I mentioned this subject in 2010, when I wrote Galley Slaves of the 17th Century, referring to L. Sprague deCamp’s The Ancient Engineers: An Astonishing Look Back at the Ancient Wonders of the World and Their Creators. From p. 81.

Most ancient rowers, Ben-Hur to the contrary notwithstanding, were free workers, and fairly well-paid ones at that. The use of slaves and prisoners as rowers did not become common until the Renaissance

From p. 352:

[in the late middle ages] was that rowers, instead of being free workers as in ancient times, became captives chained to their benches. Use of slaves and prisoners first became common in +X V . No doubt this change was fostered by the fact that multitudes of people were captured in the constant wars and piratical raids of the Christian and Muslim powers of the Mediterranean against each other. The Turks, like the Romans of the late Republic, were given to slave-raiding and slave-owning on a huge scale; while, during the Reformation, the French and Spanish kings found the galleys a useful way to dispose of Protestants.

The Roman galley slave: Ben-Hur and the birth of a factoid Abstract:

Of the repertoire of stereotypical images representing the Roman world in Western popular historical discourse, that of the chained galley slave stands out, since it is demonstrably false: the Roman empire knew no such practice. Since the notion must therefore have a post-classical origin, it may be possible to trace its roots and the means of its establishment in contemporary culture. It is argued that the motif was constructed by projecting onto classical times knowledge of the historical galley slavery of the early modern Mediterranean. Further, both the principal point of origin for the idea, and the means of its dissemination and implantation in popular historical discourse, lay not in academic scholarship but in popular fiction: they may be traced primarily to the tale of Judah Ben-Hur. A spectacularly successful Victorian novel and an equally popular stage show before the famous film versions, Ben-Hur had an unequalled impact on the entertainment, religious instruction and even formal education of several generations across the United States, the Anglophone world and beyond. Through this enthusiastic public reception, it is suggested, motifs such as galley slavery and chariot racing passed permanently into popular understandings of Rome. It is a vivid illustration of the role of fiction in the creation of popular views of history.

Mare Nostrum – Rowing Under the Romans:

Crews were sometimes conscripted or borrowed from allies, but they were not slaves chained to their oars. (A misconception arising from a mis-translation of the word for sailors who received Roman citizenship upon completion of their service). The pay and opportunity for citizenship made naval careers attractive, and volunteers were usually plentiful.

From Ben-Hur: playing to the galley:

When Ben-Hur is falsely accused of throwing a roof tile at Valerius Gratus, he is sentenced to slavery in the galleys. This is one of the film’s (and the book’s) biggest blunders. Galley slavery was hardly known in the Roman empire, and there are no records of it being used as a punishment. Not least, this was because most Roman galleys – including the triremes shown in the film – required skilled rowers. The only substantial Roman use of galley slaves was recorded in the second Punic war, more than 200 years before the period depicted in the movie. During that conflict, the Romans deployed the quinquereme, which had five men to an oar. Only one had to be a professional; the rest merely added muscle. Roman general Sextus Pompey and Emperor Augustus recruited slaves for their ships in the first century BC, but these men were freed first, then took the job by choice. Chained convicts enslaved as rowers are a modern phenomenon, first reliably recorded in 1443, when King Charles VII of France licensed a shipping magnate to pressgang vagabonds into his private fleet.

Also see Galley Slaves in the Second Punic War.

Were slave rowers ever chained to the ship?

In Classical, Hellenistic, and Imperial navies, rowers were free men. They were not chained, and fought against boarding actions when necessary. The exception was in Greece, when slave-owners might send some of their slaves to the navy in time of war, but they were treated same as the free men, including being paid by the day. One way to earn money to buy your freedom!

TEN THINGS THE ROMANS DIDN’T HAVE

Despite the famous scenes in old films like Ben Hur and Cleopatra, Roman galleys were not rowed by chained slaves. Roman oarsmen were paid professionals, and those in the navy were enlisted in the armed forces. Galley slaves did exist, but not until centuries later; medieval ships of both the Christian and Islamic Mediterranean powers used them extensively. In fact, the French were still using chained prisoners to row their galleys until the eighteenth century.

Going back before the Romans, rowers in ancient Greece were also generally free men and well paid. In the final phase of the Peloponesian war, the Spartan navy finally got the better of its Athenian counterpart because, thanks to Persian gold, it could offer higher wages to its rowers. See Myke Cole, The Bronze Lie: Shattering the Myth of Spartan Warrior Supremacy, p. 312.

Note that the absence of galley slaves in ancient Greece or Rome was not due to any particular kindness in those civilizations. For civilian shipping, the sailing season was only half the year. Slaves would have to be housed and fed during the off season. Sometimes it is cheaper to rent than to buy. For naval warfare, free rowers were a body of muscular men who could help in a fight if needed, and at the end of fighting be paid and sent on their way.