hen deciding the positions of twin screws, there is
room for variation, vertically, longitudinally, and transversely. For these
screws, the immersions inserted in the table give the vertical positions.
The immersion in A is 9 ft., showing what may be done in a deep draught
ship with a small screw. Whatever the value of deep immersion may be in
smooth water, there can be no question that it is much enhanced in a
seaway. The longitudinal positions are such that the center of the screw is
about one-fifth of the diameter forward of the aft side of the rudder post.
The positions may, perhaps, differ somewhat from this rule without
appreciably affecting the performance, but, if any alteration be made, it
would probably be better to put the screws a little farther aft rather than
forward. The forward edges of the blades are from 2 ft. to 3 ft. clear of
the legs of the bracket which carries the after bearing. The transverse
positions are decided, to some extent, by the distance between the center
lines of the engines. As regards propulsive efficiency, it would appear
that the nearer the screws are to the middle line, the less is the
resistance due to the shaft tubes and brackets, and the greater is the gain
from the wake in the screw efficiency, but, on the other hand, the greater
is the augment of the ship's resistance, due to the action of the screws.
Further, the nearer the screws are to the hull, the less are they exposed.
But experience is not wanting to show that the vibration may be troublesome
when the blades come within a few inches of the hull. The average of the
clearances between the tips of the blades and the respective hulls is about
one-eighth of the diameter of the screw.

An interesting and noteworthy fact in connection with these propellers is
the wide differences in the pitches and revolutions, though the products of
the two do not greatly vary. Such differences are extremely rare in the
mercantile marine for similar speeds, but in war ships they are inseparable
from the conditions of the engine design. As a general rule, with
(revolutions x pitch) a constant, an increase of revolutions and the
consequent decrease of pitch allow a diminution of disk and of blade
area--other modifying conditions, such as the thrust, slip, number, and
pattern of blades, being the same. The screws for E and F are interesting,
because, with practically the same speeds and slips, there is a
considerable difference in the revolutions