"Ah, that’s a good soak."
While soaking in the hot spring, Jin let his thoughts wander.
"A torpedo that uses a Force Generator to blast through the water, slicing through or punching holes in the enemy before returning... No, that won’t work. If it falls into enemy hands, they’ll just reverse-engineer it."
"But, Father, that is..."
Reiko immediately spoke up to point out the flaw. The same could be said for the golems Jin had already created.
"Ah, I see. I suppose I just need to program them to self-destruct if they’re captured."
"Exactly."
"Hmm, I don't know. I’m just not feeling it."
He felt as though he would be using them for his own convenience and then discarding them the moment things went south. Since his golems possessed their own wills, he let them make those sorts of decisions themselves, so it didn't weigh quite so heavily on his conscience.
"Right. I just need to build them so that they can never be captured by any conventional opponent in the first place."
Jin’s golems already possessed strength that wouldn't lose to any average—no, even above-average opponent.
"Let’s go with that."
His thoughts settled, and Jin climbed out of the bath before he could become lightheaded. In the changing room, Reiko wiped his back. This part of their routine remained unchanged.
Even though they were now engaged, there was little room for progress between the naturally innocent Elsa and a dyed-in-the-wool blockhead like Jin. Furthermore, Jin was an old-fashioned soul—once told by a middle school classmate that he was "like an old man"—and he held the conviction that premarital relations were out of the question.
Jin stepped out onto the veranda to cool his flushed body. Someone was already there. Naturally, it was Elsa.
"...Jin-san."
It felt somewhat distant to him, and Jin didn't particularly care for the honorific, but if he brought it up, they would only end up in another fruitless discussion. He suppressed the urge to complain.
"Are you enjoying the evening air too, Elsa?"
"Mhm."
Located on the Tropic of Cancer, Hourai Island was reasonably warm even in winter. The two huddled close together with few words, watched over only by the moon in the night sky.
...Or so it should have been.
"Father, Elsa-san, you’ll catch a cold if you let yourselves get chilled after your bath."
Reiko, the loyal automata, had called out to them, concerned for their health.
"...Shall we turn in for the night?"
"Mhm."
"Goodnight, Elsa."
"Goodnight, Jin-san."
The next day, Jin finally set to work on building the submarine.
"By the way, Jin... san."
Elsa called out to him, having seemingly thought of something.
"Are you properly managing the material inventory on Hourai Island?"
"Inventory... Laojun keeps track of it. I think."
Which was a polite way of saying that Jin did not keep track of it himself.
"Mhm, thought so. I just asked him. According to the records, Jin-san, you have consumed twenty years' worth of 64 light silver, fifteen years' worth of iron, and eighty years' worth of magi-crystals in just the last year and a half."
The previous night, Elsa had received some schooling from her mother, Mine.
She had said, "To be a good wife, you need a sense of economic sense."
Consequently, Elsa had apparently been looking into the material stockpiles since morning.
"Ah... that much?"
"...In a normal country, the economy would have collapsed several times over."
Such consumption was only possible because of the highly exceptional environment of Hourai Island, where mining had continued non-stop for a thousand years since the time of the Predecessor.
"I don't intend to nag you about it. I believe almost all of it was for necessary things."
Jin hadn't been wasteful; Elsa understood that much.
"But it’s better if you’re at least aware of it."
"You’re right. I get it. Thanks."
"Mhm."
"So, if I keep consuming materials at my current rate, even the mining from the underground golems on Hourai Island will eventually run dry..."
Jin seemed to reflect on his actions, if only a little.
"Alright! Once we build the submarine, let’s go explore for seafloor resources!"
And thus, his objective became clear.
"Okay, let's use yesterday's discussion as the basis for the design."
To withstand water pressure, the structure couldn't be a simple monocoque. He would combine structural members, prioritizing robustness, airtightness, and watertightness.
"Would it be too heavy if we used adamantite?"
"I think 64 light silver would be better."
Having said her piece, Elsa was now fully focused on helping Jin build the submarine.
"Father, could we not also use the shells of the Gigant Peeder?" Reiko suggested.
The shell of a Gigant Peeder was weaker than adamantite but stronger than light silver. Furthermore, it was lighter than light silver. The drawback was the limited flexibility in processing it. Since the shells came in thin plates, they were difficult to use for pillars or beams.
"Oh, I forgot about those. Alright, let’s use that for the outer plating."
Jin decided to use them for the exterior hull.
"The windows will be ten centimeters thick... No, let's use three layers of Ground Spider resin."
