Analytical studies and harbor tests were conducted to determine the feasibility of harvesting oil beneath the surface of the water with the use of inclined planes. The analytical and laboratory investigations provided basic information to design and build units and showed that this kind of device could harvest both light and heavy oils between 3/4 knot and 2 knots. Information was obtained regarding the geometry of the device. Tests showed that oil could be collected in waves without seriously affecting efficiency. A 22-foot-long unit was designed, built, and domonstrated in Boston Harbor. The results showed that the fixed-plane concept is highly effective in areas where the vessel can travel through the slick. Recovered oil is virtually water free and the unit recovered between 70% and 85% of the oil presented to it. The fixed inclined plane Submerged Hydrodynamic Oil Concentrators demonstrator unit works between 3/4 knot and 2 knots. (Modified author abstract)