Another winter is upon the family. The wind and rain pound against the rock-walled cave. Although they are safe and dry inside with plenty of provisions, they grow depressed. They are used to being outside, and the long hours in the cave make them feel very dreary.

To occupy their minds, the father suggests they make another boat, tightly constructed but very light and modeled after a Greenlander's kayak. Everyone but Elizabeth is excited and quickly jumps to the task. Elizabeth constantly fears they will drown; the lighter the boat, she imagines, the more easily it will tip.

Attempting to allay her fears, the father promises this boat will be much sturdier than any of the others. He uses whalebones for the ribs of the kayak and splits bamboo for the sides and  deck. The entire boat will be covered except for a square hole where the sole rower will sit.

It takes the entire rainy season to finish major construction of the kayak. When it is done, the clouds have broken and the weather is good enough to venture out. Outside, they encase the kayak in seal skin and make the entire vessel watertight with a coating of sealing mixture. Although curious in appearance, the kayak is light enough to be lifted with one hand. When they place the boat in the water, it floats easily. Since it was Fritz's idea to make the kayak, his father gives his eldest son the right to ownership. However, before Elizabeth allows Fritz to make his first journey, she insists on him waiting until she has constructed a "swimming dress" to make him buoyant should his kayak capsize.

Although the father assists his wife in making the outfit, he admits he has no proof that it will do what she has intended. It looks like a "double waistcoat" to which they have applied a solution of India rubber, thus making the outfit completely airtight. They configure it with a double lining that is then inflated. Of course, when Fritz puts the outfit on, his brothers cannot stop laughing.

Fritz paddles the kayak to Shark Island with his family following in the larger boat. The kayak proves to be sleek and fast. Once on the island, despite their earlier humor, the younger boys all ask their mother to make similar "swimming dresses" for them.