Complex sentence: Ancient navies generally preferred to rely on free men to man their galleys and slaves were usually not put at the oars except in times of pressing manpower demands or extreme emergency, and in some of these cases they would earn their freedom by this.
Split into simpler sentences: Ancient navies generally preferred to rely on free men to man their galleys. Slaves were usually not put at the oars except in times of pressing manpower demands or extreme emergency, and in some of these cases they would earn their freedom by this.