This Report Has Be£n Delimited and Cleared for Public Relea3e under Dod Directive 5200,2ü and No Restrictions Are Imposed upon Its Use and Disclosure,

Previous theories and empirical formulas for the lift of flat planing surfaces are reviewed, and the resemblance of the planing surface to the airfoil noted. A simple expression, which converges to the correct limits for exactly known cases, is assumed for the ratio of planing to airfoil lift, and the planing lift is then estimated by using airfoil experimental data. The resulting calculated values are in satisfactory agreement with planing experiments.