The Umi-Bune (海船), literally &ldquo;sea boat,&rdquo; was a type of large Japanese seagoing vessel used during the Kamakura period (1185&ndash;1333). It represented an early stage in Japan&rsquo;s maritime engineering, bridging the gap between simple riverboats and more advanced coastal ships. The Umi-Bune was primarily designed for coastal trade and transport, moving goods and people between ports along the Japanese archipelago rather than for open-ocean voyages. Structurally, the Umi-Bune was what ship historians call a semi-structured vessel. Its hull began as a massive dugout log or several logs joined together, forming the boat&rsquo;s central body. This core was then built up with wooden planks and reinforcing beams to create a stronger, more seaworthy frame. The design offered greater cargo capacity while retaining the simplicity and strength of a dugout base. Large Umi-Bune are estimated to have carried up to 200 koku, or roughly 30 tons, of goods. The stern of the vessel featured a rudder operated by a rope-lifting system, not a hinged mechanism like in later European ships. This system allowed the crew to raise or lower the rudder in shallow waters. The ships were likely propelled by oars or sculling poles, though some reconstructions suggest a single straw-mat sail may have been used for wind assistance on longer routes. Because of the vessel&rsquo;s shape and weight distribution, it was best suited to protected coastal waters rather than the open sea. Historical evidence for the Umi-Bune comes mainly from artistic depictions, scroll paintings, and written descriptions, since no complete remains have ever been found. Modern ship modelers and maritime historians have reconstructed its form based on these visual sources, resulting in varying interpretations of details such as hull curvature, plank arrangement, and sail rigging. The most consistent conclusion is that the Umi-Bune was a robust, practical workhorse of early Japanese maritime trade, representing a key step toward the more advanced coastal ships of later periods.