Best form of Vane. - A B, Fig. 703,
is the float of an undershot water wheel, receiving the impact of a thin
Drawing vj the velocity of water jet, and vf that of the
float tangentially, the completion of the parallelogram gives vr,
the relative velocity, in magnitude and direction, to which the float
should be made tangential. This form of vane is due to Poncelet,
and the action is essentially as at F, Fig, 702.
Water Wheels - the earliest forms
of water motor-consist of (1) those rotated by water falling down the rim,
known as weight machines ; (2) those actuated by water impact on
their lower floats, and called impulse machines. Overshot
and breast wheels belong to the former, and undershot wheels to the latter
The Undershot Wheel is shewn in Fig. 707. The form of float has
been drawn at Fig. 703, and there, only remains to add that, with Poncelet's
improvements in floats and race, the water
leaves the wheel with little absolute velocity, and the efficiency
is about .66, a great improvement over that of the old radial-float wheel,
which was only .3. As the water never fills the vanes, there is no
pressure, but pure impulse only, and the efficiency is therefore constant
under varying sluices. Horse-power may be reckoned from head or velocity
(see pp. 719 and 720). The circumferential velocity is about
.55 of that due to head, and the jet thickness is about 8 or 10 ins.
The wheel is suitable for falls up to 6 feet, and the diameter may be four
times the fall.