The ships were clinker-built, utilizing overlapping wooden strakes. The knaar, a relative of the longship, was a type of cargo vessel. It differed from the longship in that it was larger and relied solely on its square rigged sail for propulsion. The cog was a design which is believed to have evolved from (or at least been influenced by) the longship, and was in wide use by the 12th century. It too used the clinker method of construction. The caravel was a ship invented in Islamic Iberia and used in the Mediterranean from the 13th century. Unlike the longship and cog, it used a carvel method of construction. It could be either square rigged (Caravela Redonda) or lateen rigged (Caravela Latina). The carrack was another type of ship invented in the Mediterranean in the 15th century. It was a larger vessel than the caravel. Columbus's ship, the , was a famous example of a carrack. Arab age of discovery The Arab Empire maintained and expanded a wide trade network across parts of Asia, Africa and Europe. This helped establish the Arab Empire (including the Rashidun, Umayyad, Abbasid and Fatimid caliphates) as the world's leading extensive economic power throughout the 8th–13th centuries according to the political scientist John M. Hobson. The Belitung is the oldest discovered Arabic ship to reach the Asian sea, dating back over 1000 years. Apart from the Nile, Tigris and Euphrates, navigable rivers in the Islamic regions were uncommon, so transport by sea was very important. Islamic geography and navigational sciences were highly developed, making use of a magnetic compass and a rudimentary instrument known as a kamal, used for celestial navigation and for measuring the altitudes and latitudes of the stars. When combined with detailed maps of the period, sailors were able to sail across oceans rather than skirt along the coast. According to the political scientist John M. Hobson, the origins of the caravel ship, used for long-distance travel by the Spanish and Portuguese since the 15th century, date back to the qarib used by Andalusian explorers by the 13th century. Control of sea routes dictated the political and military power of the Islamic nation. The Islamic border spread from Spain to China. Maritime trade was used to link the vast territories that spanned the Mediterranean Sea to the Indian Ocean. The Arabs were among the first to sail the Indian Ocean. Long-distance trade allowed