The concept behind fans is to lower pressure on one side of the radiator so air will flow from high pressure (front of radiator) to low pressure (engine side). The shrouding makes this process much more efficient. If it is not shrouded, the air can just blow in circles in the engine compartment.
I like the Lincoln fan, although I doubt it moves as much air as claimed. Moving 4,500-5,000 CFM would require probably around 40-45 Amps
. The Spal 17-inch is enough, which pulls around 22 amps.
Brian