IMPROVEMENTS IN THE CONSTRUCTION OF SHIPS' PROPELLERS AND OTHER LIKE ARTICLES WITH HELICAL SURFACES
Year: 1878

page_text: LETTERS PATENT to Robert Henry Armit, of Craven Street, in the County of
Middlesex, a Retired Lieutenant of Her Majesty's Navy, for the Invention of
"IMPROVEMENTS IN THE CONSTRUCTION OF SHIPS' PROPELLERS AND OTHER
LIKE ARTICLES WITH HELICAL SURFACES."
PROVISIONAL SPECIFICATION left by the said Robert Henry Armit at the
Office of the Commissioners of Patents on the 14th March 1878.
ROBERT HENRY ARMIT, of Craven Street, in the County of Middlesex, a Retired
Lieutenant of Her Majesty's Navy. "IMPROVEMENTS IN THE CONSTRUCTION OF
SHIPS' PROPELLERS AND OTHER LIKE ARTICLES WITH HELICAL SURFACES."
My Invention relates to the construction of screw propellers, fans, and other like
articles with blades formed in a peculiar manner, and is a further developement of
the principles involved in the construction of the screw propellers described in the
Specifications of former Letters Patent, dated the 15th September, A.D. 1876,
No. 3618, and 9th January, A.D. 1878, No. 111.
The manufacture of screw propellers with the blades shaped as described, and
shewn in the said former Specifications, is attended with certain difficulties which
the present Invention is designed to obviate, as herein-after described.
a
It is well known to all familiar with the making of screw propellers that the
ordinary and most convenient manner of forming the moulds of the propeller blades,
previous to casting them, is by a process termed "sweeping;" but the shape of the
propeller blades, as shewn in the said former Specifications, can only be produced
in this manner with difficulty, such blades requiring the use of special patterns
which are very costly and inconvenient; therefore I have designed the following
method of forming these improved cycloidal propellers, whereby I obviate these
difficulties without sacrificing any of the advantages of the cycloidal form of the
blades, and which I will now proceed to explain.

page_text: According to my Inventions, as described in the said former Specifications, I
only used a "right cycloid" which is thus developed.
Referring to the diagram, Figure 1, if the circumference of a circle (horein-after
termed the generating circle) be rolled upon a right line, until the point A in that
circumference in contact with the said line passes entirely around the circle and
comes again in contact with it, that point will describe a curve AGACA, termed
a cycloid. The line ABA, on which the generating circle rolls, and which may
equal in length the diameter of the screw, is the base of the cycloid which starts
from the axis or centre of the screw, and the diameter BA of the said circle drawn
perpendicularly to this base at the centre or middle point of the latter is the axis
of the cycloid. Taking the said axis A. of the screw as a centre, a circle B x y z,
is drawn around the same, whose radius equals the distance from the said contre A
of the screw to the middle point of the base of the cycloid, and this circle will
represent the diameter of the propeller.
The width of the blade is the chord of an arc wzd, or part of the said circle
representing the diameter of the screw, and this width being determined the shape
of the edges to AdA of the blade are delineated by the cycloid in the plane of
the face of the propeller boss, or extending beyond this plane as may be required.
The external points or corners of the said blades may be curved or rounded, as
shown at w and d.
In the operation of "sweeping" my improved cycloidal propeller it will be
understood that I commence in the usual manner by using iron or other suitable
plates, and complete the formation of the propeller by means of templates. One
template is placed on a right or straight line in the centre of the blade, extending
from the boss to the periphery of the propeller and vertical to the axis of its
rotation. The templates for the remaining portions of the blades will be developed
by or formed from the natural curves assumed by the blade, in consequence of its
having been delineated by a cycloid in the plane of the faces. of the boss or outside
of this plane.
The position of the straight line on the blade, above mentioned, may vary
according to circumstances and the form of blade required.
One very important feature of this Invention is that I now make use of both
the "prolate" and "curtate" cycloids, as also of the "epicycloid" and "hypo-
cycloid," according to the shape. I desire my. blades to assume. I am therefore able
to impart cycloidal curves to any shaped blade, and to gain from 10. to 15 per cent.
on screws formed on the principle of the archimedean curve or screw thread; thus
the spiral of Archimedes may be regarded as an "epitrochoid." "The curve traced
out by a point, retaining a fixed position with respect to a straight line which rolls
without sliding on a circle in the same plane, as line and point may be regarded
as an epitrochoid whose generating circle has an infinite radius. Archimedes
defines this curve as "the curve traversed by a point moving uniformly along a
straight line which revolves uniformly around a centre," and the difference between
these spirals when applied to mechanical purposes is that the cycloidal screw
thread, described in the said former Specifications, reverses the curves of the
archimedean thread or spiral, that is to say, that where the archimedoan screw
thread is concave the cycloidal thread is convex, and vice versa, forming a true
"line of sines" "or companion to the cycloid."
I wish it understood that I may use any desired number of blades, of any form
or shape, governed by the cycloidal principle on my improved propellers, and these
blades may be cast or otherwise formed separately from the boss and attached
thereto by screws or other suitable means, so that in case of fracture or injury of
a blade the same may be readily removed and replaced by a new blade if necessary,
or the blades may be cast solidly with the shaft without any boss whatever. The
blades may also be made to incline aft or forward.
The boss may be of any shape or form, but the smaller the boss, consistent with
strength and safety, the more efficient will be the propeller,

