Why did ancient galley fleets require so many rowers?

Ancient galley fleets required large numbers of rowers primarily because naval warfare demanded high speed, agility, and the ability to maneuver in any direction at any time. Unlike merchant vessels, which prioritized cargo capacity and relied on sails, warships were designed as long, narrow vessels to maximize their effectiveness in combat.1

To achieve the necessary speed and maneuverability, these ships utilized human muscle power as their primary propulsion. By arranging rowers in multiple, vertically superimposed banks—such as in the bireme (two banks) and the trireme (three banks)—shipbuilders could increase the number of oarsmen without making the hull excessively long, which would have made the vessel difficult to handle in open water.2314

This design allowed for a concentrated application of power, enabling ships to reach speeds of up to 9 knots. Because these light, oar-powered warships had limited space for provisions and lacked the endurance of sailing ships, they were restricted to short voyages and required frequent stops at coastal bases.23