CERTAIN IMPROVEMENTS IN THE CON- STRUCTION OF SCREW PROPELLERS
Year: 1854

page_text: LETTERS PATENT to James Pritchard, of Portsea, in the County of
Hampshire, for the Invention of "CERTAIN IMPROVEMENTS IN THE CON-
STRUCTION OF SCREW PROPELLERS."
Sealed the 12th June 1855, and dated the 18th December 1854.
PROVISIONAL SPECIFICATION left by the said James Pritchard at
the Office of the Commissioners of Patents, with his Petition, on the
18th December 1854.
I, JAMES PRITCHARD, of Portsea, in the County of Hampshire, do hereby
declare the nature of the said Invention for "CERTAIN IMPROVEMENTS IN THE
CONSTRUCTION OF SCREW PROPELLERS" to be as follows:-
These improvements relate to a new form or construction of screw propellers,
differing from those at present in use.
In constructing a screw propeller according to my Invention, in place of
every part of the several blades being perpendicular or at right angles to the
central axis, as those at present in use (and consequently a true screw, and
would work in a thread cut in wood or iron), I construct the propeller blades
with the middle portion only of each blade perpendicular, or at right angles
to the central axis, in such manner that the forward and back edges of the
blade of a screw, in place of being perpendicular to the central axis, spring
horizontally from the periphery of the axis of the propeller, the forward edge
springing from one side of the axis, and the other or back edge springing.

page_text: from the other or opposite side of the axis, and consequently at a point or
points distant from the central axis. This arrangement gives a twist to the
blade of the propeller, which causes each face or propelling side of the blade
from the axis to the periphery to be concave, while the blade from the forward
edge to the back edge is convex, and consequently not a true screw, and would
not work in a thread cut in wood or iron. The forward and back edges of
each blade are of a curved form, each edge being part of a circle struck from
or upon a line drawn parallel to the outer periphery of each blade, such line
being between the outer periphery and the root of the blade where it joins
the axis of the propeller, being longest from the forward edge to the back
edge in that part of the blade where the line is placed. I make each blade
strong in substance near the axis, and diminish the same to a thin edge
towards the outer periphery of the blade, and towards the forward and back
edges of the same.
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said James Pritchard in the Great Seal Patent Office on the
18th June 1855.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JAMES
PRITCHARD, of Portsea, in the County of Hampshire, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Eighteenth day of December, in the year of our
Lord One thousand eight hundred and fifty-four, in the eighteenth year of
Her reign, did, for Herself, Her heirs and successors, give and grant unto
me, the said James Pritchard, Her special licence that I, the said James
Pritchard, my executors, administrators, and assigns, or such others as
I, the said James Pritchard, my executors, administrators, and assigns,
should at any time agree with, and no others, from time to time and at
all times thereafter during the term therein expressed, should and lawfully
might make, use, exercise, and vend, within the United Kingdom of Great
Britain and Ireland, the Channel Islands, and Isle of Man, an Invention
for "CERTAIN IMPROVEMENTS IN THE CONSTRUCTION OF SCREW PROPELLERS," upon
the condition (amongst others) that I, the said Jaines Pritchard, by an
instrument in writing under my hand and seal, should particularly describe
and ascertain the nature of the said Invention, and in what manner the
same was to be performed, and cause the same to be filed in the Great Seal

page_text: of the said Letters Patent.
NOW KNOW YE, that I, the said James Pritchard, do hereby declare
the nature of my said Invention, and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement, that is to say:-
My Invention has for its object a better construction or form of screw pro-
pellers than those at present in use.
In order that, my Invention may be fully understood, I have hereunto
annexed a Sheet of Drawings, with letters of reference marked thereon,
corresponding with letters of reference in this Specification. The same letters
refer to the same parts in the several Figures. And in order to show my
improved propeller more clearly, I have described it somewhat virtually, inas-
much as that it appears in the following description to be made in portions
or sections, whereas in fact it is made in one casting.
The way I construct my improved screw propeller is shown in the Drawing
Figure One, the black lines showing the ordinary screw propeller, and the red
lines showing my improved screw propeller. I first form each of the blades of
the propeller as a true screw A, B, C, D, Figure One, of larger dimensions
than is required for my improved screw. I then cut away the section or por-
tion of the blade G, H, I, J, near the root, as shown in the Drawing, Figure
One, and bring the blade from which the section or part G, H, I, J, has been
taken down upon the periphery of the axis of the propeller at I, J, causing the
blade to over-reach the line of radius A, L, B, L, C, L, and D, L, as shown in
Figures Two and Three. Having brought down the blade, reduced as herein
described, to the axis of the propeller, I proceed to form the forward edge E, G,
of the reduced blade E, F, G, H, each of which I construct in a curved form,
each edge being part of a circle struck from or upon a line Q, R, drawn
parallel to the periphery of each blade, such line being between the periphery
E, F, and the root of the blade G, H, where it joins the axis, the propeller
being longest in that part of the blade Q, R, where the line is placed, and
the periphery E, F, of the blade E, F, G, H, is at every point equidistant
from the central axis, as shown in Figure One. The forward and back
edges, and the periphery of the blade are gradually reduced from the centre
to a thin edge, and P, P, is the only part at right angles with the central axis.
This arrangement gives a twist to the blade of the screw propeller the same
as in a true screw, with the exception of the section or portion G, H, I, J, as
herein-before described, being taken away, and thereby reducing the leverage
of the blades and saving the power of the engine.

page_text: Having thus described the nature of my said Invention, and in what manner
the same is to be constructed, I declare that what I claim as my Invention is,
first, the cutting out or removing a section or portion of each blade, and
bringing the blade down again upon the axis of the propeller, in the manner
herein described.
Secondly, I claim the circular form of the front and back edges, as con-
structed in the manner herein described.
In witness whereof, the said James Pritchard has hereunto set his
hand and seal, this Sixteenth day of June, in the year of our Lord
1855..
JAMES PRITCHARD.