For the next four hundred years, the peace of mare nostrum was never seriously challenged. The activites of the Roman navy were confined to protecting the extensive overseas trade of the empire and the lines of communication across the Mediterranean, the English Channel and the Black Sea. We know little about such trading ships, though modern underwater archaeology has shown us much more than the scant evidence engraved on monuments which have survived. What size, for instance, were the freighters on the grain run to Africa? Wrecks which have been excavated suggest something between 100 and 300 tons. Probably the ships averaged 80 feet in length and were sailing ships rather than long, narrow war galleys. The reason there were so many wrecks was largely because these ships sailed with a single square sail on the mainmast, with a small foresail called an artemon on a spar projecting over the bows for steering purposes. The Romans never learned either the fore-and-aft or the lateen rig which would have made their vessels more manageable; nor did they contribute anything to the art of navigation.
After the barbarian invasions split the Roman empire into two, the western Mediterranean became the haunt of the barbarians, while Byzantium dominated the eastern half. One reason for the long survival of the eastern empire -- it lasted until 1453 -- was the efficiency of the Byzantine navy, with its fleets of two masted dromons with rams armed with Greek fire, a mixture of naptha oil and saltpetre fired through tubes in the bows. But the barrier of Islam prevented sea-borne trade with the east. It was, however, from the Arabs that a new feature appeared in Mediterranean shipping -- the adoption of the triangular lateen sail.