Since Germany declared unrestricted submarine warfare in February 1917, the losses of Allied merchant ships increased from 300,000 tons in January to 400,000 tons in February, then surged to 500,000 tons in March, and peaked at 850,000 tons in April.
Among British merchant ships venturing out to sea, one in every four ships, on average, was sunk.
In order to protect their own merchant ships and maintain their maritime trade routes, the British Admiralty began implementing convoy escort systems. This involved grouping a dozen or more merchant ships together into convoys, escorted by destroyers or cruisers, for travel between the United States and Britain.
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On the vast ocean, a massive fleet consisting of more than thirty vessels was heading towards the British Isles. Within this fleet, there were oceanic tankers of over 20,000 tons, as well as large cargo ships of over 10,000 tons. They formed a compact formation resembling a circle, surrounded by cruisers and destroyers with formidable firepower and high speeds. These warships looked new and formidable, their guns vigilant as they scanned the sea, ready to unleash fierce artillery fire at the slightest provocation.
This convoy escort formation seemed flawless at first glance. However, if one were to nitpick, the lack of accompanying aircraft or airships was the biggest flaw of this British convoy.
Zhāng Hǎinuò noticed this early on. It was precisely because of this observation that he was able to calmly command U171 to approach the attack position on the surface before the British convoy arrived and then submerge—had there been even one British airship accompanying the convoy, his U171 would have had to dive much further away, delaying the attack and potentially compromising subsequent tactical maneuvers due to battery depletion.
"Angle!"
"015!"
"Target 1, distance 3300, bearing 15, speed 12!"
This time, it was the gunnery officer, Hackett, reporting the angles, while Zhāng Hǎinuò, the only person aboard the submarine capable of observing the surface situation, focused on his periscope. Through it, he aimed at a white cargo ship, its hull looking sturdy enough to carry a significant load of goods—perhaps much-needed grain or ore for Britain, or other critical supplies, details Zhāng Hǎinuò didn't bother to ascertain.
"Target 2, distance 3100, bearing 15, speed 15!"
This time, Zhāng Hǎinuò didn't need Hackett to report the angle, as his periscope hadn't moved yet, and another ship had entered his view. It was a typical British destroyer, displacing around 1000 tons, much smaller than the German torpedo boats, with a sleek hull. Armed with a single gun on the forecastle, a simple bridge aft, three funnels amidships, a forward-facing gun, and two torpedo tubes, and a rear-facing gun on the stern, the destroyer appeared less menacing compared to larger cruisers.
As the submarine's operations officer and assistant to the captain, Cook diligently recorded the data reported by Zhāng Hǎinuò, his duty being to calculate the submarine's firing parameters. Of course, all the data kept changing every second, something he had to factor in during his calculations.
Zhāng Hǎinuò turned the periscope clockwise slightly. This time, he saw a larger warship than the previous destroyer, about twice as long, with a higher freeboard, and armed with more powerful guns—two single guns on the forecastle, secondary guns and torpedoes amidships, and two single guns on the stern. From this distance, Zhāng Hǎinuò estimated its main guns to be 152mm caliber, a significant upgrade in firepower compared to the destroyer's guns. He also noticed a British squadron command flag flying from the cruiser's mast—identifying various ship flags was a course at the Kiel Naval School's submarine specialization training. While it might seem trivial to other students, it was a valuable addition for Zhāng Hǎinuò.
"Angle!"
Cook stared at the periscope's readings. "018!"
"Target 3, distance 3000, bearing 18, speed 15!"
Next, Zhāng Hǎinuò didn't continue searching for other targets or their related parameters but ordered the periscope to be retracted.
He remembered the instructor saying that during combat, the periscope should be exposed to the surface for as little time as possible to avoid being detected by the enemy. "Periscope! Torpedoes 1 and 2 set to a depth of 3 meters, no spread! Torpedoes 3 and 4 set to a depth of 2 meters, spread 3! Torpedo 5 set to a depth of 2 meters, no spread!"
After issuing this series of commands, Zhāng Hǎinuò stared at his watch, simultaneously asking Hackett, "Time until Target 3 reaches bearing 5?"
"From just now, 4 minutes and 20 seconds!"
Zhāng Hǎinuò didn't speak further but waited silently. Although he hadn't launched a torpedo in the past few days since entering the Atlantic, he had figured out many things. An outstanding submarine commander shouldn't engage in direct confrontations with enemy escort ships but should exploit the submarine's advantages to launch surprise attacks on enemy vessels, achieving lethal hits and then withdrawing swiftly. This kept the enemy escorts in a passive state, rather than exposing the submarine to vulnerabilities while traveling at full speed and depth, unable to retaliate.
At an unexpected moment and place, Zhāng Hǎinuò felt this was the essence of Hessen's operational philosophy. He couldn't merely imitate Hessen's actions and tactics but needed to use his own intelligence to analyze the battlefield and devise clever strategies.
At this moment, Zhāng Hǎinuò, unlike a few days ago in the North Sea, was no longer clueless. He believed that although the listening devices installed on British ships had improved in accuracy, they were still in a passive search phase. Otherwise, U171 wouldn't have easily dodged the British in the previous "feigned death" maneuver!
In a state of intense concentration, the next four minutes for Zhāng Hǎinuò passed almost by the second. Just as the second hand completed four revolutions and returned to its original position, he erupted like Hessen:
"Raise the periscope, torpedo tubes ready for immediate launch!"
After lowering the periscope, Zhāng Hǎinuò made a slight adjustment, aligning his view to 5 degrees. A few seconds later, a white ship silhouette appeared in his view. Without hesitation, he ordered the launch of Torpedoes 1 and 2.
After a pause of about two seconds, he shouted, "Torpedoes 3 and 4, launch!"
Soon, Edwyn's voice came over the intercom, "Torpedoes 1 and 2 launched! ... Torpedoes 3 and 4 launched!"
At this point, Zhāng Hǎinuò no longer felt any excitement. He calmly commanded, "Retract the periscope, dive to a depth of 40
meters!"
63 seconds later, the first explosion was heard from the surface, followed by the second three seconds later, and then the third, two seconds after that. At that moment, Zhāng Hǎinuò exulted inwardly:
"Yes! I'm finally in business!"