You need a fan shroud. Running a fan without a shroud is just wasting energy.
For a mechanical fan, the shroud should only cover up about a 1/2 inch of the fan blades. Fans fling a lot of air off the end of the blades, shrouding that area decreases the effectiveness of the fan. The shroud design also needs to allow for engine movement so the fan blades do not hit the shroud.
Electric fans are a bit different as they usually have a housing already surounding the fan blades. This does not mean you can mount the fan directly to the radiator.
A proper shroud seals against the outter edges of the radiator so that the fan draws air from the entire radiator. Some designs include spring loaded flaps on the shroud that open when air volume at highway speeds surpasses the draw of the fan. One design that looks good, but doesn't work very well, is to slap a piece of sheet metal against the radiator, cut a hole the diameter of the fan, then weld a ring around the hole to enclose the fan. Even thought the fan is sealed against the radiator, the fan ends up only drawing air from directly infront of it, the corners of the radiator see very little air flow as they are effectively blocked. You want a large plenum that the fan can draw air from. With air coming from all parts of the radiator, the fan sees less static load and pulls more air.
