20 feet, and from 2 to 20 feet wide, although, as before stated, there
is scarcely a limit under 100 feet diameter for special purposes.

In designing this class of wheels special attention should be given to
the concentration and increase of the velocity of the current by wing
dams or by the narrowing of shallow streams; always bearing in mind that
any increase in the velocity of the current is economy in increased
power, as well as in the size and cost of a wheel for a given power.

The blades in the smaller size wheels should be 1/4 of the radius in
width, and for the larger sizes up to 20 feet, 1/5 to 1/6 of the radius
in width and spaced equal to from 1/4 to 1/3 of the radius.

They should be completely submerged at the lowest point.

For obtaining the horse power of a current wheel, the formula is

Area of 1 blade × velocity of the current in ft. per sec.
----------------------------------------------------------
400

× by the square of difference of velocities of current and wheel
periphery = the horse power; or

A × V 2
------ × (V - v) = h. p.
400

[TEX: \frac{A \times V}{400} \times (V - v)^2 = h. p.]