1. Marciano? 2. Louis? 3. Shavers? Fitz was considered the most ferocious puncher of his era, and he proved it against heavyweights rather than fighters of his own weightclass. Killed at least two men with punches, for that matter. But was his power in the same league as some of history's hardest heavyweight punchers? Would such a thing be possible? Discuss.
In terms of raw power, I would guess not. But of course, knockout power is a combination of raw PSI and accurate targeting of tender parts of the anatomy. Fitzsimmons had the benefit of learning the bareknuckler's emphasis on soft targets early in his development in the sport. With small gloves just barely padded enough to protect his fists and excellent raw power in his own right, an accurate blow from Fitz to your jaw, your neck, your ear, your short ribs, and of course your solar plexus would crumple a man just as fast as a Foreman overhand right to the top of the skull even if Fitzsimmons couldn't match a big heavyweight for raw power.
In terms of raw powere it is hard to judge. In terms of the efectivness and finishing capability of a single blow I think that he could be compared to those three. Nobody ever knew the soft bits of the body better than Ruby Rob.
My guess would be that it doesn't matter in practical terms. He's powerful enough that his most powerful shots would do for anyone, and in that respect he's probably in th class of the guys mentioned. If you want my best guess i'd say he's probably fourth if we made a list, but from what I understand pretty much anyone is going if he does his buisness.
Realistically in this day and age Bob Fitzsimmons is a super welterweight. So I think the better question is, did he hit as hard as Felix Trinidad or Thomas Hearns?
It's possible that Fitz hit as hard as Marciano, but in my opinion not Shavers or Louis (though some do allege otherwise). Fitz isn't that different in size from the Rock, fought similarly sized opponents, and left a comparable trail of battered heavyweight bodies in his wake. Allowances must be made for Fitz's smaller gloves, but unlike some modern fighters, Marciano's weren't much larger anyway.
The thing that particularly impreses Fitzsimmon's heavyweight power upon me is the way he efortlesly blasted guys who Jeffries et al couldn't knock a dent in. I beleive that he was an exceptional finisher even by heavyweight standards.