Abstract: A hydrofoil section comprises first and second faces that create, in operation at speeds above a ventilation speed, a ventilated cavity defined by a first cavity face which departs from the first hydrofoil face and a second cavity face which departs from the second hydrofoil face. Each cavity face represents a free surface and each face separating from the said free surface at a discontinuity on that surface, the separated faces forming a continuation of the faces of arbitrary shape and enclosed by the free surfaces without contacting the said free surfaces. Below the speed at which full ventilation occurs the arbitrarily shaped portion of each face is configured to provide a modified flow configuration resulting in changed lift and or drag and or pitching moment under partial, or unventilated operation.