Set back plates are popular here, most commonly used on "bass boats" used in shallow water fishing. Moving the motor 12 inches behind the transom also compensates for a shallower shaft to be used because it give the water a chance to "recover" from being displaced by the transom and coming into contact with the prop.
Taken from http://continuouswave.com/whaler/cet...ceaPage41.html
It is often desirable to move the mounting point of outboard engines aft from the hull's original transom by as much as 36 inches. The two principal benefits of this are:
The propeller runs in cleaner (i.e., less-turbulent or less aerated) water that exists behind the boat.
The engine can be raised (sometimes several inches) which reduces the hydrodynamic drag by having less of the engine lower unit submerged in the water.
The net effect of these changes is to increase the propeller efficiency (from operating in cleaner water) and to decrease drag (from reduction of submerged lower unit). This increases the potential top speed of the boat, and improves the overall fuel economy.
In addition, there are some more subtle performance benefits to this mounting technique:
Bow lift (in static trim) may be gained from the changed weight distribution aft. When underway this in turn may allow the engine trim to become more vertical, resulting in the propeller thrust becoming more aligned with the forward motion of the boat.
The engine thrust will be applied with a longer moment arm to the boat's center of lateral resistance and the boat's center of gravity, producing greater leverage of the engine thrust on both the boat's course and the boat's trim.
Operation in reverse will be improved because of the increased distance between the propeller and the hull.
Radically reduced immersion of the propeller may permit use of more efficient propeller designs, further increasing efficiency.
Underwater drag of the lower unit may be reduced when its trim becomes more nearly vertical because of improved flow of water around the "bullet" of the gearcase.
Some of these performance gains are very slight and are really of concern only to the competitive racer, but taken en toto there is a definite gain to be had from mounting the engine behind the boat. Side by side comparison of boats with and without bracket mounted outboard engines often shows marked improvement in performance and fuel economy for the bracket model.
An additional benefit of bracketed engine mounting is it leaves the boat transom intact. There is no cutdown of the transom for engine mounting, nor are the gunwales or aft deck cut away to form a well. The result is the strength of the hull is increased. The full transom will also keep following seas out of the boat and it will prevent shipping water aboard when rapidly reversing, as when backing down on a fish.
While it might at first seem strange to mount the engine outside of the boat, the performance reasons detailed above make a case for doing so. When the engine is moved behind the boat, it releases valuable space inside the boat to other uses. The boat's cockpit becomes bigger and more useable.
On top of all these sound reasons, there is the "cool" factor. Boats with engines mounted on setback brackets just look cool. This is probably because many full-transom, engine bracketed boats are premium designs, and the appearance of a bracket mounted engine is thus associated with high-quality, high-performance boats.