IMPROVEMENTS IN SCREW PROPELLERS
Year: 1886

page_text: I, CHRISTIAN HOEHLE, of the City and County of Philadelphia, in the State of
Pennsylvania, United States of America, Civil Engineer, do hereby declare the
nature of my invention for IMPROVEMENTS IN SCREW PROPELLERS and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement :--
My, invention relates to certain novel features in the construction of screw
propellers applied to vessels for their propulsion with a view of enhancing their
efficiency and perfection of operation and more particularly for increasing the speed
of the vessel.
The screw propeller wheel may generally be defined as consisting of helical or
twisted blades secured to a hub set upon a shaft or axis revolving beneath the
water at the stern of the vessel, but its efficiency as heretofore constructed has only
been partially satisfactory in its action, due principally to the considerable loss of
power arising from its irregular and improper construction, and in practice I have
found that two causes may be assigned for its lack of comparative merit and of its
failure to meet fully the requirements demanded, first, to improper pitch given to
the blades of the wheel at the different points, and, second, to the improper shape
of the cross sections of the blades. For example, by giving to the screw or blade a
uniform or constant pitch the opening in the wheel near the hub is contracted and
the water thereby prevented from passing freely through the wheel, while from the
improper shape of the cross sections the water is prevented from leaving the wheel
in the same direction that it euters it, and again, in the screw propeller as now
ordinarily constructed it is the outer parts of the blades that perform the specific
part of the work, while at the same time they are compelled to overcome the
resistance of the vessel as well as the resistance caused by the parts of the blades
nearest the hub, and it is the object of my invention to overcome these objectionable
features; to which end it consists in so constructing the blades of the propeller
wheel that the pitch of each blade will be smallest at its periphery and increase
gradually at first therefrom, and then more decidedly as the hub of the wheel is
approached, as hereinafter more fully explained.
In the accompanying drawings illustrating my invention:
Figure 1 represents a plan view of a four bladed screw propeller wheel of my

page_text: Fig. 2 is an end view partly in section, showing the thickness of the blade of
the wheel in the middle at the different radii.
Fig. 3 is a cross section of two adjoining blades of the wheel at the same radius.
Fig. 4 is a view showing the cross sections and the corresponding pitches of one
of the blades of the wheel at different radii; and
Fig. 5 is a view of a cross section of one of my improved blades and also
showing in dotted lines a cross section of one form of an ordinary screw propeller
blade.
Referring to the drawings A, represents the hub of the wheel and B, B1, B2,
and B³, the four blades rigidly secured in any suitable manner to the hub. Two
of the adjoining blades B and B¹, are shown in section in Fig. 3, with a cylindrical
surface as indicated by the dotted line d d, in Fig. 2, whose axis coincides with
the axis of the wheel A, laid out flat.
fk, (Fig. 3) represents the distance between the two sections measured
perpendicularly to the axis of the wheel and also may represent the circular
velocity of the wheel at the cross section.
fr, represents the velocity of the water relatively to the wheel and parallel with
its axis, and k r, the resultant of the two, the direction and velocity of the water
entering the wheel.
The perpendicular represented by fs, is the width of the current C, passing
between the two blades B and B', and the velocity fr, is a varying quantity which
at the hub of a well designed wheel is never less than the velocity of the vessel,
while at the periphery it is somewhat less than the velocity of the vessel plus the
slip of the wheel.
•
In order that the water may pass freely through the wheel the width of the
current C, must be one and the same on entering the wheel in passing through it
and on leaving it, so to accomplish this I construct the pitch of the cross sections
by drawing the line k n, in such a manner that the perpendicular ft, is equal
to fs, plus the thickness of the cross section did, of the blades B and B. The
lines forming the cross sections consist of the two straight lines as indicated
by ddaud dd, parallel to the line k n, and the two curved lines similarly
indicated by d d and d¹ d'11. The straight lines being tangent to the curved
lines at the points d and d, and hence it will be seen that by forming the
cross sections in the above manner the current will pass through the wheel as
indicated by the arrows in Fig. 3, while the width of the current C, will not in the
least be contracted in the wheel, and therefore the water will leave the wheel in
the same direction that it enters it. This would not be the case under such a
construction of screw propeller blade as is shown in cross section partly in dotted
Jines in Fig. 5, the contour of which particular blade is clearly indicated
by dad and d¹, and hence it will be readily understood from this view
that the water would not leave the wheel in the same direction that it entered it.
By repeating the operation herein before explained with the cross sections a a,
b b, c c, d d, and e e, I obtain the cross sections a¹ all, b1 b, c' c¹¹, d¹ d", and e¹ ell,
with their respective pitches f q, f p, ƒ o, ƒ n, and fm, as clearly shown in Fig. 4,
and hence it will be observed that the pitch fq, is smallest at the periphery of the
blade and that it increases gradually at first therefrom and then more decidedly as
-the hub of the wheel is approached.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what
I claim is
1. A screw propeller in which the pitch of each blade is smallest at the periphery
and increases gradually at first therefrom and then more decidedly to the hub,
substantially as described. .
2. A screw propeller in which the cylindrical cross sections of the blades are
each composed of two semi-segments joined together and so arranged that the

page_text: straight lines of the respective semi-segments form the continuations of the curved
lines and are tangent thereto, substantially as described.
3. A screw propeller in which the pitch of each blade is smallest at the periphery
and greatest at the hub and the cylindrical cross sections thereof in form that of
two semi-segments joined together and so arranged that the straight lines of the
respective semi-segments form the continuations of the curved lines and are tangent
thereto, substantially as and for the purposes described.
In witness whereof, I, the said Christian Hoehle, have hereunto set my hand
and seal this twentieth day of April in the year of our Lord one thousand
eight hundred and eighty-six.
Witnesses:
JOHN WALTER DOUGLASS.
THOMAS MILLER SMITH.
CHRISTIAN HOEHLE. (LS)
Carpmael & Co.,
Agents for the Applicant.