Abstract: End wall compartment closures for the stern parts of inflatable sponsons or pontoons for rubber boats are disclosed to enhance the gliding motion or planing action of the boat through the water and prevent distortion of the sponsons under variable inflation pressures while increasing the load carrying capacity and stability of such craft. In the several embodiments each of the elongated tubular sponsons that extend along the sides of a rubber boat is provided with a rigid-non-deformable closure wall that is essentially vertical to the longitudinal axis of the sponson or is in the form of an annular or tubular part or insert in the stern of the sponsons. The tubular inserts can be pot-shaped with circumferential grooves or ribs on the sides and have a bottom wall which forms the end compartment wall or closure wall for the sponson.