Wave resistance: the effect of varying draught

1, In previous studies in the theory of wave resistance, while the water-plane section of the model was of a reasonably ship-like form, the draught was assumed to be infinite. In the following paper the model has the same simple lines and has vertical sides, but the draught is finite. The investigation shows how the resistance at different speeds depends on the draught, but it was undertaken specially for other reasons. In view of certain applications, it was important to find how the interference effects due to bow and stern waves are affected by varying draught. It is shown now that these become less prominent with diminishing draught, but the maxima and minima occur at practically the same positions. Further, when the ratio of draught to length is of the order of the values in actual ship models, one is in a position to attempt a comparison between the absolute values of theoretical and experimental results. Curves are shown in fig. 2 (p. 590) for the variation of resistance with velocity in three cases—when the draught is infinite, and when it is one-tenth and one-twentieth of the length of the model. The latter values cover approximately the usual ratios in practice. On the same diagram are reproduced experimental curves for three models of different types, the data being reduced to the same non-dimensional co-ordinates. Making allowance for the differences of form between these models and for the simplified form for which the calculations have been made, the results show that the calculated values are of the right order of magnitude over a considerable range of velocity. Differences in the two sets of curves, such as the greater prominence of interference effects in the theoretical curves, are discussed.