My 2 cents:
1.Google: count rumford
2.It's my opinion that the smoke shelf is needed to prevent downdrafts. If the chimney is heating up some and it isn't getting a downdraft from above, you don't need one.
Most guys that I know don't use one, although some do have them in side draft hoods that they have made.
The most common successful solution that I know of is the have a straight, vertical chimney on 10 inches MINIMUM ID. The top of the chimney needed to be higher than 2 feet above the peak of the roof that it emerges from. A simple horizontal flat cap on top, about 4 or so inches above the opening keeps the rain out, for the most part. The hood over the tuyere needs to be far enough above the forge table that it does not obstruct the blacksmith frome seeing or manipulating the fire and the work. I'd think that 18 inches or so would be a minimum. My own forge's hood only has the volume about 3 5 gallon pails. It works fine to contain even the start-up smoke if I start the fire correctly. But I have seen hoods that have 5 or 10 times that volume that can't contain the start-up smoke if the draft is poor (due to bends in the chimney, not long enough chimney, etc.. Once the fire is going and the chimney has heated up by 50 °F or so above ambient, the chimney will draw pretty well, if designed as above, but the first few moments of the fire may not create a good draft. Some people stuff a lit sheet of newspaper up the flue as they light forge up to start the draft. Knowing HOW to start the fire can also limit the amount of smoke that is released in starting the fire. There shouldn't be any smoke in the shop if its all done properly. For instance, carefully sorting all the green coal out of yesterday's coke will give you a much cleaner start.
I know several people who were sold on side draft forges that had smoke shelves for a while, but have since switched to conventional overhead hoods, which don't have soke shelves and are happier campers.
Powered hoods: If you are considering a powered hood, make sure that the fan motor is outside of the pipe, otherwise, it will probably overheat and fail.
Pete Stanaitis