page_text: The pitch of cycloidal blades may vary. The diameter, length, and pitch of
cycloidal screw propellers must depend on the weight of the vessel and the available
horse power of the engines.
Moreover, I wish it understood that I do not bind myself to any particular ortho-
graphic projection of the blades, as my cycloidal screw thread enables me to take
such portion thereof as I may deem necessary, and to shape it or cut it down until
I have left such area of blade as I may require in each individual instance.
The companion to the cycloid or curve of sines, whose hollow surface as a rule
forms the leading driving surface of the leading quadrant of my blades is obtained
as follows:-Referring to Figure 2, let AB be a fixed diameter of a circle AQB,
and through any point Q on AQB draw MQP perpendicular to ACB, and equal
to the arc AQ; the locus of this point p is the companion to the cycloid APD,
having AB as axis. If CO c, the line of centres of semi-cycloid APD, be bisected
in O, the curve passes through 0 because CO = quadrant AQC¹. Drawing pm,
Qn, perpendicular to CO c, I have mO = CO-Cm= ACAQ arc QC-
then pm Om :: Qn are QC¹ sine QCC: circumferential measurement of
QCC Hence the part ApO of the companion to the cycloid is a curve of
sines, and when produced to Op' D this latter will be found to bear precisely the
same relation to the line Oc which the part Apo bears to OC. Thus the entire
curve is a curve of sines of which the central point O is to be found in the centre
of my blades, at which point a straight edge will lie fint along the blade but not
at any other point. A straight edge when applied to a cycloidal blade at the
point O will lie flat along the surface of the said blade either from boss to peri-
phery or from edge to edge, according to the construction and form of the said
cycloidal blade.
The curve of sines or companion to the cycloid may be used to delineate both
the leading and the trailing edges of the blades of a screw propeller. Any portion
of the cycloid may also be used. In each case the horizontal section of the blade,
parallel to the axis of rotation will form a "curve of sines or companion to the
cycloid."

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Robert Henry Armit in the Great Scal Patent Office on the 14th
September 1878.
ROBERT HENRY ARMIT, of Craven Street, in the County of Middlesex, a Retired
Lieutenant of Her Majesty's Navy. "IMPROVEMENTS IN THE CONSTRUCTION OF
SHIPS' PROPELLERS AND OTHER LIKE ARTICLES WITH HELICAL SURFACES."
My Invention relates to the construction of screw propellers, fans, and other like
articles with blades formed in a peculiar manner, and is a further developement of
the principles involved in the construction of the screw propellers described in the
Specification of former Letters Patent, dated the 15th September, A.D. 1876,
No. 3618, and 9th January, A.D. 1878, No. 111.
The manufacture of screw propellers with the blades shaped, as described and
shewn in the said former Specifications, is attended with certain difficulties which
the present Invention is designed to obviate, as herein-after described.
It is well known to all familiar with the making of screw propellers that the
ordinary and most convenient manner of forming the moulds of the propeller blades,
previous to casting them, is by a process termed "sweeping." But the shape of
the propeller blades, as shewn in the said former Specifications, can only be pro-
duced in this manner with difficulty, such blades requiring the use of special
patterns, which are very costly and inconvenient. Therefore I have designed the
herein described method of forming these improved cycloidial propellers, whereby I
obviate these difficulties without sacrificing any of the advantages of the cycloidal
form of the blades, and which I will now proceed to explain.
According to my Inventions, as described in the said former Specifications, I
only used a "right cycloid," which is thus developed.
Referring to the diagram, Figure 1, if the circumference of a circle (herein-after
termed the generating circle) be rolled upon a right line until the point A in that
circumference, in contact with the said line, passes entirely around the circle and
comes again in contact with it, that point will describe a curve AGA'CA* termed
a cycloid. The line ABA² on which the generating circle rolls, and which may
equal in length the diameter of the screw, is the base of the cycloid which starts
from the axis or centre of the screw, and the diameter BA' of the said circle drawn
perpendicularly to this base at the centre or middle point of the latter is the axis
of the cycloid. Taking the said axis A of the screw as a centre, a circle B x z is
drawn around the saine, whose radius equals the distance from the said centre A
of the screw to the middle point of the base of the cycloid, and this circle will
represent the diameter of the propeller.
The width of the blade is the chord of an arc wzd or part of the said circle
representing the diameter of the screw, and this width being determined the shape
of the edges wA and dA of the blade is delineated by the cycloid in the plane of
the face of the propeller boss or extending beyond this plane, as may be required.
The external points or corners of the said blades may be curved or rounded, as
shewn at w and d.
In the operation of sweeping my improved cycloidal propeller it will be under-
stood that I commence in the usual manner by using iron or other suitable plates,
and complete the formation of the propeller by means of templates. One template
is placed on a right or straight line in the centre of the blade, extending from the
boss to the periphery of the propeller and vertical to the axis of its rotation. The
templates for the remaining portions of the blades will be developed by or formed
from the natural curves assumed by the blade, in consequence of its having been
delineated by a cycloid in the plane of the faces of the boss or outside of this
plane.
The position of the straight line on the blade above mentioned may vary
according to circumstances and the form of blade required.

