Ancient watertight bulkheads power modern icebreaker
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China’s first domestically built polar icebreaker, Xuelong 2, appears to be a triumph of modern engineering as it slices through Antarctic ice fields.
Yet hidden within its steel hull is a technology far older than radar systems or ice-strengthened plating: the watertight-bulkhead system developed for Chinese wooden junks more than a millennium ago.
When Xuelong 2 was delivered in 2019, public attention focused on its advanced scientific laboratories, icebreaking capability and role in China’s polar research program. Few noticed the ancient principle embedded deep within its structure.
From bow to stern, the vessel is divided by nine watertight bulkheads into 10 independent compartments. If one cabin is damaged or flooded, the others remain sealed, allowing the ship to maintain stability even under extreme polar conditions.
This same concept once kept Chinese wooden ships afloat on stormy seas. During the Song (960-1279) and Yuan (1271-1368) dynasties, ocean-going vessels sailing the Maritime Silk Road relied on the watertight-bulkhead technology to survive long-distance voyages.
The massive treasure fleets led by Zheng He during the Ming Dynasty (1368-1644) — dispatched by Emperor Yongle on seven expeditions across the Indian Ocean — also used the same structural principle. Even Ming-era coastal defense ships built to counter piracy depended on this technology for safety and endurance.
The watertight-bulkhead system is widely regarded as one of ancient China’s most significant contributions to global shipbuilding. Its origins can be traced back to the Eastern Jin Dynasty (AD 317-420). By the Song Dynasty, it had reached a high level of maturity.
Quanzhou, southeast China’s Fujian Province, became a major international maritime hub in the Song Dynasty. In 1974, the remains of a Song Dynasty cargo ship were unearthed in Quanzhou Bay, revealing fully developed watertight bulkheads.
In the late 14th century, the craft spread further north to Zhangwan Town in Fujian’s Ningde, where it has been practiced continuously for more than 650 years. There, generations of shipwrights refined and preserved the technique, passing it down through families and apprenticeships.
Materials play a crucial role in the construction of watertight-bulkhead ships. Shipbuilders traditionally select wood with great care, balancing strength, flexibility and resistance to water. Red pine from northeast China is commonly used for keels, as it resists rot even after prolonged immersion. Camphor wood and Chinese fir are favored for bulkheads, while hardwoods such as ironwood are used for rudders and other high-stress components.
Building a ship using this technology involves a long sequence of precise steps. Construction begins with shaping the keel — a ship’s spine — which determines its width, depth and overall proportions. Carpenters then install ribs and hull planks, using mortise-and-tenon joints and rabbeted seams to ensure tight connections. Bulkheads are fitted carefully to form sealed compartments within the hull.
The most critical step is caulking. Instead of modern sealants or metal fasteners, traditional shipwrights use a natural mixture of tung oil, lime powder and oakum. Workers first apply lime as a base, then pack oakum into seams, and finally seal everything with layers of oil paste.
The entire construction process is directed by a master craftsman, who oversees a team of carpenters working in close coordination. For centuries, shipyards in coastal Fujian — particularly in Quanzhou, Jinjiang and Ningde — thrived on this craft. Knowledge was transmitted orally from master to apprentice, often within the same family.
With the rise of steel-hulled vessels in the 20th century, however, demand for traditional wooden junks declined sharply. Rising labor costs and the scarcity of suitable timber further increased construction expenses. Today, only three master shipwrights are believed to have full command of the watertight-bulkhead technology.
Recognizing its cultural and historical value, China listed the technology as a national intangible cultural heritage in 2008. In 2010, it was added by the United Nations Educational, Scientific and Cultural Organization to the List of Intangible Cultural Heritage in Need of Urgent Safeguarding.
In Zhangwan Town, one of the craft’s last strongholds, master shipwright Liu Xixiu continues to keep the tradition alive. Born in 1957, Liu comes from a family with a shipbuilding lineage spanning 20 generations and more than six centuries. He began learning the craft at 19, following his father into a local shipyard. Over more than four decades, Liu has participated in the construction of over 300 vessels and drafted more than 20 ship designs.
In 2024, one of his designs — the Funing ship — was completed. Built using traditional watertight-bulkhead technology, it is considered the world’s largest antique-style sailing vessel of its kind.
From ancient wooden junks to modern polar icebreakers, the enduring logic of watertight compartments continues to bridge past and present.
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