IMPROVEMENTS IN THE CONSTRUCTION OF SCREW PROPELLERS
Year: 1876

page_text: LETTERS PATENT to Adolphe Aubert, of Paris, Boulevard de Strasbourg, No. 23,
Engineer, for the Invention of "IMPROVEMENTS IN THE CONSTRUCTION OF
SCREW PROPELLERS."
Sealed the 28th March 1877, and dated the 19th October 1876.
PROVISIONAL SPECIFICATION left by the said Adolphe Aubert at the Office
of the Commissioners of Patents on the 19th October 1876.
"
ADOLPHE AUBERT, of Paris, Boulevard de Strasbourg, No. 23, Engineer. IM-
PROVEMENTS IN THE CONSTRUCTION OF SCREW PROPELLERS."
This Invention relates to improvements in the construction of screw propellers
of the kind described in the Specification to my Patent of the 14 September 1871,
No. 2416, and consists in forming the propelling surface by means of an eccentric
directrix in such a manner as to equalise the work or render it nearly uniform over
the whole blade, and to produce a propelling thrust almost directly in the line of a
vessel's movement without such torsive action on the column of displaced water as
generally results from the use of ordinary screw propellers. For this purpose I generate
the curved surface of the propeller blade in the following manner:-Looking at the
front view of a blade, from the centre or axis I describe a circle representing the outer
edge of blade. At a previously determined distance from the centre of this circle
I draw a straight line, and from the point of intersection thereof with the said circle
I set off an arc corresponding to the fraction of the pitch adopted for the proposed
screw. Then taking the before-mentioned distance of the straight line from the
centre of the circle as the radius of a circle representing the boss of the screw, I set
off upon this circle from the point where the straight line meets it tangentially an
arc, the length of which is variable, according to the relation that is to exist between
the pitch of the screw at the periphery and that at the boss, such relation being
determined by the proportion between the length of the outer pitch and the external
diameter. It has been found that this relation should be in inverse ratio to that
between the length of the outer pitch and the diameter. I then draw a straight
line through the extremities of the inner and outer arcs set off as above described,
and I extend this line till it meets the tangent at a point beyond its point of
contact, I take this point which is at a distance from the axis, as the centre round
which the generatrix of the blade curve is supposed to revolve, and I trace the

page_text: curvature of the blade surface by using as generatrix a straight line guided by the
two pitches, and always radiating from the excentric point determined as above
set forth. The form presented by the vertical projection of the blade is readily
determined from the above.
The curve described by the intersections of the generatrix with the surface of
boss being considered as directrix, and the relations which the pitch at the boss
should bear to that at the periphery being known, it results that the value of the
angles which the generatrix should form in relation to the vertical planes inter-
secting the blade is readily determined.
From the above-described construction, it follows that both in plan and in
elevation of the blade the relations between the angles and surfaces are variable.
For the purpose of reducing the surface of the blade to a minimum without
impairing its efficiency, the back edge thereof is rounded off by a curve starting
from about the middle of the length of the blade and extending inwards to about
one-third the full width of the blade at the periphery as determined by the above-
described method of construction.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Adolphe Aubert in the Great Seal Patent Office on the 10th April
1877.
ADOLPHE AUBERT, of Paris, Boulevard de Strasbourg, No. 23, Engineer. " IM-
PROVEMENTS IN THE CONSTRUCTION OF SCREW PROPELLERS."
This Invention relates to improvements in the construction of screw propellers
of the kind described in the Specification to my Patent of the 14th September 1871,
No. 2416, and consists in forming the propelling surface by means of an eccentric
directrix in such a manner as to equalise the work or render it nearly uniform over
the whole blade, and to produce a propelling thrust almost directly in the line of a
vessel's movement without such torsive action on the column of displaced water as
generally results from the use of ordinary screw propellers. For this purpose I
generate the curved surface of the propeller blade in the following manner:-
Looking at the front view of a blade, from the centre or axis I describe a circle
representing the outer edge of blade. At a previously determined distance from
the centre of this circle I draw a straight line, and from the point of intersection
thereof with the said circle I set off an are corresponding to the fraction of the
pitch adopted for the proposed screw. Then taking the before-inentioned distance
of the straight line from the centre of the circle as the radius of a circle representing
the boss of the screw, I set off upon this circle from the point where the straight
line meets it tangentially an arc, the length of which is variable, according to the
relation that is to exist between the pitch of the screw at the periphery and that at
the boss, such relation being determined by the proportion between the length of
the outer pitch and the external diameter. It has been found that this relation
should be in inverse ratio to that between the length of the outer pitch and the
diameter. I then draw a straight line through the extremities of the inner and
outer arcs set off as above described, and I extend this line until it meets the tangent
at a point beyond its point of contact. I take this point which is at a distance
from the axis, as the centre round which the generatrix of the blade curve is
supposed to revolve, and I trace the curvature of the blade surface by using as
generatrix a straight line guided by the two pitches and always radiating from the
eccentric point determined as above set forth. The form presented by the vertical
projection of the blade is readily determined from the above.
The curve described by the intersections of the generatrix with the surface of
the boss being considered as directrix, and the relations which the pitch at the boss
should bear to that at the periphery being known, it results that the value of the

