The principal method of predicting ship horsepower is still by the use of scale models in a towing tank. However, the problem of the extrapolation of the data from model to ship is an extremely difficult one which is still not satisfactorily solved. This paper discusses the types of scale effect which arise in the interpretation of model resistance and propulsion data. It suggests that although satisfactory engineering solutions can be obtained by the use of gross overall correlation coefficients derived from a comparison of ship trial results with predictions from model tests, yet many unknown areas remain in our basic knowledge of how correctly to extrapolate model data. The need is emphasized for further experimental and theoretical work in the model field, to clear up such questions as blockage effects, turbulence stimulation, and the division of resistance into its several components, and for full-scale trials in which all the quantities measured on the model will also be measured on the ship. Such trials would give for the first time information on the scale effects on wake, thrust deduction, propeller open efficiency, and overall propulsive efficiency. Until we know the extent of such effects, we lack guidance as to where to look for further improvement in ship performance.

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