Stepping back in time a little, we return to February 1st—the day the Granat was launched in the Shouro Empire.
The location was Port Rock Town, in the Kingdom of Elias.
"Marcia! I’ve made a monumental discovery!"
Rodrigo, Marcia’s father, came rushing into the shop with incredible momentum.
"F-Father? What’s happened?"
Marcia looked completely bewildered, but Rodrigo grabbed her by the hand and started pulling.
"Just come with me! You’re going to be amazed, I promise!"
Practically dragging her behind him, Rodrigo hurried out of the shop.
"Baccus, Jeremy, I’m leaving the shop to you...!"
Marcia barely managed to shout those words over her shoulder as she was whisked away.
"...So, what is it, Father?" Marcia asked as they walked side by side.
"Ah. You know I’ve been researching the submerged waterwheel, right?"
"Submerged waterwheel" was the provisional name for a propulsion system where the waterwheel, which usually sat with more than half its diameter above the waterline, was placed entirely underwater to generate thrust.
"Yeah. Traditional waterwheels are inefficient, so the idea was to eliminate that waste and see if it could power large ships, right?"
"Exactly! And I’ve found a revolutionary method!"
"Heh... that’s amazing!"
Marcia knew her father possessed a certain brilliance for sudden inspiration. It was that very talent that had allowed him to design the catamaran in the past.
"Here we are."
They arrived at the dock where Rodrigo had recently been conducting various experiments with ship prototypes. There was no one else around.
A prototype boat about three meters long was bobbing in the water. At its stern, a literally submerged waterwheel was attached.
"Yeah, I know you’ve been working on this boat, Father. But from what I can see, nothing has changed."
It had all started when he took a waterwheel of the same type used on the Cygnus—the boat Marcia had used to win the Golem Boat Race—and tried rotating it while it was fully underwater.
Naturally, in that state, it was impossible to obtain any satisfactory thrust.
Because of that, Rodrigo had been repeating experiments by altering the shape of the waterwheel—changing the curvature and size of the fins.
"Just you watch."
Leaving Marcia on the dock, Rodrigo climbed into the prototype and started the waterwheel rotating.
"Ah!?"
Marcia’s eyes went wide. As the waterwheel spun, the prototype boat began to move sideways rather than forward.
To be precise, because the waterwheel moved laterally, the stern followed suit. As a result, the prototype began to fishtail, pivoting around the bow.
"H-How is that happening?"
She could understand it moving forward or backward. But moving sideways... Marcia narrowed her eyes, focusing on the waterwheel.
"...Are they bent strangely?"
The fins of the waterwheel were twisted, looking almost spiraled.
"You noticed? Well, while I was out running tests, I apparently hit a reef and snagged the waterwheel. When I did, the stern suddenly started swinging to the side."
The eight fins had been twisted out of shape, making them look almost like a screw.
"If it stays like this, it moves sideways. Which means if you mount the waterwheel at a right angle, you can make it move forward and backward!"
Realizing the potential, Marcia cried out in excitement.
"That’s exactly right!"
Rodrigo nodded deeply, a proud smile spreading across his face.
And so, they began the development of a "new submerged waterwheel"—the screw propeller.
"Hmm, it seems we don't need many fins."
"Yeah, if there are too many, it’s hard to keep the balance."
For several days after that, Marcia and Rodrigo devoted themselves to developing the new propulsion system. The fact that Arrow—whom Jin had upgraded—could now use beginner-level Craft Magic proved to be an enormous help.
"The curve only needs to be slight, it seems."
"Indeed. If the curve is too sharp, the water resistance is too great; if it's too shallow, the thrust is low."
In this way, after much trial and error, they finally succeeded in creating what was essentially a screw propeller on February 6th.
Marcia and Rodrigo immediately set about trying to install it on the prototype boat, but...
"Hmm, the shaft will be cantilevered. We should make it a bit thicker for strength."
"Father, the bearing seems like it’s going to be difficult."
"Indeed. If we do this... no, that won't work!"
They ran into significant engineering hurdles.
"Agh... if only Jin were here!"
The complaint slipped from Marcia’s lips.
It was understandable. Three days had already passed since the prototype screw had been finished.
That was how difficult it was to actually mount it on a ship.
After all, a gap was always necessary between the screw’s shaft and the bearing to reduce friction, and water would leak into the hull through that very gap.
In effect, it was no different from having a hole in the bottom of the boat. They were unable to devise a way to solve this.
After agonizing over it for three days, it was no wonder Marcia couldn't help wishing for Jin's assistance.
"Marcia, I understand how you feel. I suspect he might even know about this method already. But that's exactly why we should finish the prototype ourselves."
Admonished by Rodrigo, Marcia looked down, her face flushing.
"Yeah... you're right. Sorry, Father. I shouldn't have been so weak."
Two more days passed. It was now February 11th.
"This is it!"
The day finally came when Rodrigo’s persistence bore fruit.
"Father, you did it!"
It was close to what is called an "outboard motor" on modern Earth.
Namely, they decided to combine the screw and the golem drive unit that powered it into a single unit, which was then attached to the stern of the boat.
As for the water resistance of this outboard motor, it wouldn't be a problem if the materials and structure were carefully selected. Even in the Golem Boat Race, there had been entries like Reinhard’s Lorelei and various underwater golems.
However, their implementation differed slightly from the version found on modern Earth.
While on modern Earth, the direction of the ship is typically changed by pivoting the outboard motor itself, they used a method where the unit was fixed to the stern, and steering was performed normally using a rudder from the pilot’s seat.
It was used more like a modular power unit than a steerable outboard motor.
Even so, the talent of Rodrigo and Marcia, who had reached this point on their own in such a short time, was truly remarkable.
"My Lord, it appears Master Rodrigo has discovered the principle of the screw propeller."
On Hourai Island, this was the day the installation of markers across various parts of Ars had begun.
"What was that?"
"It happened by chance; he noticed that thrust was generated in a lateral direction after hitting a reef and bending the waterwheel’s fins."
"Haha, I see. That makes sense."
On Earth, it started by cutting out part of a spiral pump, or something like that, Jin recalled.
"I’d like to invite them into the Jin Family as well..."
"If it is those two, they are more than qualified."
"You're right. The time is... a bit past 6:00 PM. That would make it just before 3:30 PM in Port Rock Town."
At that, Jin decided to head to Port Rock Town immediately.
"Elsa, I’m going to go bring those two here. Please let the other members know. Just whoever is available is fine."
"Mm, understood."
"Reiko, let's go."
And so, Jin and Reiko used the Warp Gate to jump to Port Rock Town.