| | ABS.
-----------------------------+-------+---------+----------+-------
Smooth London tube, 13/4 in. | 71 | 62 | 94.25 | 49 deg.
mean radius. | | | |
| | | |
Rough wrought iron tube, | 78 | 52 | 98.75 | 56.5 deg.
13/4 in. | | | |
| | | |
Smooth leaden tube bent to a | 71 | 40 | 81.5 | 60
sharp right angle. | | | |
-----------------------------+-------+---------+----------+------

The third column is obtained by constructing a parallelogram of forces,
where impact and reaction form the measures of opposing sides, and it
furnishes the resultant due to both forces. The fourth column gives the
inclination ABS, at which the line of impact must incline toward a plane
surface RS, Fig. 3, so as to produce this maximum resultant perpendicularly
upon it; as the resultant given in column 3 indicates the full practical
effect of impact and reaction. When a stream has its direction changed to
one at right angles to its original course, and as such a changed direction
is all that can be hoped for by ordinary screw propellers, the figures in
column 3 should bear some relationship to such cases. Therefore, it becomes
an inquiry of some interest as to what angle of impact has been found best
in those screw propellers which have given the best results in practical
work. Taking one of the most improved propellers made by the late Mr.
Robert Griffiths, its blades do not conform to the lines of a true screw,
but it is an oblique paddle, where the acting portions of its blades were
set at 48 deg. to the keel of the ship or 42 deg. to the plane of rotation.
Again, taking a screw tug boat on the river Thames, with blades of a
totally different form to those used by Mr. Griffiths, we still find them
set at the same angle, namely, 48 deg. to the keel or 42 deg. to the plane
of rotation. An examination of other screws tends only to confirm these
figures, and they justify the conclusion that the inclinations of blades
found out by practice ought to be arrived at, or at any rate approached, by
any sound and reliable theory; and that blades of whatever form must not
transgress far from this inclination if they are to develop any
considerable efficiency. Indeed, many favo