IMPROVEMENTS IN SCREW PROPELLERS
Year: 1873

page_text: LETTERS PATENT to John Purves, of Liverpool, in the County of
Lancaster, Engineer, for the Invention of "IMPROVEMENTS IN SCREW
PROPELLERS."
+
Sealed the 13th February 1874, and dated the 17th December 1873.
PROVISIONAL SPECIFICATION left by the said John Purves at the
Office of the Commissioners of Patents, with his Petition, on the
17th December 1873.
I, JOHN PURVES, of Liverpool, in the County of Lancaster, Engineer,
do hereby declare the nature of the said Invention for "IMPROVEMENTS”.
IN SCREW PROPELLERS," to be as follows:-
When screw propellers are of large or considerable size, as those for
ocean steamers, the blades require to have great strength at the root,
sometimes as much as twelve inches in thickness for a blade about
four feet wide at the root. The said thickness has heretofore been
obtained by making the section of each blade the segment of a circle,
the face of said blade being the straight chord and the back of the
blade being the arc of the segment. To move blades so constructed

page_text: through the water at a speed much greater than that of the ship
requires a great expenditure and consequent loss of power.
Now the object of my Invention is to reduce the amount of the loss
of power.
For that purpose I reduce the cleavage obstruction by
forming the blades in such manner that the necessary thickness for
strength is divided between the two sides of the blade instead of being
all on the back of the blade, that is to say, I make both surfaces convex,
and thereby secure efficient propulsion. And as it is only near the boss
that the greater thickness need exist I extend the double convexity
only to about mid length of the blade.
I have obtained the most satisfactory results by dividing the thickness
at the root between the sides in the proportion of six-tenths on the
back to four-tenths on the face. The curvature on the face is by
preference run out to be nothing at about mid-length of the blade.
The divisions mentioned are exclusive of the fillet, which is added
on both sides.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said John Purves in the Great Seal Patent Office on
the 17th June 1874.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, Joпn
PURVES, of Liverpool, in the County of Lancaster, Engincer, send
greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Seventeenth day of December, in the year of
our Lord One thousand eight hundred and seventy-three, in the thirty-
seventh year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said John Purves, Her special license
that I, the said John Purves, my executors, administrators, and assigns,
or such others as I, the said John Purves, 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 "IMPROVEMENTS IN SCREW PROPELLERS,"

page_text: upon the condition (amongst others) that I, the said John Purves, my
executors or administrators, by an instrument in writing under my,
or their, or one of their hands and seals, should particularly describe
and ascertain the nature of the said Invention, and in what manner
5 the same was to be performed, and cause the same to be filed in the
Great Seal Patent Office within six calendar months next and imme-
diately after the date of the said Letters Patent.
NOW KNOW YE, that I, the said Jolin Purves, do hereby declare
the nature of my said Invention, and in what manner the same is to be
Operformed, to be particularly described and ascertained in and by the
following statement, reference being had to the accompanying Sheet of
illustrative Drawings and to the figures and letters marked thereon, that
is to say.:-
When screw propellers are of large or considerable size, as those for
ocean steamers, the blades require to have great strength at the root,
-sometimes as much as twelve inches in thickness for a blade about four
feet wide at the root. The said thickness has heretofore been obtained
by making the section of each blade the segment of a circle, the face.
of said blade being the straight chord and the back of the blade being.
the arc of the segment. To move. blades so constructed through the
water at a speed much greater than that of the ship requires a great
expenditure and consequent loss of power.
Now the object of my Invention is to reduce the amount of the loss
of power. For that purpose I reduce the cleavage obstruction by
forming the blades in such manner that the necessary thickness for
strength is divided between the two sides of the blade instead of being
all on the back of the blade, that is to say, I make both surfaces convex,
and thereby secure efficient propulsion. And as it is only near the boss
that the greater thickness need exist I extend the double convexity only
to about mid-length of the blade.
I have obtained the most satisfactory results by dividing the thickness
at the root between the sides in the proportion of six-tenths on the
back to four-tenths on the face. The curvature on the face is by
preference run out to be nothing at about mid-length of the blade.
The divisions mentioned are exclusive of the fillet, which is added
1

page_text: DESCRIPTION OF THE DRAWINGS.
Fig. 1 is a front view of the boss and one blade of a propeller
constructed in accordance with my Invention; the outside line A shows
the actual breadth of the blade, the inside line B shows the apparent
breadth; Fig. 2 is a vertical section through the blade and boss; 5
Figs. 3, 4, 5, 6, 7, 8, and 9, are sections, showing the shape and angular
position of the blade at the lines C, D, E, F, G, H, and I.
The face of the blade at the root is formed convex, in the proportion
of four at the front to six at the back, the said convexity running out
to nothing about midway of the length of the blade; a is the convex 1
portion added to the face of the blade.
Although I have shown and described the convexity of the face and
back of the blade to be in the proportion of four to six, this proportion
may be varied without departing from my Invention.
Having now described the nature of the said Invention, and par- 1
ticularized the same in such manner that others will be enabled to use
my said Invention, I claim, constructing screw propellers, substantially
as described and illustrated for the purpose set forth.
In witness whereof, I, the said John Purves, have hereunto set.
my hand and seal, the Thirteenth day of June in the year of our 2
Lord One thousand eight hundred and seventy-four.
In the presence of
JAMES JOHNSON,
Patent Office,
Clayton Square,
Liverpool.
JOHN PURVES. (L.S.)