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The optimum circulation distribution and hence the maximum theoretical thrust obtainable for contra-vanes fitted behind propellers is markedly dependent on the number of guide vanes. The outer portion of the vanes, even if projecting considerably beyond the edge of the propeller slipstream, still contribute appreciably to this theoretical gain of thrust. But, owing to the always existing friction of the vanes, the limit of the optimum vane length lies at relatively small diameters. A large hub unloads the vanes. Hence, the guide vanes are best attached to suitable parts of the bodies which would, in any case, be subjected to the slipstream.