Building a Surfboard Frame from Cardboard

When I decided to build the internal frame of a surfboard out of cardboard, I chose to incorporate a traditional Japanese bamboo-weaving technique known as Yotsume-ami (four-eye weaving).

The first thing I needed was a large amount of cardboard, so I visited several supermarkets and collected boxes that were no longer needed. As I cut them apart with a utility knife and scissors, I began to notice that not all cardboard was the same. Some sheets were thicker, others softer, and each had different characteristics.

Looking at the product labels on the boxes, I realized why. Boxes designed for heavy products were much stronger, while those used for fragile goods had different levels of flexibility and rigidity. I also discovered that the corrugated layer between the two outer sheets—the fluted core that gives cardboard its cushioning effect—came in several different patterns.

After comparing many types, I selected cardboard that was about 5 mm thick: neither too soft nor too stiff. My goal was to find the best balance between strength and light weight.

I then built a small prototype measuring approximately 20 cm × 30 cm × 3 cm. Holding it in my hands, I pressed down on it and twisted it repeatedly to test its strength.

My honest impression was that, if properly assembled, the structure could withstand a considerable amount of force. However, I also discovered that its strength depended greatly on the size of the square openings created by the woven framework.

To find the ideal size, I built several test pieces with different dimensions and even stood on them with my full body weight. After many experiments, I concluded that squares measuring about 3 cm on each side provided the best balance of strength and weight.

Through this process, I completed a rectangular cardboard core measuring 20 × 30 × 3 cm.

Now came the next challenge.

How could I transform this rectangular block into the curved shape of a surfboard?

My first attempt was to cut it with a utility knife, but it simply didn't work. Next, I tried several different types of saws. They performed better than the knife, but none of them could produce the smooth curves I needed.

A feeling close to panic slowly began to grow inside me.

"How can I carve the graceful curves of a surfboard out of this solid rectangular core?"

As sweat poured down my face, my thoughts drifted toward despair.

For weeks, I had worked on this project with the hope of opening a completely new door in surfboard construction. If I gave up here, everything I had accomplished would be wasted. More than that, I felt as though it might even change the course of my own life.

"There has to be another way..."

Then, suddenly, an idea appeared.

"Wait a minute... It's difficult to cut vegetables that are hollow inside, but it's easy to cut those that are solid. That's it! If I fill these 3-centimeter square spaces with something, cutting the board should become much easier."

That desperate idea arrived just in time.

But it immediately raised another question.

What should I fill those 3-centimeter spaces with?

A new challenge had just begun.