After breakfast, Marcia went to the Ermee River to conduct an experiment. She is accompanied by Hitoshi and Reiko.

The purpose of the day was to determine the position of the propulsor.

"Normally we put it on the stern of the boat. ......"

Marcia begins to consider the position of the propulsor while the raft is placed on the riverbank.

Marcia said that the bow would be better for the directional stability in the current.

"Hmmm, that may be true. But ......"

The force of the propulsor pushing the hull has <> (strength) and <>.

Also, the force of the river current to flow the hull has a <> and a <>.

What happens if the > is not in the same .

If the of the force is the same, or if the of the force is small, there is little effect. This is because a hull is designed to be stable.

However, if the of the force is opposite and the < of the force is large, the ship will <.

One of the ways to prevent this is to position the propulsor in the direction of travel.

This is called a tractor type, and most propeller-driven airplanes use this method.

On the other hand, the screw of a ship is called a pusher type.

"...... but as far as I know, I've never seen such a ship. I think there are some pitfalls."

Jin has built model ships when he was in modern Japan, but he is an amateur when it comes to actual ships.

Since this is a problem related to fluid dynamics, it is not surprising that Jin, who is not an expert in this field, does not know about it.

Simply put, the efficiency of a tractor-type ship is reduced when the water flow generated by the screw hits the hull.

This is less of a problem for airplanes, which are airborne, because the physical constants such as the density and viscosity of air are smaller than those of water.

"Well, since Jin says so, there must be something to it."

"Now, the only thing I can think of right off the top of my head is that if we use screws, the water flow would hit the hull of the boat. That would be a drag, right?"

But Jin, with his experience and sense, had guessed almost the right answer.

"Indeed. If we assume that the position of the screw is shallow so that it does not rub against the river bottom, the water current created by the screw will certainly hit the hull.

Marcia also understood the disadvantages of such a situation with her imagination backed by experience.

"No, we've stumbled on it."

Marcia's head was in her hands.

"Oh, ...... something, I'm sorry."

Jin apologized to Marcia when he realized that he had made a mistake first thing in the morning.

"No, it's not Jin's fault. She was just giving me advice so that I wouldn't have to do anything unnecessary.

Marcia laughed it off.

Then Rodrigo came in. He asked Marcia what she had to say,

"If we are going to use the Magi Water Jet, a magical water propulsion system, I think the only way is to use a trimaran.

She says, "If we are going to use the Magi Water Jet, then the only way is to use a trimaran.

"Oh, I see, that's the way to go. That's true, that's the best way.

Marcia agrees.

We had ruled out trimarans and catamarans in order to reduce the width of the river, but now we were considering adopting them from a different point of view.

"......Then, if we make it a trimaran and put a magical water propulsion engine "Magi Water Jet" in the leading hull,......I think this will work, don't you?"

"Yes, that sounds good."

Jin agrees. Jin agrees, because in that position, the water flow would be less likely to interfere with the hull.

"That will reduce the load capacity a little, but I think it is unavoidable for safety reasons.

Rodrigo was of the opinion that the advantages were the most important.

"All right, then, let's build an experimental ship.

Marcia declared and headed for Nido Castle. She goes to the workshop there to build a simple trimaran and experiment with it.

"...... By the way, where is Tiberio?"

Marcia asked Jin on the way back.

"After yesterday's conversation, he said he still had a lot to learn, so I told him about the library in the castle. I've been reading books about ships there.

But I'll invite you to help me build an experimental ship later," Jin said.

* * * *

The workshop in Potrock in the Kingdom of Elias was left in the hands of the shopkeepers, Arrow, Arco, and Vail, so that Marcia and the others were assisted here by a craftsman (Smith) 20 on loan from Jin.

But, but....

Today we have Jin.

"Is this it?"

"Uh-huh. Maybe it should be a little smaller?

"Yes."

"Oh, that's fine."

Like this, the experimental boat was completed in about 20 minutes.

The experimental boat is a trimaran of 2.5 meters in length and 1.5 meters in width. In terms of size, it can accommodate one person, or two if necessary.

Three hulls, each 1.5 meters long and 50 centimeters wide, are arranged at the apex of an isosceles triangle.

The material used for the experiment is . The material is more durable than FRP because it has been treated with <> by Jin.

"......It's Jin after all. ......"

Marcia said nostalgically, remembering the time when she, Jin, Reiko, and I were aiming for the championship at Potrock.

Then Tiberio rushed in.

"......This is ......!　Uh, <>, you say?"

It seems that the effect of reading the book in the library has been felt immediately.

"Yes, it is. It's also called a trimaran. I made a prototype because I thought it would have a lot of advantages for operating in rapids.

All that was left was to install the propulsion and control systems, Jin said.

"Reiko?"

"Yes, Father."

Reiko, who had been silently assisting Jin, called out to him.

"Get the propulsion unit ready."

"Yes, here."

The Magi Water Jet, a magical water propulsion system, was already in place. I wondered how long it had been in the works, but it seems that Reiko and Smith20 had been working on it while Jin was building the hull.

"Marcia, put it on."

"Okay."

Jin is only helping to save time. The actual work is left to Marcia and Rodrigo.

Marcia and Rodrigo, who are used to doing this, quickly install the Magi Water Jet, a magical water propulsion system, and connect it to the control system.

A simple deck cover was put over the boat, and the work was completed in an hour.

"That's it. Jin, let's start experimenting!

Marcia is enthusiastic.

The experimental boat weighs about 50 kilograms, and Craftsman (Smith)20 easily lifts it over his head and carries it to the Herme River.

Jin, Reiko, Rodrigo, and Tiberio follow. Only Marcia returned to her room at Nidojo to change into a wetsuit.

Five minutes later, Marcia was seen boarding an experimental boat on the Herme River.

"Wow, that's so cool!

"I want to go on board!"

Girls from the village of Kaina, who had been collecting Magi Sand in the Erme River, gathered around.

"Oh, isn't that the boat Barbara and the others asked for?

Patty, one of the girls who had arrived, said.

"That's right. When it is completed, it will be easier to transport the magi sand.

"Wow, that's exciting!

With the girls' voices in the background, Marcia operated the experimental ship and worked hard to collect data.