French courts and Spanish wartime practice supplied many galleys with convicts or prisoners of war. A large body of coerced rowers could turn against the crew during boarding, especially if an attacker offered them freedom. Starving or mistreating them reduced the ship's motive power, but feeding them well made a potentially hostile group harder to control. Roman rowers were paid, while enslaved men used by Rome, Carthage, or the ancient Greeks could be freed in return for service. Those arrangements gave rowers a reason to remain effective and loyal.
Notable Point · 2:54-9:14
Why coerced rowers made unreliable propulsion
Galley-slave systems reduced cash costs but created security, nutrition, endurance, and loyalty problems aboard fighting ships.