Experimental and Analytical Studies of Dihedral Hydrofoils

Experimental and analytical studies of dihedral hydrofoils were conducted to determine the principal effects of dihedral on the performance of hydrofoils. An approximate theory is given whereby the lift of a surface-piercing foil at high speed can be determined as a function of the angle of attack solely from two-dimensional wind tunnel data. Tests on the drag of dihedral hydrofoils indicate that the profile drag varies directly as the true submerged length of the foil. A theory for the trailing vortex drag is developed for both deeply submerged and surface-piercing dihedral hydrofoils. Measured and computed values of total drag using this theory compare favorably. An analysis of the wave drag of surface-piercing hydrofoils is presented with only qualitative agreement with test results. Tests on the effect of dihedral in suppressing flow separation together with an analysis based on momentum principles indicate the advantages to be obtained by large dihedral angles. A study of the effect of dihedral on the performance ratio, L/D, indicates that the optimum dihedral angle is a function of the ratio of the profile drag to the trailing vortex drag. Finally, brief exploratory tests on flat hydrofoils in surface waves (simulating rough water) indicate that for waves of low steepness, the instantaneous angle of attack, including the angle induced by the orbital motion, can be used together with steady lift data to predict the lift variation in surface waves.