page_text: One very important feature of this Invention is that I now make use of both
the "prolate" and "curtate" cycloids, as also of the "epicycloid" and "hypo-
cycloid," according to the shape I desire my blades to assume. I am therefore
able to impart cycloidal curves to any shaped blade, and to gain from 10 to 15 per
cent. on screws formed on the principle of the archimedean curve or screw thread ;
thus the spiral of Archimedes may be regarded as an "epitrochoid." The curve
traced out by a point, retaining a fixed position with respect to a straight line,
which rolls without sliding on a circle in the same plane as line and point may be
regarded as an epitrochoid, whose generating circle has an infinite radius. Archi-
medes defines this curve as "the curve traversed by a point moving uniformly
along a straight line which revolves uniformly around a centre," and the difference
between these spirals when applied to mechanical purposes is that the cycloidal
screw thread, described in the said former Specifications, reverses the curves of the
archimedean thread or spiral, that is to say, that where the archimedean screw
thread is concave the cycloidal thread is convex, and vice versa, forming a true
"line of sines or companion to the cycloid."
I wish it understood that I may use any desired number of blades, of any form
or shape, governed by the cycloidal principle on my improved propellers, and these
blades may be cast or otherwise formed separately from the boss and attached
thereto by screws or other suitable means, so that in case of fracture or injury of
a blade the same may be readily removed and replaced by a new blade if necessary,
or the blades may be cast solidly with the shaft without any boss whatever. The
blades may also be made to incline aft or forward.
The boss may be of any shape or form, but the smaller the boss, consistent with
strength and safety, the more efficient will be the propeller.
The pitch of cycloidal blades may vary. The diameter, length, and pitch of
cycloidal screw propellers must depend on the weight of the vessel and the available
horse power of the engines.
Moreover, I wish it understood, that I do not limit myself to any particular
orthographic projection of the blades, as my cycloidal screw thread enables me
to take such portion thereof as I may deem necessary, and to shape it or cut it
down until I have left such area of blade as I may require in each individual
instance.
-
The companion to the cycloid or curve of sines, whose hollow surface as a rule
forms the leading driving surface of the leading quadrant of my blades, is obtained
as follows:-Referring to Figure 2, which is a diagram illustrating this method of
forining the blades, let AB be a fixed diameter of a circle AQB, and through any
point on AQB draw MQP perpendicular to ACB and equal to the arc AQ; the
locus of this point p is the companion to the cycloid APD, having AB as axis.
If CO c, the line of centres of semi-cycloid APD, be bisected in O, the curve passes
through O, because CO = quadrant AQC. Drawing pMQn perpendicular to
CO c, have m O = CO - Cm = AC¹ AQ=arc QC then pm: Om :: Qn :
arc QC sin. QCC': circumferential measurement of QCC¹. Hence the part
Ap of the companion to the cycloid is a curve of sines, and when produced
to Op D this latter will be found to bear precisely the same relation to the line Oc
which the part ApО bears to OC. Thus the entire curve is a curve of sines of
which the central point O is to be found in the centre of my blades, at which point
a straight edge will lie flat along the blade but not at any other point. A
straight edge when applied to a cycloidal blade at the point Ŏ will lie flat along
the surface of the said blade, either from boss to periphery or from edge to edge,
according to the construction and form of the said cycloidal blade.
The curve of sines or companion to the cycloid may be used to delineate both
the leading and the trailing edges of the blades of a screw propeller. Any portion
of the cycloid may also be used. In each case the horizontal section of the blade,
parallel to the axis of rotation, will form a curve of sines or companion to the
cycloid.

page_text: Having thus fully described the said Invention, and the manner of performing
the same, I wish it understood that I claim,-
First. In the construction of a cycloidal screw propeller, or other article or object
which has a series of cycloidal blades or surfaces, the adaptation of the process of
"sweeping" to the formation of the moulds of the blades or surfaces, as above set
forth.
Second. In the manufacture of a screw propeller, or fan, or similar article, the
method of constructing and designing the same, which comprises or involves the
employment of the companion to the cycloid or curve of sines, as illustrated in
Figure 2 of the Drawing, and as above set forth.
In witness whereof, I, the said Robert Henry Armit, have hereunto set my
hand and seal, this Fourteenth day of September, in the year of our Lord
One thousand eight hundred and seventy-eight.