Back in time, February 1st. The day the was launched in the Kingdom of Shoro.

The place is Potrock in the Kingdom of Elias.

"Marcia!　I've made a great discovery!

Rodrigo, Marcia's father, rushed into the store.

What's going on?　What's going on?"

Marcia's face was a mess, but Rodrigo took her hand and pulled her to him.

"Well, come here!　You'll be surprised!"

Rodrigo walked out of the store, dragging Marcia behind him.

"Bacchus, Jeremy, take care of the rest. ......"

That was all Marcia could manage.

"...... so what's the deal, dad?"

Marcia asked her father as they walked side by side.

"Oh. You know I've been working on submerged vehicles, right?"

Submerged turbine," was the tentative name for a propulsion system in which the turbine would be entirely submerged rather than half above water, as in the past.

"Yes. The conventional water turbine drive is wasteful, so why don't we eliminate it and use it to drive large vessels?"

"That's right. And now, we have found a revolutionary method!

"Wow, ...... that's great!"

Marcia also knows that her father Rodrigo has a flash of genius. That's why he once designed a catamaran.

Here we are.

It was a dock where Rodrigo had recently been experimenting with various prototypes of boats. The two of them are alone now.

A prototype ship about three meters long is floating. There was literally a submerged water wheel attached to the stern.

Yes, I know you've been doing a lot of work on this boat. "Yeah, I know you've been doing a lot with this boat, but it looks like nothing has changed.

It was the same type of waterwheel that had been installed on the boat that Marcia had won the Golem boat competition in the past, <>, and it was put into the water and rotated as it was.

Of course, it was impossible to obtain satisfactory thrust in such a state.

So Rodrigo repeated the experiment by changing the shape of the waterwheel ...... and bending the fins in different ways and sizes.

"Well, watch and see."

Rodrigo left Marcia on the dock and boarded the prototype. Then he spins the waterwheel. .......

"Oh!

Marcia's eyes widened. As the waterwheel spun, the prototype ship began to move sideways instead of forward.

More precisely, as the waterwheel began to move sideways, the stern moved sideways along with it, and as a result, the prototype began to around in a circle around the bow.

"Why?

I can understand if it moves forward or backward. But moving sideways? Marcia looked at the waterwheel.

"Is it bent in a ...... strange way?"

The fins of the waterwheel were bent in a twisting motion.

"You noticed. ...... Well, I was running around trying different things, and there was a reef, and I caught the waterwheel. Then all of a sudden the stern swung sideways."

The eight fins on the stern were twisted, as if they were screws.

"If it keeps going like this, it will move sideways. Then, if we bend the waterwheel at a right angle and attach it to the waterwheel, it can move back and forth!

Marcia exclaimed, realizing its effectiveness.

"That's right!"

Rodrigo nodded his head with a big smile on his face.

Thus, they set to work on the development of a new type of submerged vehicle, or screw.

"Well, it seems that the number of fins should be small.

"Yes, it's hard to keep the balance if there are too many."

Over the next few days, Marcia and Rodrigo worked on the development of their new submerged vehicle. The fact that the Arrow, which Jin had improved, was able to use engineering magic, albeit at a beginner's level, helped a lot.

"It looks like it only needs a little bending."

"Yes. The more bends, the greater the water resistance, and the smaller the bends, the lower the thrust.

After much hard work, on February 6, they finally developed the same screw as the one used in the .

Marcia and Rodrigo immediately decided to try it out on their prototype ship.

"Hmm, the shaft is cantilevered. Should we make it a little thicker for strength?

"Dad, the bearing looks difficult..."

"Well, if we do it this way, ...... won't work!"

That's going to be quite a challenge.

"Oh ...... if only Jin were here!"

Marcia can't help but blurt out, "Oh, if only Jin were here!

No wonder. It has already been three days since the prototype screw was completed.

That's how difficult it is to install on a ship.

There must be a gap between the shaft of the screw and the bearing to reduce friction, and water will enter the ship through this gap.

In other words, it is the same as having a hole in the bottom of the ship. We have not been able to devise a way to solve this problem.

It had been three days since she had heard this. No wonder Marcia said to herself, "Jin would have done something about it.

"Marcia, I know how you feel. I even think he probably knows how to do it. But that's why we're going to do this on our own until we finish the prototype.

At Rodrigo's rebuke, Marcia blushed and turned her head down.

"Yes,...... I guess so. I'm sorry, Dad. I'm sorry for being so whiny."

Two more days passed, on February 11.

"This is it!"

Finally, Rodrigo's persistence paid off.

"Dad, you did it!"

It is similar to what we on modern Earth would call an outboard motor.

In other words, the screw and the golem drive that rotates the screw were made into a single unit and attached to the stern of the ship.

The water resistance of this < is not a problem if the materials and construction are carefully examined. There are also Reinhardt's < and underwater golems in the golem boat competition.

However, their usage is different from that of the modern Earth.

On modern Earth, the direction of the ship is changed by changing the direction of the outboard motor itself, whereas the outboard motor is fixed at the stern and the steering is usually done at the cockpit.

The outboard motor was fixed at the stern, and the helm was usually handled from the cockpit.

Nevertheless, the talent of Rodrigo and Marusia to have achieved such a feat in such a short period of time and on their own is remarkable.

* * * * * * * * * * * *

In Horai Island, it was the day that they started to set up markers all over Ars.

"What?"

"Ha, I see. That's true.

On Earth, Jin cut off part of the spiral pump and thinks, ...... or something like that was the beginning.

"I'd like to add them to the family too. ......"

"Yes, they are. The time is ...... after 6:00 p.m., right? Potrock will be there at 3:30 or so.

So Jin decided to go to Potrock immediately.

"Elsa, I'll bring two people with me. Just the ones who can come."

"Okay."

Reiko, let's go.

Thus, Jin and Reiko flew to Potlock by the warp gate.