As the Western Roman Empire fell to Germanic tribes and its own internal problems, the Roman Empire in the east, called the Byzantine Empire, flourished.
The trireme derives its name from its three rows of oars on each side, manned with one man per oar. These war-ships (naves longae) rarely had more than a single sail and were propelled mainly by means of oars. The speed of the warship was dependent on how fast the rowers could work and they kept time according to the speed of the beat of a drum.
The sail and mast had to be lowered during combat. The triremes, used by the early Roman navy, were made of softwoods such as pine and fir. Larch and plane were used for the ship's interior parts. The light woods allowed emphasis on maneuverability and speed. The ship's primary propulsion came from the 170 oars arranged in three rows, with one man per oar. The triremes required 170 rowers and 10-15 sailors and 10-20 marines who fought during boarding actions.
The Byzantine navy transported soldiers and supplies to help recover the Western Empire. The navy relied on fast galleys called dromons, or racers, to accompany and protect the supply ships.
Early dromons had a single bank of oarsmen, but Byzantine shipbuilders later incorporated a second level for oarsmen.
It was only during the early Byzantine period in the eastern Mediterranean that any evidence emerges that triangular sails began to appear on the Mediterranean Sea. The square sail, though stable on heavy seas, is not very versatile to make much use of any headwinds. Square sails were still used until very recently on the sewn sambugs of Aden as well as lateen sails.
There certainly were large ships, such as the fine one sailing up the Bosporus that reportedly incensed the Byzantine emperor Theophilos (d. 842) when he learned that its owner was his wife Theodora, or the three-masted “pirate” naus reported by Anna Komnene during the First Crusade. Three-masted sailing ships certainly existed in the eleventh century, as is proved by a depiction on glazed Pisan bacini, which were glazed pottery bowls from the Muslim world placed on the facades of churches at Pisa and elsewhere to reflect sunlight and give the churches a glittering aspect. However, such large ships have left no trace in the documentary record before the thirteenth century.
Also the earliest evidence of the existence of lateens on the Mediterranean is in Greek Byzantine manuscripts of the late ninth century which show drawings of lateens. Before this, in antiquity, only the square sail was found in this sea. This would lead us to suspect that the lateen came to the Mediterranean in the wake of the Arab expansion.
It seems that during the Byzantine era, the forepart of the lateen sail was changed to a point, making it a complete triangle. This occurred first in the Mediterranean, but the Arabs of the Red Sea and Indian Ocean kept their old form. The lateen eventually reached North European waters at the end of the Middle Ages, and there developed into every sort of fore-and aft rig.