Considering the possibility of cracks or shattering, he judged that layering the resin would be safer than using a single piece. He would fill the gaps between the layers with highly transparent soft magi-resin to give the windows self-repair capabilities.
With the materials decided, construction began. Twenty Smith units and four Daidara units supported the effort.
"Jin-nii... Ah!"
Elsa had once again accidentally called him Jin-nii.
"Elsa, 'Jin-nii' really is the most pleasant to the ears after all."
"...Mhm. I'm used to saying it, too."
"...Then..."
Without either of them explicitly proposing it, they decided to stick with "Jin-nii" for the time being.
"So, what was it again?"
"...Ah, right. I suggest keeping a record of the manufacturing procedures."
This was advice born from their previous experience building a new ship by Lake Wasu in the Shouro Empire.
"Ah, that’s right....Laojun, can you handle that?"
"Yes, My Lord. I believe Reiko-san would be best suited for the task."
Laojun was being considerate of Reiko’s desire to be close to the work.
"Right, she’s closer to the actual construction. Reiko, can I ask you to do it?"
Reiko was not one to refuse a request from Jin.
"Yes, please leave it to me!"
Reiko accepted joyfully. She looked visibly happy to be of use to Jin.
"Very well, let us record everything into a dedicated magi-crystal!"
It was faster than writing on parchment, and it was easy to record the Craft Magic used. Furthermore, with this record, Laojun would be able to easily replicate the craft later.
The cooperation between Jin, Elsa, Reiko, and Laojun was highly effective. By the end of the day, the new submarine was complete.
It was 10 meters long, almost the same as the Shark. It had a total width of 3 meters and could seat eight people. The silhouette was streamlined, giving it the appearance of a sleek submersible. It featured a design that wouldn't have looked out of place in outer space, but floating on the sea, it could pass as an innovative, futuristic vessel.
The structural frame was made of 64 light silver, while the hull was a two-layer structure of Gigant Peeder shell and 64 light silver. The inner surface was coated with soft magi-resin for leak prevention and emergency repairs.
Propulsion consisted of Aqua Thrusters and a Force Generator.
For armament, it was equipped with two underwater magi-cannons (for torpedo-type shells), ten mana bomb torpedoes (propelled by Aqua Thrusters), and four adamantite guided underwater torpedoes (propelled by a Force Generator). It also featured one laser cannon and one Sonic Cannon.
The Sonic Cannon was an offensive weapon that could vary its frequency from the audible range to ultrasonic, emitting directional sound waves. This was an application of the Sonar magic Jin had used before.
Additionally, it carried one Paralyzer, one Magi-Jammer, and one ether jammer. It had two powerful searchlights and two auxiliary lights, plus two pairs of golem hands for seafloor exploration made of magi-adamantite.
For defense, it was equipped with a barrier and a solid barrier. Naturally, it also featured water-avoidance and breeze-avoidance barriers, allowing it to reach supersonic speeds in an emergency. For a quick getaway, it was equipped with a Warp Gate.
The energy source was a magi reactor, with ethenol reserved for emergencies. Of course, it carried a manacam for communication, along with a magi-detector and underwater sonar for exploration. It also featured an autopilot system based on a control core.
The thing Jin struggled with most was securing oxygen, until he hit upon a brilliant idea.
"Right! I can just use the Craft Magic 'Extraction' to perform 'Separation' on the oxygen from the carbon dioxide in the air!"
By extracting oxygen from the exhaled carbon dioxide, the oxygen concentration wouldn't rise to dangerous levels. For safety, he installed two identical units so that one could cover for the other in case of failure.
Despite the painstaking effort he put into the design, testing was still left to Reiko.
"Father, leave it to me!"
Smiling happily, Reiko boarded the submarine and spent half a day performing meticulous tests.
As a result, a gravity control magi-device was added.
This was because the specific gravity of the entire hull—which had been significantly lightened by using 64 light silver and Gigant Peeder shells—had coincidentally ended up slightly below 1. Consequently, it floated on the water effortlessly, but required gravity magic just to submerge. Jin had forgotten to account for ballast, but since the current setup was structurally superior, it was a successful outcome.
"Ah... I guess there really are things you don't notice while you're in the middle of building it..."
Jin felt a bit dejected by the oversight.
"Father, it is a prototype, so such things are to be expected. Besides, it can be easily compensated for with magic," Reiko comforted him from the pilot's seat.
Elsa, sitting next to Jin, looked a bit downcast after Reiko stole her chance to console him.
In the end, they resolved the issue by installing an additional small gravity control magi-device, finalizing the vessel's systems.