Following a general review of the analytical and experimental information on the friction loss encountered by fluids flowing in annular pipes, with particular regard to the influence of eccentricity of the inner member, experiments are described on an evaluation of the friction of water in a steel annular pipe of diameter ratios 5.8 and 3.2 in the Reynolds number range of 100,000 to 1,000,000. It is found that, on a friction factor basis (in terms of the equivalent diameter as four times the hydraulic radius), whereas at the larger ratio the concentric position of the insert increases the friction only about 2 percent, its full eccentric location results in a decrease of some 15 per cent. On a discharge basis, however, for the same head loss in a given length, the flow capacity of the pipe line is decreased 12.7 per cent in the concentric situation but only 4.5 percent with full eccentricity. This decrease is not greatly different from the 3 percent reduction in area due to the inserted smaller pipe. An analysis is included showing that for the simple insert at full eccentricity the near-full-flow capacity of a sewer is little affected. The effects of hangers such as might be employed to support inserts in sewers is found to have an appreciable effect on the flow capacity of a full-flowing sewer. (Author)