The next morning.
A new notice was posted on the bulletin board.
“Another Challenge Record entry? They’ve been frequent lately.”
“What is it? What is it?”
“Hey, move your head out of the way!”
Researchers and craftsn gathered in droves to read the contents of the new notice posted on the bulletin board.
The first thing the researchers and craftsn checked was the grade of the entry. Checking the grade first had beco a habit for everyone since the Challenge Record was established.
And the eyes of those who confird the grade all widened.
“Huh?”
“What kind of grade…”
The newly posted notice had two grades applied simultaneously.
Grade: Lone Army’s Struggle – Lower, Moving Mountains – Upper.
It was a simultaneous assignnt of two extre grades.
Surprised by this unexpected grade assignnt, people examined the content more closely.
New Challenge Record Entry Notice.
Date: 17th day of the 3rd month, Year of Gyechuk (1433).
Proposer: Jang Yeong-sil
Content: Create a propulsion device for ships by applying the shape of a windmill.
Grade: Lone Army’s Struggle – Lower, Moving Mountains – Upper
“Ah!”
“Oh no!”
“To think we didn’t co up with this, what fools we are!”
The researchers and craftsn of the team tasked with developing the propulsion device read the content and blad themselves, slapping their foreheads.
Windmills were not unfamiliar to Joseon people. It was one of the toys that every boy had played with.
Called “pallanggaebi,” “doreurami,” or “hoehoi-a” depending on the region, windmills were toys that had been passed down since before Joseon. Initially, they had a strong magical character – typically attached to rice sheaves with five grains on the first full moon – wishing for a good harvest, but later beca children’s toys.
They blad themselves for looking for answers in strange places despite having such an object that accompanied their childhood.
After this period of self-reproach, the researchers and craftsn all had the sa question.
“But why did it receive two extre grades simultaneously?”
Unable to co up with a suitable answer no matter how much they thought about it, the researchers discussed with their colleagues.
“Do you know why it received such grades?”
“I’m not sure. Do you have any idea?”
“Should we ask His Highness?”
When one researcher ntioned Hyang, other researchers simultaneously replied.
“Would he answer?”
***
To the researchers and craftsn of the research institute, Hyang was a ‘good but demanding boss’.
When they failed in their assigned tasks or missed deadlines, Hyang carefully examined the reasons. If it wasn’t due to the negligence or carelessness of those in charge, and if there were no human casualties, Hyang rarely punished them.
Instead, what Hyang wanted from those in charge was a clear lesson learned.
While Hyang seed like a good boss in this regard, he was demanding in other areas.
There were two areas where Hyang was particularly fussy.
One was the budget, and the other was imdiately asking questions about assigned tasks without even trying to figure things out.
‘Pondering why such a grade was assigned is the first step towards success.’
Because Hyang had said this, researchers had to rack their brains from the very beginning on the question of ‘why this grade?’
“His Highness always wants us to do our best.”
“Isn’t that because he trusts us so much?”
Although they had to struggle considerably from the start, the researchers believed that all of this was for their own good and did their best.
However, this was only half the truth.
“For developnt to continue across generations, we need to thoroughly digest and move forward.”
While one reason was the belief that what the researchers and craftsn had learned needed to be properly digested to be passed on to the next generation, other aspects were due to Hyang’s selfishness and limitations.
“Are they trying to just piggyback? Even in group projects, you’d get cursed at for that. Is this deeply rooted piggybacking? And…”
Hyang examined the reports with a troubled face.
“I was the kind of geek who would make things first and think later, not good at using my head like this… If I was good at this, I would have gone into science, not liberal arts, right? Why was I said to have a 19th-century inventor style?”
The assessnt of the doctor who consulted with Hyang in the 21st century was as follows:
‘An inventor type like Leonardo da Vinci, not a theorist type like Kepler or Einstein.’
‘Not good at advanced mathematics.’
Having reached his limit due to this limitation, Hyang gave up explaining under the pretext that ‘they need to find the answer on their own to make it their own’.
***
Anyway, since asking Hyang wouldn’t yield an answer, the researchers imdiately began their research.
And as soon as they started researching, they could understand why it received two grades simultaneously.
Since the content already stated ‘model it after the shape of a windmill’, shaping it was very easy.
The researchers made rapid progress while making windmills using paper, bamboo, and reeds.
“As expected, four blades receive the wind better than two. Then wouldn’t four blades be better for the propeller as well?”
“That seems right.”
“Ah! Have you ever competed in flying windmills?”
“Yes, I have.”
“Then wouldn’t it be better to make the shape of the blades similar to windmills used for high-flying competitions?”
“Oh! That’s a great idea!”
Other researchers agreed enthusiastically at one researcher’s suggestion.
There were two ways to compete with windmills.
One was to run to a designated place or stand in place facing the wind and compete to see whose windmill spun better.
The other was to carve bamboo into wing shapes, connect them to a shaft, rub them between both hands, and compete to see whose flew higher.
Progress was rapid because they had mories of windmills, a toy that both noble children and commoner or slave children had played with familiarly in their childhood.
