Hi McHunt
The guys at SABA (South African Bowhunters Association
www.sabowhunters.co.za) will be able to give you a good indication when it comes to African game. I recently did a Bowhunters Proficiency Certificate (soon to be law before hunting in SA) and we were given quite an intricate formula to determine our limitations with our particular setup. This also takes into account the reaction time - smaller game is quicker and vice versa, as well as your distance from the animal, broadhead cutting diameter, speed etc etc. The general setup guideline to ensure an ethical hunt on a Cape Buffalo is around 900gr @ 218fps or similar to ensure 95 ft.lbs or 0.87 slug feet per second. I know it all sounds very detailed - especially when it's usually the compound guys who are caught up in the speed trap - but attempting a hunt with the incorrect setup is not only unethical, but will, sooner or later, add fuel to the anti-hunting fire. Best bet is to contact Harry Marx (he compiled the Certificate manual) who is the SABA chairman, for clearer guidelines.
I am also limited to a 45# bow because of shoulder injury, but finances appear to be the biggest problem concerning the size game I can hunt.
I hope you get some help from Harry.