Were slaves whipped? Sometimes. So were sailors, students, and children.
During the days of the British colonies in America, whipping was not considered a “cruel and unusual punishment.” Indeed, that phrase did not exist.
Whipping was a moderate measure compared to the medieval penalties dealt out for capital offenses, such as being drawn and quartered or having limbs pulled apart by four horses.
The British Whipping Act of 1530 https://www.britannica.com/topic/flogging was still in force during colonial times. It authorized whipping for theft, blaspheming, poaching, and other minor offenses. Both men and women were whipped, including the insane.
So, you see, during the Antebellum and Civil War Periods, most people upheld the practice of whipping if used judiciously. Judicious use included being limited to a minor offense, by using an appropriate instrument (not the cat-o’-nine-tails), and by limiting number of lashes.
However, abolitionists made spectacular displays of escaped slaves with whipping scars as a way of convincing northerners that slave owners in the south whipped all slaves mercilessly. They omitted the fact that runaway slaves were overwhelmingly slaves of cruel masters.
In fact, many plantation owners did not whip slaves—or anyone else—at all.
As to merciless whipping, the DVD series North and South did a wonderful job of pointing out that men who were cruel to slaves were just bad men. They were cruel to everyone in their power.