This is an excerpt from J. Belk, J. Caillavet, F. Hodges, H. Razzaghi, " Water Detector," ECE 4522: Design II, Department of Electrical and Computer Engineering, Mississippi State University, Spring 2003:

The original design of the outboard motor dates back to 1881, when Gustave Trouve introduced his .Motor and Screw. at the Paris Exposition. Trouve.s design consisted of an electric motor positioned directly over a rudder. Within the rudder.s frame was a three bladed screw, or propeller, that was driven by sprocket, cogwheel, and chain [1]. In the original design, the gears that drove the .screw. were exposed to the elements. However, in 1885, Samuel H. Jones, revised Trouve.s design by creating an underwater casing that projected the propeller and housed the gears of the unit.

Today in outboard motors a portion of the engine.s housing still extends below the water line so that the propeller can be located below the surface of the water. The power head of the engine delivers power to a drive shaft that is geared to turn the propeller shaft, thus delivering thrust to propel the boat. Additionally, the gears, shafts, and supporting bearings are still contained in a lightweight housing or gear-case to protect them from the surrounding elements of the engine [2]. This casing, often referred to as the .lower unit., is sealed and filled with oil that provides lubrication to its gears.