60 
BULLETIN 718, U. S. DEPARTMENT OF AGRICULTURE. 
per minute multiplied by the height of fall in feet, multiplied by the 
weight of a cubic foot of water, and divided by 33,000, gives the horse 
power per minute. 
Amount of water required to develop a given horscpoirer with a given available 
effective head. 1 
Effective head. 
Flow of water per minute. 
10 H. P. 
20 H. P. 
30 K. P. 
40 H. P. 
50feet 
Cubicfeet. 
125 
" 104 
88 
77 
70 
63 
Cubicfeet. 
250 
208 
177 
155 
140 
125 
Cubicfeet. 
375 
312 
266 
232 
210 
186 
Cubicfeet. 
500 
60feet 
416 
70 feet 
355 
SOfeet 
311 
90 feet •:. 
280 
100 feet 
248 
1 Horsepower based In 85 per cent efficiency of the wheel. 
Water wheels are built either overshot, breast, undershot, or tur- 
bines. 
Overshot wheel. — The effective power is 60 to 75 per cent of possible 
power. The proper velocity of the circumference is 5 feet per second 
and is equal to approximately one-half the velocity of the water. 
The water velocity must be greater than the rim of the wheel. In 
falls pi from 20 to 40 feet in height the overshot wheel is more 
effective than a turbine. The buckets should have a capacity three 
times as large as the volume of water actually carried, should have 
holes in the bottom in order to allow the escape of air, and have a 
depth of from 12 to 14 inches and be 12 inches apart at least from 
centers. The speed necessary to run a sawmill with this power is 
obtained by countershaft or gears. 
Breast wheel. — The water acts by weight and impact, dropping 
vertically into the buckets. The efficiency varies from 45 to 65 per 
cent of possible power and works best in falls from 8 to 16 feet hav- 
ing a discharge from 20 to 80 cubic feet per second. The speed of 
the wheel should be such as to fill the buckets one-half their capacity 
each revolution. Other forms of breast wheel are called "high 
breast," "low breast," and "flutter" wheels. The old form of water 
wheel had radial buckets, but the modern idea is in favor of curved 
buckets which give a higher efficiency. 
Undershot wheel. — The undershot or current wheels have a low 
efficiency and are usually anchored in rapid streams in such a position 
as to be safe during floods or high water. The size of the wheel 
varies. It usually has 12 blades, each one of which is submerged 
as it passes directly under the axle. All of those wheels have become 
practically obsolete in this country. 
The Pelton turbine is a type of axial flow impulse turbine in which 
a small jet issues from a nozzle and strikes on a series of cups of 
