Abstract: Efficiency of a boat propeller is increased by curved flanges extending along the tips of the propeller blades. The flanges form projections at the blade tips which deflect water that flows radially towards the tips outward into the thrust producing water flow through the propeller. The flanges have arcuate outer surfaces that face away from the propeller hub and which have a center of curvature at the axis of rotation of the propeller. Arcuate inner surfaces of the flanges have centers of curvature which are offset from the axis of rotation. This flange configuration inhibits cavitation in the region of the flanges and also at the hub region of the propeller.