r to a smaller hull, which is
only another expression for a larger proportionate screw.

588. _Q._--When you speak of a larger screw, what increase of dimension do
you mean to express?

_A._--An increase of the diameter. The amount of reacting power of the
screw upon the water is hot measured by the number of square feet of
surface of the arms, but by the area of the disc or circle in which the
screw revolves. The diameter of the screw of the Rattler being 10 feet, the
area of its disc is 78.5 square feet; and with the amount of thrust already
mentioned as existing in the first experiment, viz. 8722 lbs., the reacting
pressure on each square foot of the screw's disc will be 108-1/2 lbs. The
immersed midship section being 380 square feet, this is equivalent to 23
lbs. per square foot of immersed midship section at a speed of 9.2 knots
per hour.

589. _Q._--In smaller vessels of similar form, will the resistance per
square foot of midship section be more than this?

_A._--It will be considerably more. In the Pelican, a vessel of 109-3/4
square feet of midship section, I estimate the resistance per square foot
of midship section at 30 lbs., when the speed of the vessel is 9.7 knots
per hour. In the Minx with an immersed midship section of 82 square feet,
the resistance per square foot of immersed midship section was found by the
dynamometer to be 41 lbs. at a speed of 8-1/2 knots; and in the Dwarf, a
vessel with 60 square feet of midship section, I estimate the resistance
per square foot of midship section at 46 lbs. at a speed of 9 knots per
hour, which is just double the resistance per square foot of the Rattler.
The diameter of the screw of the Minx is 4-1/2 feet, so that the area of
its disc is 15.9 square feet, and the area of immersed midship section is
about 5 times greater than that of the screw's disc. The diameter of the
screw of the Dwarf is 5 feet 8 inches, so that the area of its disc is
25.22 square feet, and the area of immersed midship section is 2.4 times
greater than that of the screw's disc. The pressure per square foot of the
screw's disc is 214 lbs. in the case of the Minx, and 109-1/2 lbs. in the
case of the Dwarf.

590. _Q._--From the greater proportionate resistance of small vessels, will
not they require larger proportionate screws than large vessels?

_A._--They will.

591. _Q._--Is there any ready means of predicting what the amount of thrust
of a screw will be?

_A._--When we know the a