The effect of the free surface on the pressure distribution on the upper side of a shallow-running hydrofoil is considered from a general point of view. Previous theoretical and experimental work is reviewed in order to compare the range of flow variables for which each treatment of the surface proximity problem is valid. A qualitative theoretical expression for the pressure is developed. This result shows the relative importance of the pertinent parameters and it is shown to agree qualitatively with previous experiments as well as with new pressure measurements made in the Free Surface Water Tunnel. The above considerations reinforce the view generally held in the past, that the methods of potential theory when properly applied to hydrofoils at shallow submergences may be expected to lead to valid and useful results
