On August 8th, under the relentless glare of the summer sun, Jin and his team arrived at Rihoto Village.
They had come to install their prototyped Air Jets onto a boat for a field test.
"I’m certainly looking forward to this!"
"Indeed, my Lord."
Paerova Sondvik and Jed Alamold had accompanied them.
The arrival of such high-ranking officials caused quite a stir in Rihoto Village. While Paerova and Jed were busy receiving formal greetings like "Welcome, Lord Administrator, we are honored by your presence," Jin, Eira, and Kachea slipped away to the pier.
They began browsing the various vessels moored there, ranging from small skiffs to larger craft.
"Hmm, there aren’t many good options here..." Jin muttered.
"Why fuss over whether the boat is any good, Jin? Just pick the newest-looking one and be done with it," Eira said bluntly.
"She has a point, Jin-san. This is only a prototype test, after all," Kachea added.
Jin reconsidered their advice and nodded. "You're right. In that case... let's go with that one."
He pointed to a boat that appeared to be the newest in the lot.
"Yeah, that looks fine."
"The size seems right, too. I’m sure Lord Paerova will give us permission immediately if we ask."
As if on cue, Paerova appeared, accompanied by Jed and the village chief. "Jin-dono, Eira-dono, Kachea—have you found something?"
"Ah, yes. We believe that boat over there would be suitable."
"Hmm, that one? Village chief, what do you say? I explained our requirements on the way here."
"I have no objections, my Lord."
"Very well. Jin-dono, it is yours to use."
"Thank you very much."
＊ ＊ ＊
"...This is... truly remarkable..."
"It is just as you say, sir..."
"..."
Paerova, Jed, and the village chief stood in stunned silence, watching as Jin put "a bit" of his real effort into modifying the boat. Eira and Kachea, having grown accustomed to his pace over the last few days, didn't even blink.
"There, that should do it."
The vessel Jin had finished modifying was a wide, shallow-draft boat, roughly four meters long and 1.5 meters wide. He had mounted the two prototype Air Jets to its sides.
He intended to move the boat using "Wind Thrusters" rather than "Aqua Thrusters." Because he was using magic circles instead of magi-crystals, the control was inevitably coarse; therefore, he had opted for engines that ejected air rather than water.
Another factor was his inability to incorporate a barrier function to prevent the intake of solid objects. In the air, the most common solids would be dust, and even if a bird were to somehow get stuck, it would be a simple fix.
Finally, he attached a magi-circuit plate engraved with a "water-repelling barrier" magic circle to the hull. This would significantly reduce water resistance.
"Lord Paerova, we’re about to start the test run. Would you like to come aboard?" Jin called out from the boat.
The boat was a five-seater. With Jin, Eira, and Kachea already on board, there were two seats left.
"Hmm, I would love to, but I doubt Jed would permit it."
"Forgive us, Jin-dono, but we shall observe from the shore," Jed added.
"Understood."
Jin took his seat in the center. Eira sat near the bow, and Kachea settled at the stern.
"Wake up!"
Jin activated the Air Jets. The boat began to glide forward slowly.
"Oh! It's moving! It’s actually moving!" Eira cheered.
"Kachea, steer just like I showed you."
"Yes!" Sitting at the stern, Kachea shifted the rudder slightly, guiding the boat away from the bank.
"Alright, let's pick up some speed."
Because the current Air Jets lacked precision control, the airflow was adjusted with a lever rather than a verbal command. Jin made a mental note to eventually implement a system using wire cables or rods, similar to an accelerator.
Even with the current setup, the output was impressive. The boat managed to travel against the river's current at about ten kilometers per hour. At this rate, they could likely make it as far as Rireazu Village in the middle reaches of the Noshina River.
"I wish I could make it a bit more powerful, though..."
Output was proportional to input, and in this case, the input was free ether. While Jin had managed to replicate an ether converter using magic circles, its efficiency was only a fraction—perhaps one-fiftieth to one-hundredth—of what it would be with a magi-crystal.
"Even if I wanted to improve the magi-circuit plates, I don't have the right materials. This might be the limit."
"Nonsense, Jin! This is already quite a feat," Eira said. "Look at the shore. Paerova looks like he’s dying to hop on."
Eira’s bluntness remained unchanged; as long as she knew she wasn't being overheard, she dropped the honorifics for everyone. Jin gave a wry smile as he looked back at the shore.
"He really does. Still, I should try to improve this a little more."
"Ho? If you're saying that, you must already have a plan, right?"
"Of course. I want to increase the diameter a bit to raise the velocity of the ejected air."
Naturally, this type of propulsion couldn't exceed the speed of the air it expelled. To make the boat faster, Jin had to increase the jet speed.
"Let's try it. Forming."
Jin expanded the diameter by about twenty percent. Given the balance between material thickness and structural strength, this was about as far as he could push it. And yet...
"Whoa!"
The increase in speed was palpable.
"Since the diameter is 1.2 times larger, the area is 1.44 times larger. That means the volume of air intake is 1.44 times greater, which should result in a 1.44 times increase in jet velocity," Kachea calculated instantly.
"Yeah. Real-world results might not match the math perfectly, but there’s no doubt we’re moving about 1.4 times faster now."
The wind rushing past them felt significantly stronger.
"I want to make sure we can travel upstream to the middle reaches," Jin said.
Kachea immediately began running the numbers. "Assuming the river flow here is three kilometers per hour, our current speed is roughly fourteen kilometers per hour. The current in the middle reaches is about fifteen kilometers per hour, so we’ll need a boat that can hit at least twenty kilometers per hour."
"I see. That much?"
"In that case, we’d need the diameter ratio to be about 1.2 times what it is now."
By dividing 20 by 16, it was clear they needed an area ratio of 1.25. For the diameter, the square root was approximately 1.118. She had suggested 1.2 to account for potential loss in efficiency.
"Kachea, you're amazing," Eira said from the bow, sounding impressed. "I can't believe you can crunch those numbers so fast."
"Alright, let's head back."
"Yes!"
Kachea turned the rudder hard. The flat, shallow-draft boat remained stable throughout the maneuver. After a 180-degree turn, the boat ran with the current, reaching a speed of seventeen kilometers per hour.
"That settles the prototype for the Air Jet. Now we just need to refine the controls."
"We should think about what kind of setup would be easiest to handle!" Eira suggested.
"Exactly."
Jin agreed with her. He had never actually piloted a motorboat before, so while the controls might end up being a bit unconventional, he decided that usability was the only thing that mattered.