoorlie tracked the I-174 for two hours, dropping 36 depth charges in four separate patterns. Finally, when they lost contact and saw oil on the surface, they concluded the sub had been sunk. The Royal Australian Navy was wrong, however. The I-174 had sustained damage, but Nambu had escaped and soon resumed combat operations.
Nambu’s success made him a hero to his crew. He’d attacked an escorted enemy convoy in broad daylight and gotten significant results. In fact, Nambu’s attack proved to be the most successful by a single Japanese submarine off Australia’s east coast. It also turned out to be the last two ships sunk in the region by a Japanese sub.
As Japan’s defense perimeter continued to weaken, I-boats were recalled to defend the South Pacific. This meant fewer combat subs were available to sink merchant ships off Australia. The I-174’s run was cut short for this reason. But Nambu wasn’t just being recalled to defend the empire—he was being given the most despised assignment in the Sixth Fleet sub force.
Nambu was becoming a mole.
- Nambu remembers it as a large, armed commercial vessel rather than a U.S. Army transport, but Allied records are more accurate in this instance; the I-174 was probably too far away to accurately identify the type of ship.
- Though Nambu believed he’d hit the ship, there is no Allied record confirming a ship was attacked on this date at this location.
- Allied naval records show that Nambu fired upon the American Liberty ship SS John Bartram. All four torpedoes missed.
- The LST-469, though damaged, remained afloat and was eventually towed back to Sydney. It belonged to MacArthur’s newly formed Seventh Amphibious Force, which was short of ships. Loss of the vessel forced the last-minute elimination of troops and cargo from the assault convoy destined for MacArthur’s first amphibious operation, the occupation of Kiriwina and Woodlark islands. See David Stevens, “The Naval Campaigns for New Guinea,” Journal of the Australian War Memorial.