The trading dynasty of the Song developed the first junks based on Southeast Asian ships. By this era they also have adopted the Malay junk sail. The ships of the Song, both mercantile and military, became the backbone of the navy of the following Yuan dynasty. In particular the Mongol invasions of Japan (1274–84), as well as the Mongol invasion of Java (both failed), essentially relied on recently acquired Song naval capabilities. Worcester estimates that Yuan junks were 11 m (36 ft) in beam and over 30 m (100 ft) long. In general they had no keel, stempost, or sternpost. They did have centreboards, and watertight bulkhead to strengthen the hull, which added great weight. Further excavations showed that this type of vessel was common in the 13th century. By using the ratio between number of soldiers and ships in both invasions, it can be concluded that each ship may carry 20-70 men.