The paper describes part of an investigation aimed at developing a multi-megawatt pulsed travelling-wave tube. To this end, a multistart helix structure which may be water cooled has been considered both theoretically and experimentally.Theoretical relationships have been determined for the axial series impedance of a sheath helix surrounded by a perfectly conducting cylinder, and for the phase velocity of the propagating modes. Results of experiments on four- and eight-start helix structures are found to give reasonable correspondence with the theoretical results for equivalent bounded sheath helices.