2 RELATIVE RESISTANCES OF SOME MODELS WITH BLOCK 



Plate I gives the body plans of the 5 models of .60 block and having midship 

 section coefficients varying from .86 to i.io, the midship section coefficient being 

 indicated below each body plan. The lines are not, perhaps, the best to be obtained 

 for speed. Conventional lines were aimed at, although, of course, the higher mid- 

 ship section coefficients are materially greater than used in practice. 



Plate 2 shows the water lines and sectional area curves for the 20 models. 

 The body plans for the .60 block models as shown in Plate i may be regarded as the 

 parent lines for the 15 models of the other block coefficients. The process of tran- 

 sition is simple. Starting with the .60 block model, imagine it expanded or con- 

 tracted transversely until the midship section area is that for the new block coeffi- 

 cient. This expanded or contracted model will not have the curve of sectional area 

 desired, but by shifting sections forward or aft as required the form with the de- 

 sired curve of sectional area is readily obtained. 



It will be observed in Plate 2 that the curves of sectional area have a small or- 

 dinate at the forward perpendicular. In actually making the models the corners 

 shown are rounded off on the model, the amount removed, however, being in- 

 finitesimal. 



Plate 3 shows contours of wetted surface coefficient plotted over the range of 

 midship section coefficient and block coefficient of the 20 models. The wetted sur- 

 face coefficient is the well-known coefficient C in the formula 



Wetted surface=Ci/DL 



where D is displacement in tons in salt water, and L is water line length in feet. 



The variation of C in Plate 3 is as might be expected from what we already 

 know of the effect of variation of form upon wetted surface. If we keep m 

 constant and vary the block coefficient, b, there is very little change in the wetted 

 surface; it decreases from b = .55 to about b = .65, but from .65 to .70 remains 

 practically constant. If we keep the block coefficient constant and vary the midship 

 section coefficient there is appreciable variation of wetted surface for all blocks, 

 the maximum for a range from an m value from .86 to i.io being about 6 per 

 cent. The fact that the wetted surface coefficient diminishes from about 16.3 when 

 m equals i.io, to about 15.3 when m is equal to .85, or thereabouts, should not lead 

 us to conclude that a smaller value of m will involve further material diminution 

 in the wetted surface coefficient. As a matter of fact, 15.3 is very close to the mini- 

 mum wetted surface coefficient obtainable, and if we continued to decrease m we 

 would find the wetted surface beginning to rise again. 



The 20 models were run in the usual way in the full depth of the model basin 

 — 14 feet in the center — and were afterwards tested, with a false bottom in place, 

 in water 20 inches deep. The results are given in Plates 4 to 12. 



Plates 4, 5, 6 and 7 give results as curves of effective horse-power in deep water 

 for 500- foot ships displacing 17,850 tons upon the lines of the models. One figure is 

 given for each block coefficient, the 5 curves in each figure referring to the 5 models 

 of varying midship section coefficient. 