page_text: angles which the generatrix should form in relation to the vertical planes inter
secting the blade is readily determined.
From the above-described construction, it follows that both in plan and in
elevation of the blade the relations between the angles and the surfaces are variable.
For the purpose of reducing the surface of the blade to a minimum without
mpairing its efficiency, the back edge thereof is rounded off by a curve starting
rom about the middle of the length of the blade and extending inwards to about
one-third the full width of the blade at the periphery as determined by the above-
lescribed method of construction.
.
igs. 1 and 2 shew in elevation and plan the relation between the angles included
at the centre and at the circumference by the directrixes of one of the blades, and
the successive positions of the generatrixes in the direction from the front to the
back.
Figs. 3 and 4 shew in plan and elevation the geometrical construction of the
propeller, shewing the curve F, G, by which the fraction of the total pitch is reduced
when required to a less surface.
Figs. 5 and 6 shew a plan and elevation of the screw propeller.
Having drawn a line A, B, parallel to the diameter a, b, at a variable distance
B, O, from the centre, I set off from the point A at the outer circumference, an
arc A, C, corresponding to the fraction of the pitch adopted for the proposed screw.
Taking the distance B, O, as the radius of the circle for the boss of the screw, I set
off upon the circle the are B, D, which is variable, according to the relation that is
to exist between the pitch of the screw at the periphery and that at the boss, I
then draw the line C, D, extending the same to the point O¹ where it meets the
tangent A, B, which point becomes the centre round which the generatrix turns in
moving from front to back. This centre O¹ consequently is situated in front and
eccentrically of the centre of rotation of the propeller. The development in vertical
projection may be readily deduced from the preceding description.
The curve described on the boss by the intersection of the generatrixes being
considered as the directrix, and the proportion between the pitch at the boss and
that at the circumference being known, it results that the value of the angle which
the generatrix should form with the plane perpendicular to the axis, and conse-
quently with those plans corresponding to the different sections from front to back
is readily determined. It should be observed however that it is important for the
useful effect of the screw that the greater or less obliquity of such angle should be
in correct relation to the other proportions of the screw. It will be seen that the
blade of the screw thus formed describes round the axis an oblique rotating move-
ment following an eccentric direction, from which it follows that instead of first
acting on the water at its extremity it acts upon it at its base near the boss so as to
impart to the centre of the column of water a more or less accelerated velocity which
compensates for the disadvantage resulting from the insufficiency of the length of
leverage, and which in addition tends to bring the centre of action to a more distant
point of the length of the lever. It will also be seen from the above that in the
configuration of the propeller, both in plan and in elevation, the relation between
the angles and the surfaces are variable. The proportions of these several elements
should in effect be co-ordinated according as experience may require, according to
the relation which exists between the length of the pitch of the screw as compared
with its diameter. If we consider the section line taken transversely through the
blade as the diagonal of a parallelogram, and we set off on this diagonal the
resultant of the two forces represented by the two adjacent sides of the parallelo-
gram, it will be seen that according as the proportion of the pitch is greater or
smaller the propelling force will be exerted in a direction more or less advantageous
to the reaction from which the useful effect results, and it is for this reason that in
order to exert on all the threads of liquid corresponding to the divers points of the

page_text: propelling surface a thrust or a mechanical effect suchthat all the threads of liquid
shall be brought as it were into a compact bundle of cylindrical form, it is necessary
to give to the force of eccentric expansion from the axis to the extremity a more or
less energetic initial impulse, according to the different relations which exist
between the pitch and the diameter. Experience has shewn that this relation
between the length of pitch at the boss and that at the circumference should follow
a progression the inverse of that which exists between the length of the extreme
pitch and the diameter.
One of the essential conditions for the useful effect of the propeller is the proper
extent of surface or fraction of pitch which should be adopted in relation to its
different applications. In case the fraction of pitch requires to be reduced to a
small surface without altering the principal conditions and proportions above
described, I effect such reduction according to a curve F, G, Figs. 3 and 4, on the
back edge of the blade, while retaining the geometric construction of the blade as
described with reference to Figs. 1 and 2. By this means I am enabled to give the
propelling surface the required proportions and extent without interfering with the
general principles of construction above set forth.
Having thus described the nature of my Invention and in what manner the same
is to be performed, I claim, the before-described improved construction of screw
propellers wherein the blades are so formed as to describe an oblique rotary move-
ment following an eccentric direction, while the length of the pitch at the boss
differs from that at the circumference in an inverse ratio to the proportion between
the extreme pitch and the outside diameter of the propeller, and while the angles.
formed by the generatrixes with the surface of the blade are variable.
In witness whereof, I, the said Adolphe Aubert, have hereunto set my hand
and seal, this Sixth day of April, in the year of our Lord One thousand eight
hundred and seventy-seven.
Witness:
J. ARMENGAUD JEUNE,
Civil Engineer,
23, Boulevard de Strasbourg,
Paris.
A. AUBERT. (L.S.)