On U-100 submarine, Captain Joachim Schepke watched as the three transport ships in the distance collided with each other. Excitedly, he slapped the bulkhead. "Ah, what a stroke of luck! One shot, three hits!"
With such a dense formation, if their submarine approached and launched a torpedo, it would undoubtedly hit the mark. However, Joachim wasn't foolish. In this transport convoy, there were more than ten destroyers alone. Getting too close would be dangerous.
During World War II, submarine performance was poor. They were slow, spent most of their time on the surface, and relied on sonar and visual searches, using straight-running torpedoes for attacks. Once detected by destroyers, they would be swarmed by anti-submarine task forces. Running away or fighting back was futile; they were like lambs to the slaughter.
It's important to note that the submarines during World War II couldn't truly be called submarines. They were more like boats that could submerge. Hence, the term "submersible boat" would be more appropriate. At that time, most submarines had a hull shape similar to conventional ships, resembling boats. These boat-type submarines often operated on the surface, much like surface vessels. Therefore, a key technical characteristic of boat-type submarines was that their surface speed exceeded their submerged speed.
Their surface speed was generally only about ten knots, and their underwater speed was even slower—full speed at 7 to 8 knots. Underwater travel relied on battery power, which would drain after one or two hours. In contrast, destroyers could reach speeds of 30 knots or more, even exceeding 35 knots. Submarines could run for a while, but the destroyers would catch up in no time.
Submarines were helpless against the destroyers above them. Even if they retaliated with straight-running torpedoes, destroyers could easily evade them due to their high speed and shallow draft. Destroyers employed S-shaped maneuvers or a net-style search with multiple ships, bombarding submarines with depth charges (the most commonly used anti-submarine weapons, along with anti-submarine torpedoes and missiles).
At such times, the submarine could only silently dive and hope that the depth charges wouldn't hit them or that the destroyers would leave soon. However, World War II submarines could only stay submerged for about twenty hours at most. If they maneuvered at high speeds, they'd need to surface for recharging after just one or two hours. As long as the destroyers had patience, they could wear down the submarine.
When the submarine surfaced, surrender was the only option. While World War II submarines had deck guns, engaging in a shootout with destroyers was futile. Destroyers had more weapons, powerful firepower, excellent maneuverability, and strong damage control capabilities. As long as they didn't hit a critical area, they could float even after taking hits. Submarines couldn't do the same; their fragile hulls suffered damage from even minor impacts.
Even in the future, submarine technology developed rapidly. Conventional submarines equipped with AIP (Air-Independent Propulsion) significantly improved underwater endurance and noise reduction. Nuclear submarines became immensely powerful "ocean black holes" with unlimited range, great diving depths, and high speeds.
But those destroyers weren't standing still either. Their technology tree was growing in sync. Compared to the World War II destroyers, not only had their tonnage increased and their weaponry multiplied, but their detection capabilities had also significantly improved. Most importantly, they now carried 1 to 2 anti-submarine helicopters onboard. The combination of anti-submarine aircraft and destroyers posed a formidable threat, even to nuclear submarines.
Therefore, Joachim had no intention of recklessly risking their position. He returned to the cabin and loudly ordered, "Send a report to headquarters, requesting wolf pack support." Although the distance was too great to see clearly, Joachim believed that the large vessel out there was at least 10,000 tons, which satisfied him.
Suddenly, an unexpected explosion rocked the entire transport convoy.
"What's happening?!"
"Why did it suddenly explode?!"
"Is it a submarine attack?!"
The outer perimeter destroyers were certain. "We haven't detected any submarines nearby! Nor have we seen any torpedo traces."
Humbert, responsible for the escort, felt a faint unease. The cargo ship that exploded was carrying various steel and metal materials—no high-explosive materials or anything like that. Even if it were a boiler explosion, it wouldn't be so intense. Clearly, they had encountered an external attack.
But even German submarines couldn't silently infiltrate the core of the convoy and launch torpedoes, right? Could it be a mine? But even mines followed the same principles as submarines. Why did it collide with the central transport ship instead of the numerous vessels on the periphery?
Baffled and with no follow-up attacks, Humbert gradually eased his anxiety.
Meanwhile, the German wolf pack command received a telegram from U-100 submarine. Reconnaissance planes took off from Brest, flying over the vast North Atlantic, searching for this transport convoy.
Most of Germany's reconnaissance planes were modeled after Japan's Nakajima C6N "Saiun" high-speed reconnaissance aircraft.
On May 29, 1944, in the skies southwest of Saipan Island, three U.S. Grumman F6F "Hellcat" carrier-based fighter planes were closely pursuing a Japanese reconnaissance aircraft. During their routine patrol, they spotted this Japanese plane returning from a high-altitude reconnaissance mission over the Mariana Islands, where the U.S. fleet was anchored. The American fighters quickly engaged in pursuit. However, despite the sleek design and comparable engine power of the slender single-engine reconnaissance aircraft to the F6F, the American pilots couldn't catch up. They could only watch helplessly as the Japanese reconnaissance plane disappeared into the distance. After successfully evading the American fighters, the Japanese crew, led by Captain Masahiko Chihaya of the 121st Naval Air Squadron, sent a famous telegram to their base:
"I have shaken off the pursuing Grumman" (meaning there were no Grumman fighters chasing them anymore).
This remarkable aircraft that could effortlessly maneuver over U.S. fleet and logistics supply areas in the later stages of the Pacific War, leaving American fighters powerless, was the Nakajima C6N "Saiun" high-speed reconnaissance plane.
The C6N was an outstanding single-engine reconnaissance aircraft, boasting a top speed of 650 km/h (after the war, the U.S. conducted its own fuel tests on a C6N-11 model and achieved nearly 700 km/h, but the production version, with additional equipment like radios, weighed more and had a reduced speed of 609 km/h). With auxiliary fuel tanks, it could achieve an astonishing range of 5,308 km.
The C6N featured a high-wing loading design to enhance high-speed performance. To compensate for the reduced wing area's impact on lift, the designers incorporated a two-stage large-area leading-edge slat system, ensuring excellent low-speed maneuverability for carrier takeoffs and landings.
However, the most critical aspect was Japan's limited resources at the time. To minimize aircraft production costs, the C6N used extensive metal skin, which also strengthened the airframe structure and significantly reduced the number of rivets needed for production. (During World War II, Japan's military aviation industry often measured aircraft production costs and labor hours based on the number of rivets, with an exchange rate of 1 rivet = 1 yen.) While the Zero fighter weighed 2.4 tons and had 220,000 rivets, the heavier C6N (combat weight of 4.5 tons) required only 100,000 rivets.
Despite being heavier than the Zero, the C6N's focus on cost-effectiveness led to a new design that drastically reduced rivet usage, resulting in lower production costs and improved efficiency. This resource-conscious approach aligned perfectly with Germany's situation, and naturally, the Germans wasted no time in adopting it for their own use.