The restriction L # 2pRo /K given above on the pitch L of
the screw needs some discussion. In order for water to be
trapped in the screw, it is necessary that the sinusoidal curve
in Fig. 5 defining the outer edge of a blade tilts downward as
it crosses the axis of the screw. In terms of the angles u and
a in the figure, it is necessary that u # a or, equivalently,
tan u # tan a. Now tan u = K by definition, and tan a =
Ro(2p/L) since the sinusoidal curve has amplitude R0 and
period L. The condition tan u # tan a is thus K # Ro(2p/L),
which is just L # 2pRo /K.
Fig. 5 also shows the angle b that the sinusoidal curve defining
the inner edge of a blade makes with axis of the screw.
Because the inner sinusoidal has amplitude Ri and period L,
it follows that tan b = Ri(2p/L). If u # b, so that the inner
sinusoidal curve tilts downward as it crosses the axis of the
screw, then the horizontal water level in a bucket is tangent
to this inner sinusoidal curve. As above, the condition u # b
can be expressed as L # 2pRi /K.
If L lies in the interval (2pRi /K, 2pRo /K), then the horizontal
water level is right at the point where the inner and
outer sinusoidal curves cross the axis of the screw. At that
point the inner sinusoidal curve has positive slope (with respect
to the horizontal) and the outer sinusoidal curve has negative
slope. The buckets of water that form in this case are
rather small and in, in fact, will not be in contact with the
inner cylinder. Summarizing, the horizontal water level of a
bucket is