Galleys are large seagoing vessels propelled primarily by oars in battle and equipped with sails for cruising. The Egyptians, Cretans, Byzantines, Arabs, and other ancient peoples all used galleys for both war and trade. However, galleys came to be known as warships.
The galley was the standard European battle vessel until the late 16th century, when the sail-powered and more heavily armed galleon began to replace it. The earliest galleys about which much is known were Greek and Phoenician warships of early classical times. The largest of these were biremes, apparently first introduced by the Phoenicians around 700 BC, who improved upon an Egyptian design. The galley had a narrower underwater hull with a high length to beam ratio. This resulted in a slender and graceful vessel, increasing power and momentum, which was important for ramming.
The Greek galley possessed a single mast with a broad rectangular sail that could be furled. The mast was stowed or lowered when rowing into the wind or in battle. The Greek galley was a true seagoing warship. It typically possessed a bronze-shod ram.
The next evolution in ships was the uniremes. This ship had a single bank of oars, undecked or partially decked. It was fast and graceful, with a high curving stem and stern. In Homeric times the unireme sometimes carried an embolon (a beak or ram).
In the 8th century BC, the bireme was created. Descendant from the unireme, it was about 25m (80') long, with a maximum beam of about 10'. The bireme had 2 banks of oars - hence its name. The Phoenician bireme had a single pole mast with a square sail and steering oars to port and starboard, with two banks of oars staggered on either side of the vessel; the upper bank situated above the lower to permit the oars of the upper bank to clear the rowers below. The vessels used for trade in the Mediterranean were of approximately 30 m (100 ft) in length. Larger Tarshish ships with which they ventured out through the Straits of Gibraltar and up to the Scilly Isles were also built. Phoenician war galleys were low at the bow and high at the stern. They possessed a heavy pointed ram at or below the waterline.
By the 6th century BC trireme were in use. The trireme had 3 banks of oars, and a full spar deck instead of the centre-line gangway of the early bireme. In the 5th century the triremes measured a length of 40 m (125'), a beam of 6 m (20') and a draft of 1 m (3'). They were manned by 200 officers, seamen, and oarsmen (about 85 per side), with a small band of heavily armed marines. The trireme could reach 7 knots under oars.
The trireme had scant room for provisions. It was used for short trips from island to island. Even the largest triremes put into shore and beached, stern first for the night, resuming passage in the morning, weather permitting.
By the time of the Persian Wars (480 BC), the Greeks were also using triremes, galleys with three banks of oars permitted by the employment of an outrigger, the rowers of which sat above and outside of the other two. References to even more banks - ex. the quinquireme - are believed to indicate a ship of very large size but with no more than two or three banks of oars. Their exact design is unknown, but most likely, the oarsmen were arranged in staggered and overlapping layers, with each one pulling his own oar. This arrangement would have maximized the number of oarsmen while keeping the hull narrow for speed.
Quadriremes appeared in the middle of 4th century BC with 4 banks of oars. Quinqueremes appeared soon after. The naval arms race in the eastern Mediterranean resulted in even larger multi-banked ships. Macedonia created an 18-banked craft requiring crews of 1800 men. Ptolemaic Egypt had 20- and 30-banked craft. Ptolemy III had a 40 bank vessel created. It measured over 120 m (400') in length, with 4000 rowers. However, the vessel was never actually used.
Rowers, from the ancient world and into early Roman times, were free crewmen, not slaves. Banks of oars typically represents the number of rowers per oar (especially after triremes).