“At this rate, we’ll have results soon, right?”
“That’s right. Indeed, Lone Army’s Struggle – Lower grade.”
While the researchers were optimistic about the results, it wasn’t long before they understood why it had received the “Moving Mountains – Upper” grade.
***
The first thing that blocked their progress was finding the ‘optimal blade angle’.
“Simply put, these tilted blades push the water… so the angle should be…”
While researching to find the optimal angle, another variable erged.
‘Even with the sa angle, the efficiency changes when the shape and size of the blades change.’
“Damn it! Let’s give it a try, shall we!”
As variables appeared sneakily to trip them up, the researchers and craftsn cursed, rolled up their sleeves, and threw themselves into design and experintation.
The researchers and craftsn carved wooden boards in various ways to create dozens of types of screw propellers.
They made versions where the shaft connection part and blades were made separately and combined, versions where the connection part and blades were carved as one piece, versions with narrow and wide blade widths, and versions with varying numbers of blades such as 4, 6, and 8 blades, among others.
Before long, the workshop and warehouse of the Developnt Departnt were filled with so many wooden screw propellers that it was difficult to count them.
Amidst all these trials and errors, another variable tripped them up.
It was vibration.
In one corner of the Developnt Departnt’s laboratory was a spring chanism made by Jang Yeong-sil. The researchers investigated the performance by attaching screw propellers to the shaft connected to the spring chanism.
Wiiing!
As the screw propeller connected to the spring chanism began to rotate at high speed, the power shaft started to vibrate slightly up and down.
“Huh? Why is the shaft shaking? Hey, everyone!”
A sharp-eyed researcher noticed the anomaly and called his colleagues.
The colleagues also confird the vibration, but here the researchers made a fatal mistake.
“Isn’t it vibration caused by the spring unwinding? We had sothing similar with the water wheel before, rember?”
“Is that so?”
“Let’s quickly attach it to a ship and experint.”
And so, a model ship equipped with the 8-blade screw propeller that had shown the best efficiency was floated in the water tank.
“Start!”
As the experint using the model ship began, the researchers’ faces turned pale.
“Why is the ship doing that!”
The ship began to shake back and forth, unable to move straight, and started moving diagonally while tilting to one side.
“Stop! Stop!”
The researchers urgently stopped the experint and thoroughly examined the ship to find the cause, but they couldn’t easily find the reason.
“We have no choice. Let’s stop here for now and write up a report to submit.”
***
Hyang, upon receiving the report, had to forcibly restrain himself from speaking.
‘The ship’s crooked movent is clearly due to torque, and the vibration problem… Could it be because they made an even number of blades? As far as I know, ship propellers always had an odd number of blades, right?’
In fact, large ships use two screw propellers to prevent the ship from deviating from its straight course due to the reactive torque pair (Heeling torque/mont) acting between the ship and the propeller.
And ship propellers are made with an odd number of blades. The reason is that with an even number, resonance frequencies occur as many tis as the number of factors, but with an odd number, only one resonance frequency occurs, creating various problems including vibration.
‘That’s why even in the 21st century, all sorts of cutting-edge technology was mobilized just to make that seemingly simple screw propeller, and in extre cases, Japan even took a hit, right?’
During the Cold War era, Japan’s Toshiba violated COCOM (Coordinating Committee for Multilateral Export Controls) contracts and exported ultra-precision large machine tools to the Soviet Union. As a result, NATO countries, including the United States, began to have trouble tracking Soviet submarines.
Upon discovering that Toshiba was the culprit, the United States imdiately imposed an export ban on Toshiba, and from this point, the decline of Japanese electronic products and semiconductors began.
This happened long before Hyang was born, but Hyang rembered it because when Japan excluded Korea from its whitelist and imposed trade sanctions, the internet ntioned this incident, saying, “The Japanese make even bigger mistakes!”
Therefore, Hyang had to forcibly restrain his itching mouth.
‘This is sothing they need to ingrain in their bodies through trial and error! Only then will they be able to use it better in the future!’
Issues like torque and resonance were applicable in various fields, so it was essential to understand them before moving on.
***
While they were pondering over this intractable problem, another variable tripped up the developers.
This ti, it was the issue of the material used to make the screw propeller.
In the course of repeated experints, the wooden screw propellers broke faster than expected.
“Wood won’t do! Let’s try other materials!”
And so, screw propellers were made of iron and copper.
Due to manufacturing technology issues, the prototypes made were quite large, and as a result, Jang Yeong-sil had to make a model ship 1/10 the size of an actual ship.
The model ship, made larger and even equipped with a small steam engine, was experinted with on the Han River, and similarly failed.
In addition to the basic problems of vibration and deviation, this ti the blades of the screw propeller were also deford.
Finally, the researchers could understand why this task was graded “Moving Mountains – Upper”.
***
While the researchers were struggling like this, Jang Yeong-sil again sought out Hyang.
“Your Highness, I have an idea.”
“Let’s hear it right away!”
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