In my first Raoul and Bess novel, set in a medieval-style culture, Raoul (the hero) whips Bess (the heroine) with his belt on two separate occasions. Each time he gives her a restricted number of strokes because he’s a gentleman and doesn't want to hurt her too much.
A belt is generally considered more personal than most other instruments (although it could never give as intimate a spanking as the bare palm), and as such it’s appropriate for when Raoul punishes Bess privately.
The second novel in the ‘medieval’ series has Raoul giving Bess a public punishment for which he uses a whip. Because he doesn't want to mark her, instead of using the harsh whip that is generally used on miscreants at the castle, he makes his own out of softer material.
Purely in the interests of research, my partner bought a 'beginner's' whip and has been trying it out on my bare buttocks. It has multiple suede fronds which strike a large area – the whole of my bottom, or my lower bottom plus top of my thighs – and yet force is transmitted only through a few of the strands to create a narrow band of sting.
Each individual stroke doesn’t hurt that much, and I don’t believe it’s because my partner’s being especially gentle. However the after-sting is noticeable and the cumulative effect could be impressive.
It creates a stripy pink colour as well, apparently. I wish I had a better mirror to view the effect!