An investigation was made at subcritical speeds in Langley tank no.1 of the flow field behind a hydrofoil having an (3-inch chord and an aspect ratio of 10, and operating at a depth of 0.75 chord below the free-water surface. The downwash and water surface profiles were Measured behind the hydrofoil over a range of lateral and longitudinal positions of interest for tandem hydrofoil applications. The experimental data were compared with theoretical predictions based on two-dimensional flow.nAs predicted by theory, the displacement of the free-water surface and the angles of downwash varied directly with lift coefficient. The angles of downwash varied exponentially with depth below the water surface as would be expected for gravity waves. In the region investigated, the surface wave can be predicted by two-dimensional theory from the trailing edge to the point of maximum upwash, but only at low subcritical speeds and near the center line.
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