Abstract: A T-step hull form for monohull planing vessels includes a forward section, an aft section, a transom, a step, and a skeg. The step is transversely oriented and separates the forward section from the aft section, and the skeg extends longitudinally along the length of the hull form from the step rearwardly to the transom, dividing the aft section into a starboard tunnel portion and a port tunnel portion. The starboard and port tunnel portions are ventilated at their respective outboard sides, at the transom, or both. Vents may pass transversely through the skeg. Tunnel flaps may be disposed in each of the tunnel portions and are hingeably attached at the step to allow for adjustability of the step's geometry. The addition of the skeg, which extends from the step to the transom, provides additional longitudinal surface area for the hull form, and in turn increases lateral stability.