Systems, apparatus, and methods for hydrofoil assemblies with planing blades that may be adjusted, and securely maintained in varying tilts with respect to a support member, using a curved indexing system with a curved ridge and a counterpart groove that utilize interacting position retaining elements to retain a desired tilt in different adjustable positions. In one illustrative system, a planing blade may have a convexly curved ridge disposed on a surface thereof with a series of transverse grooves disposed in the curved ridge. A support member may have a counterpart concavely curved receiver with a series of counterpart transverse grooves formed therein. When a user places the planing blade in position with the convex ridge contacting the concave receiver, the blade may be tilted to a desired position. The counterpart transverse grooves interconnect to provide an indexed positive interlock, securely maintaining the blade in the desired position.