IMPROVEMENTS IN SCREW PROPELLERS FOR PRO- PELLING VESSELS THROUGH THE WATER
Year: 1876

page_text: LETTERS PATENT to Robert Henry Armit, of No. 8, Victoria Chambers, West-
minster, in the County of Middlesex, a retired Lieutenant in Her Majesty's
Navy, for the Invention of "IMPROVEMENTS IN SCREW PROPELLERS FOR PRO-
PELLING VESSELS THROUGH THE WATER."
Sealed the 13th March 1877, and dated the 15th September 1876.
PROVISIONAL SPECIFICATION left by the said Robert Henry Armit at the
Office of the Commissioners of Patents on the 15th September 1876.
ROBERT HENRY ARMIT, of No. 8, Victoria Chambers, Westminster, in the County
of Middlesex, a retired Lieutenant in Her Majesty's Navy. "IMPROVEMENTS IN
SCREW PROPELLERS FOR PROPELLING VESSELS THROUGH THE WATER."
I construct a screw propeller according to my Invention as shewn in Figures 1
and 2, the former being a perspective view, and the latter an end view thereof; and
Figure 3 shews a sectional view. The parts of the blades a x b, c y d, represented
by the letters a xz, cy z, are both of the same shape or nearly so, as are also
the portions b xz, and dy z, of the same screw. These parts should be either
both concave or both convex, that is to say, when the part a x 2, is concave,
the part cy z, should likewise be concave, and when the part xz b, is convex,
the part dys, should likewise be convex, or vice versa, as shewn in Figure 1.
The points a, b, c, and d, have an outward curve or bend to regulate the path of
the screw, and increase its hold on the water. On the screw being made to revolve
the point a cleaves the water, which is then thrown by the surface a xz on to the
surface b xz, and passing over the edge or rim bz forms the negative slip of the
water, which is taken up by the part cy z of the blade c y d.
In some cases I flange or partly flange the periphery or outer edge of the blades.
0 SPECIFICATION in pursuance of the conditions of the Letters Patent filed by the
said Robert Henry Armit in the Great Seal Patent Office on the 15th March
1877.
ROBERT HENRY ARMIT, of No. 8, Victoria Chambers, Westminster, in the County
of Middlesex, a retired Lieutenant in Her Majesty's Navy. "IMPROVEMENTS IN
SCREW PROPELLERS FOR PROPELLING VESSELS THROUGH THE WATER."

page_text: My Invention of improvements in screw propellers for propelling vessels through
the water consists in the construction thereof with blades of the form, and so
inclined round the boss thereof that the water through which the vessel is driven
by means of the rotation of the screw shall have a good passage through the
convolution or spiral turn of the screw in a line of the axis of rotation as shewn in
Figures 2, 3, 4, 5, 6, 7, 8, 9, and 10 in the Sheet of Drawings filed herewith. The
parts of the two blades a, a, b, and c, y, d, Figure 1, as represented by the letters
, x, z, c, y, z, should be both of the same shape or nearly so, as should also the
parts b, a, z, and d, y, z, of the same screw. Those opposite parts or quadrants should
be either both concave or both convex, that is to say, when the part a, x, z, is 1
concave, the part c, y, z, should be likewise concave, and when the part x, z, b, is
convex the part d, y, z, should likewise be convex, or vice versa, as shewn in
Figure 1. On the screw being made to revolve the point a cleaves the water which
is then thrown by the surface u, x, z, on to the surface b, c, z, and passes away
over the edge or rim b, z.
In some cases I flange or partly flange the leading driving periphery or outer edge
of the blades.
Figs. 2, 3, and 4, in the Sheet of Drawings are respectively end, side, and cross
sectional views of a screw propeller with two blades constructed according to my
said Invention, whose length of boss should equal, but not exceed the diameter of
the screw.
Figs. 5, 6, and 7, are respectively end, side, and cross sectional views of the screw
propeller according to my said Invention, whose length of boss is shewn to be equal
to one-half part of the diameter of the screw propeller, but may be more or less
according to the requirements of the vessel to which it is attached.
Fig. 8 is an end view of a three bladed propeller according to my said Invention,
whose length of boss is shewn to be equal to one-third part of the diameter thereof,
but may be more or less as required.
Fig. 9 is an end view of a four bladed screw propeller according to my said
Invention, whose length of boss is shewn to be equal to one-fourth part of the
diameter thereof, but may be more or less as required.
Fig. 10 is a sectional view of a two bladed screw propeller according to my said
Invention, in which the face of the driving side of the leading part of the blade is
convex, while the driving part of the following quadrant is concave.
In Fig. 10 I have intentionally shewn the blades so formed that they would
enter the water with a convex surface instead of a concave surface as shewn in
Figs. 2, 3, 4, 5, 6, 7, 8, and 9, but in all cases I prefer to have the face of the
entering quadrant of the blade concave on its driving side.
In forming the pattern of or in setting out for casting a screw propeller according
to my said Invention, I describe a circle on any given radius, and at the centre of
that circle I set out the face of the boss or axis z of the propeller proportioned to
any given scale, such as the diameter x, y', in Figure 1; I then divide the major
circle into quadrants, the four radii uniting at the centre of the boss. From
the points x, y, I describe four seini-cycloids in such manner that one semi-
cycloidal are lies in each quadrant and joins the circumference of the major circle
so as to divide it into four involutes, or equal semi-cycloids in opposite directions,
the extremity of the latter being in contact with the vertex of the former. This
construction causes each blade of the screw propeller to terminate in points, and
when seen in end view the blades will be found to assume a crescent shape, owing
to the cycloidal arc formed of the two involutes united at the points x and y
form the periphery of each blade. The opposite semi-cycloids springing from
each side of the boss leave a circular open space w-w¹, on each side of the said boss,
through which space the water passes away in a line with the axis of rotation when
the ship is set in motion by the rotation of the screw.
to
The pitch of the blade of my screw propeller can be determined by multiplying
the length of radius by or 3,-1416. The length to be multiplied by this number
can be readily ascertained by measuring the horizontal distance between any two

page_text: facing points of blades lying on the same side of the axis of rotation. The pitch
may vary, and in some cases be made courser at the root than at the periphery of
the blade. The diameter of the screw propeller must necessarily be regulated by the
tonnage of the vessel, and the horse power of the engines.
Fig. 1 is given to illustrate the method of setting out my screw propeller. In
this Figure Z is the centre of the shaft or boss, from the faces of which spring the
cycloidal ares y', a, and x¹, c, on the one side and those marked a, b, and y¹, d, on
the other side. The two involutes a, a, b, x, join at the point x to form the
cycloidal arc a, a, b. In like manner I construct the blade c, y, d, and I so set out
a two bladed screw propeller having waterways w, w, to allow of the discharge of
the water through which the propeller is made to pass.
Having described the nature of my said Invention and the manner in which the
same may be performed, I wish it to be understood that I claim as my Invention
screw propellers constructed according to the description herein-before contained,
and as shewn in the said Sheet of Drawings filed herewith.
In witness whereof, I, the said Robert Henry Armit, have hereunto set my
hand and seal, this Fourteenth day of March, in the year of our Lord, One.
thousand eight hundred and seventy-seven.
KUB. H. ARMIT.
(L.S.