After breakfast, Marcia headed for the Erme River to conduct her experiment, accompanied by Jin and Reiko.
The objective for today was to determine the mounting position for the propulsion system.
"Normally you'd put it at the stern, but..."
While the raft sat on the riverbank, Marcia began her evaluation.
Considering directional stability within the current, Marcia suggested that placing it at the bow might be better.
"Hmm, that might be true. However..."
The force with which a propulsion system pushes a hull has both 'magnitude' and 'direction.' Similarly, the force of the river's current trying to sweep the hull away also has 'magnitude' and 'direction.'
What happens when the 'point where the force is applied' is offset? If the 'direction' of the forces is the same, or if the 'magnitude' is small, there isn't much impact. Hulls are designed to be stable, after all.
However, if the 'directions' are opposite and the 'magnitude' is large, the boat will 'rotate.'
One way to prevent this is to move the mounting position of the propulsion system toward the front of the direction of travel. This is known as a tractor configuration, and most propeller-driven aircraft utilize it. In contrast, a ship's screw is typically a pusher configuration.
"...But as far as I know, I've never seen a boat like that. I feel like there's a pitfall somewhere."
Even though Jin had built model ships back in modern Japan, he was practically an amateur when it came to actual vessels. This was a problem involving fluid dynamics; it was no surprise that Jin, not being an expert in the field, didn't know the specifics.
To put it simply, in a tractor-type boat, efficiency drops because the water stream generated by the screw strikes the hull directly. In the case of an aircraft, the physical constants—such as the density and viscosity of air—are much smaller than those of water, so it doesn't pose nearly as much of a problem.
"Hmm, if you say so, Jin, there must be something to it."
"What comes to mind right now is that if we use a screw, that water stream will hit the hull head-on. That would create a lot of resistance, wouldn't it?"
Through a mix of experience and intuition, Jin had hit upon the nearly correct answer.
"That's true. If we assume the screw's position is kept shallow to avoid scraping the riverbed, the water stream it creates would definitely slam into the hull."
Marcia also understood the disadvantage through her own imagination, which was backed by years of experience.
"Ugh, we've hit a snag right out of the gate."
Marcia clutched her head.
"Ah... sorry about that."
Realizing he had shot down her idea first thing in the morning, Jin apologized.
"No, you're not at fault, Jin. You gave me advice so I wouldn't waste time on something useless."
Marcia laughed it off.
Just then, Rodrigo arrived. After listening to Marcia's explanation, he offered his own insight.
"If you're using a Magi Jet Engine, a trimaran seems like the only viable option."
"Oh, right! I forgot about that. Yes, that does seem like the best approach."
Marcia agreed.
They had previously excluded trimarans and catamarans to keep the boat's width narrow enough for the river, but now they were reconsidering the design from a different perspective.
"...In that case, if we make it a trimaran and place the Magi Jet Engine on the leading hull... wouldn't that work?"
"Yeah, that looks promising."
Jin agreed. At that position, the water stream would be unlikely to interfere with the main hull.
"It will reduce the cargo capacity slightly, but given the safety requirements, it's probably unavoidable," Rodrigo added, prioritizing stability and safety.
"Alright, then let's build an experimental boat!"
Marcia declared and headed back toward Jin Castle. They intended to use the workshop there to construct a simple trimaran for testing.
"...By the way, where is Tiberio?" Marcia asked Jin on the way back.
"He said that after listening to yesterday's exchange, he realized how much he still has to learn, so I showed him the castle's Library. He's currently devouring books about ships."
However, Jin mentioned that they should call for him once they began constructing the experimental craft.
＊ ＊ ＊
The workshop in Port Rock in the Kingdom of Elias was currently being managed by the staff along with Arrow, Alco, and Veil. Therefore, the one assisting Marcia and the others in the Jin Castle workshop was Smith 20, whom Jin had lent to them.
However.
Jin himself was also present today.
"Is this about right?"
"Ye-yeah. Maybe a bit narrower?"
"Got it."
"Ah, that's perfect."
In this manner, the experimental boat was completed in about twenty minutes.
Though it was for experimental use, it was a proper trimaran with a total length of 2.5 meters and a width of 1.5 meters. It was sized for one person, or two at a squeeze. The design featured three hulls—each 1.5 meters long and 50 centimeters wide—arranged at the vertices of an isosceles triangle.
The material was magi-resin. Because Jin had applied Toughening to it, it was far more durable than even a professional FRP vessel.
"...It really is incredible when Jin's involved..."
Marcia suddenly remembered the time in Port Rock when she, Jin, and Reiko had aimed for victory together, and she spoke with a touch of nostalgia.
Tiberio came running up to them just then.
"...This is...! Uh, was it called a 'trimaran'?"
It seemed the time spent in the Library had already paid off.
"That's right. It's also called a three-hulled ship. We made a prototype because it seems to have several advantages for operating in rapid currents."
Next came the installation of the propulsion and control systems, Jin explained.
"Reiko."
"Yes, Father."
Jin called out to Reiko, who had been quietly acting as his assistant.
"Prepare the propulsion system."
"Yes, right here."
The Magi Jet Engine was already prepared. While Jin had been forming the hull, Reiko and Smith 20 had apparently been assembling the engine.
"Marcia, you do the honors."
"Alright!"
Jin's help was intended solely to save time; he left the actual assembly to Marcia and Rodrigo. With practiced movements, the father-daughter duo efficiently installed the Magi Jet Engine and connected it to the control system.
After fitting a simple deck, the work was finished within the hour.
"There! Jin, let's test it right away!"
Marcia was eager to get started.
The experimental boat weighed about 50 kilograms. Smith 20 easily hoisted it over his head and carried it down to the Erme River. Jin, Reiko, Rodrigo, and Tiberio followed behind. Marcia took a moment to return to her room in Jin Castle to change into her wetsuit.
Five minutes later, Marcia was aboard the experimental boat as it floated on the Erme River.
"Wow, how cool!"
"I want to ride it!"
The village girls from Kaina Village, who were nearby collecting magi-sand, gathered around to watch.
"Wait, is that the boat Barbara-san and the others asked for?" asked Patty, one of the girls who had approached.
"That's right. Once it's finished, it'll make transporting magi-sand a lot easier."
"Wow, I can't wait!"
With the cheers of the girls at her back, Marcia operated the experimental boat, focusing her full attention on collecting data.