fleets as well as the size of individual vessels increased in size, which required more rowers. The number of benches could not be increased without lengthening hulls beyond their structural limits, and more than three oars per bench was not practicable. The demand for more rowers also meant that the relatively limited number of skilled oarsmen could not keep up with the demand of large galley fleets. It became increasingly common to man galleys with convicts or slaves, which required a simpler method of rowing. The older method of employing professional rowers using the alla sensile method (one oar per man, with two to three sharing the same bench) was gradually phased out in favor of rowing a scaloccio, which required less skill. A single large oar was used for each bench, with several rowers working it together and the number of oarsmen per oar rose from three up to five. In some very large command galleys, there could be as many as seven to an oar. All major Mediterranean powers sentenced criminals to galley service, but initially only in time of war. Christian naval powers such as Spain frequently employed Muslim captives and prisoners of war. The Ottoman navy and its North African corsair allies often put Christian prisoners to the oars, but also mixed volunteers. Spain relied on mostly servile rowers, in great part because its organizational structure was geared toward employing slaves and convicts. Venice was one of few major naval powers that used almost only free rowers, a result of their reliance on alla sensile rowing which required skilled professional rowers. The Knights of Saint John used slaves extensively, as did the Papal States, Florence, and Genoa. North African ghazi corsairs relied almost entirely on Christian slaves for rowers. Sails In ancient galleys under sail, most of the moving power came from a single square sail. It was rigged on a mast somewhat forward of the center of the ship with a smaller mast carrying a head sail in the bow. Triangular lateen sails are attested as early as the 2nd century AD, and gradually became the sail of choice for galleys. By the 9th century, lateens were firmly established as part of the standard galley rig. The lateen rig was more complicated and required a larger crew to handle than a square sail rig, but this was not a problem in the heavily manned galleys.