In vacuo elastic wave propagation characteristics in an infinite cylindrical shell stiffened by an infinite helical rib are investigated analytically. Equations of motion of the helical rib and of the cylindrical shell in helical coordinates are developed. They are combined into a single set of equations using appropriate boundary conditions. Dispersion curves of the elastic waves propagating in the shell are obtained for different pitch angles. Comparisons with wave propagation characteristics in an unstiffened shell are discussed.