TIDAL POWER TURNBULL 



533 



After this for 6% hours the low-level basin will rise, but the high- 

 level basin will also rise after 4.1 hours (as the flood tide will then 

 be filling it for 1.9 hours) and we have the head decreasing from 27% 

 feet to 22 feet, but afterwards increasing to 23% at 5.2 hours. 



After this the head will gradually decrease to 13% feet at 9 

 hours, when, the tide in the external bay having sufficiently ebbed, 

 the water from the tailrace will be again discharged into it and the 

 head will steadily rise to its previous maximum of 28% feet at low 

 tide, as shown by the curve of heads. 



The average head in this case from low tide to low tide works 

 out at 22% feet. 



POWER AVAILABLE 



The selection of a 6-inch drop per hour in the high-level basin 

 is merely for the purpose of an illustration, but other drops may 

 be considered, and in order to ascertain the most suitable drop for 

 the initial and final developments (which are referred to later) 

 we must consider the limits of good turbine operation, with variable 

 heads, and plot out the gross horsepower curves against various 

 hourly drops under the conditions, first, of the initial development 

 (here taken at 90,000 horsepower, and the present basin ratio of 

 5.5:1) and second, of the final development (here taken at 200,000 

 horsepower, and the improved basin ratio of 2:1). 



These data have been worked out (fig. §a) for both basin ratios 

 and the respective areas, and when we select an initial develop- 

 ment of 90,000 horsepower (which is that indicated by the present 

 population) we note that this corresponds to an hourly drop in the 

 high-level basin of 4.4 inches, a minimum head, as per cent of maxi- 

 mum, of 55 per cent, and a minimum head, as per cent of average, 

 of 69 per cent. And for a final development of 200,000 horsepower, 

 with basin ratio improved to 2:1, the hourly drop would be 10% 

 inches, the minimum head, as per cent of maximum, 56.7 per cent, 

 and a minimum head, as per cent of average, of 68 per cent. The 

 actual heads in feet for the two developments are given in Figure 65, 

 plotted against the time of a complete cycle, and we then have for 

 a comparative table the following: 



Maximum head feet 



Minimum head do 



Average head ... do 



Minimum, as per cent of maximum.. per cent.. 



Minimum, as per cent of average do 



Initial Final 

 develop- develop- 



ment, 

 <J0,000 

 horse- 

 power 



29.6 

 16.25 

 23.55 

 55 



ment, 

 200,000 

 horse- 

 power 



26.3 

 14.9 



21.94 



56.7 



68 



1454—25- 



-35 



