PLANING SURFACES AND HYDROFOILS

This chapter discusses the phenomena of planning surfaces and hydrofoils. Planing or gliding is a motion of a body over a free water surface in which the basic supporting force is not the hydrostatic “Archimedes” buoyancy force but the hydrodynamic lift produced by the displaced water. The planing or gliding principle is currently employed for the high-speed motion of comparatively small vessels and the takeoff and landing of seaplanes. The planing surface throws a high-speed sheet of spray ahead and in part to the sides of the surface. Generally, the sides and trailing edge of a planing surface are sharp-edged, and viscosity plays a role only in the thin boundary layer on the surface. This layer is swept from the planing surface at the sharp edges without any noticeable deformation of the main flow. In many practical applications of hydrofoils, cavitation is generally avoided because it materially reduces the foil's effectiveness. Hydrofoil systems have been successfully used on many passenger vessels, and the widespread use of foil systems for ships of all sizes seems to be a promising prospect.