A bireme is an ancient Hellenistic-era warship

Hellenistic-era warships

From the 4th century BC on, new types of oared warships appeared in the Mediterranean Sea, superseding the trireme and transforming naval warfare. Ships became increasingly bigger and heavier, including some of the largest wooden ships ever constructed...

 with two decks of oar

Oar

An oar is an implement used for water-borne propulsion. Oars have a flat blade at one end. Oarsmen grasp the oar at the other end. The difference between oars and paddles are that paddles are held by the paddler, and are not connected with the vessel. Oars generally are connected to the vessel by...

s, probably invented by the Phoenicia

Phoenicia

Phoenicia , was an ancient civilization in Canaan which covered most of the western, coastal part of the Fertile Crescent. Several major Phoenician cities were built on the coastline of the Mediterranean. It was an enterprising maritime trading culture that spread across the Mediterranean from 1550...

ns. It typically was about 80 feet (24.4 m) long with a maximum beam width of around 10 feet (3 m). It was modified from the penteconter

Penteconter (ship)

The penteconter, alt. spelling pentekonter, also transliterated as pentecontor or pentekontor was an ancient Greek galley in use since the archaic period....

, a ship that had only one set of oars on each side, the bireme having two sets of oars on each side. The bireme was twice the triaconter's length and height, and thus employed 120 rowers. The trireme

Trireme

A trireme was a type of galley, a Hellenistic-era warship that was used by the ancient maritime civilizations of the Mediterranean, especially the Phoenicians, ancient Greeks and Romans.The trireme derives its name from its three rows of oars on each side, manned with one man per oar...

, keeping the length of the bireme, added a tier to the height, the rowers being thus increased to 180. It also had a large square sail.

These ships were frequently used by the Romans, as during the second of Caesar's invasions of Britain

Caesar's invasions of Britain

In his Gallic Wars, Julius Caesar invaded Britain twice, in 55 and 54 BC. The first invasion, made late in summer, was either intended as a full invasion or a reconnaissance-in-force expedition...

. The bireme eventually evolved into the trireme

Trireme

A trireme was a type of galley, a Hellenistic-era warship that was used by the ancient maritime civilizations of the Mediterranean, especially the Phoenicians, ancient Greeks and Romans.The trireme derives its name from its three rows of oars on each side, manned with one man per oar...

. A unit commandant (who was given a tent on the open deck) directed a group of marines. The bireme was recorded in ancient history on 8th-century Assyrian reliefs. The name bireme comes from "bi-" meaning two and "-reme" meaning oar.

Medieval galleys were described as "bireme" or "trireme". The terminology can lead to confusion, since the terms are also used for rowed warships of the Greco-Roman period built on entirely different design principles.

In 1275 Charles of Anjou king of Sicily, issued an order for the construction of several galleys that provide the earliest evidence for the dimensions of the bireme galleys. Because of increased weight and breadth, which brought increased friction through the water, a trireme galley was not dramatically faster than a bireme. But the change to trireme produced more significant developments than a gain in tactical speed over short distances. Early bireme galleys escorted merchant ships but were rarely used to carry goods. A few Genoese freight contracts of the mid-thirteenth century record charters for bireme galleys.