42 FOREST UTILIZATION 



For example, if cross section of a race is = 2 sq. ft., water velocity 

 =r 660 ft. per minute, height of water fall 30 ft., then the 

 power is : 



2 X 30 X 600 X 62.5 



= 75 H. P. 



330CO 



Water wheels are either vertical, i. e., overshot, breast or under- 

 shot wheels, or horizontal wheels, i. e., turbines. 



I. Overshot wheel. Effective power is 60% to 70% of pos- 



sible 'power. The proper velocity at the circumference 

 is 5 feet per second and at best if it is equal to 0.55 of 

 velocity of water. 



In falls of 20 feet to 40 feet and over, overshot wheels are 

 more effective than turbines. 



The buckets, framed by the shrouding, should be curved or 

 elbowed and not radial. They should have a capacity 

 three times as large as the volume of water actually 

 carried, a depth of 10 inches to 12 inches and a distance 

 apart, from center to center, of 12 inches. 



Ventilated buckets, having holes in the bottom and allowing 

 air to escape, are said to have a better effect. 



It is difficult to transform the slow speed of an overshot 

 into the rapid speed required for a circular saw. Trans- 

 formation is either by countershaft or by cog wheel. 



II. The breast wheel has an effective power of from 45% to 



65%, is best applied to falls of from 5 feet to 15 feet 

 and to a discharge of from 5 to 80 cubic feet per second. 

 While in the overshot the water works by weight only, 

 it works in the breast wheel largely by impact. 

 The velocity of wheel should be such as to fill the 

 buckets to 0.5 or 0.6 of their volume. The buckets here 

 are usually called blades and must be ventilated. 

 The wheel runs in a curb or mantle, formed by the inclined 



and cased end of the sluiceway. 



The distance of the blades, from center to center, should equal 



the depth of the shrouding, both being from 10 inches 



to 15 inches. The clearance between the curb and the 



shrouding must be at least half an inch. 



"High breast" wheels are semiovershot and "low breast" 



wheels are semiundershot wheels. 



The "flutter" wheel is a low breast wheel of small diameter 

 and high speed. It is largely used in western North 

 Carolina for saw-mill purposes where water is plenti- 

 ful and fall about 12 feet. 



III. Undershot or current wheels have an efficiency of from 

 27% to 45% only and are usually kept anchored in rapid 

 streams, so as to be independent of water gauge. No. 

 buckets, but long blades instead. 



