AN IMPROVED SCREW PROPELLER FOR VESSELS
Year: 1873

page_text: LETTERS PATENT to John Isaac Thornycroft, of the Firm of John
I. Thornycroft and Co., Steam Yacht and Launch Builders, of
Church, Wharf, Chiswick, in the County of Middlesex, for the In-
PROVISIONAL SPECIFICATION left by the said John Isaac Thornycroft
at the Office of the Commissioners of Patents, with his Petition,
on the 31st October 1873.
I, JOHN ISAAC THORNYCROFT, of the Firm of John I. Thornycroft
and Co., Steam Yacht and Launch Builders, of Church Wharf, Chiswick,
in the County of Middlesex, do hereby declare the nature of the said
Invention for "AN IMPROVED SCREW PROPELLER FOR VESSELS," to be as
follows:-
.
This Invention relates to screw propellers of the class in which the
blades are inclined in a rearward direction, their outer extremities ter- :
:

page_text: minating to the rearward of their roots or points of junction with the
boss of the propeller.
My propeller is designed to counteract the centrifugal tendency of
the water, driving it backward in what may be termed hollow cylinders
or concentric annular volumes. To this end I curve each blade in the
direction of its length in such manner that assuming the blade to be
cut in a plane passing through the axis of the propeller, and through
the centre of the blade in a direction parallel to the said axis, the
section of the blade will be convex on its driving face, that is, the face
towards the axis and concave on the reverse face, that is, on the outer
side or surface, and I make this curvature of the blade such as throughout
its length to counteract the centrifugal tendency of the water.
In the direction of the screw's diameter I gradually increase the
pitch of the blades from the boss towards the centres of the blades,
at or about which I give them the greatest pitch, and thence I again
gradually diminish the pitch towards the outer ends or extremities of
the blades, and in the direction of the screw's axis I increase the
pitch of the blades towards the after or rearward end of the screw.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said John Isaac Thornycroft in the Great Seal Patent
Office on the 30th April 1874.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
ISAAC THORNYCROFT, of the Firm of John I. Thornycroft and Co.,
Steam Yacht and Launch Builders, of Church Wharf, Chiswick, in the
County of Middlesex, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Thirty-first day of October, 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 Isaac Thornycroft,
Her special license that I, the said John Isaac Thornycroft, my executors,
administrators, and assigns, or such others as I, the said John Isaac
Thornycroft, my executors, administrators, and assigns, should at any

page_text: 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 the Isle of Man,
an Invention for "AN IMPROVED SCREW PROPELLER FOR VESSELS," upon the
condition (amongst others) that I, the said John Isaac Thornycroft,
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 the same
was to be performed, and cause the same to 'be filed in the Great Scal
Patent Office within six calendar months next and immediately after
the date of the said Letters Patent.
NOW KNOW YE, that I, the said John Isaac Thornycroft, do hereby
declare the nature of the said Invention, and in what manner the same
is to be performed, to be particularly described and ascertained in and
by the following statement thereof, and the Drawings therein referred to
(that is to say) :-
This Invention relates to screw propellers of the class in which the
blades are inclined in a rearward direction, their outer extremities ter-
minating to tho rearward of their roots or points of junction with the
boss of the propeller:
My propeller is designed to counteract the centrifugal tendency of
the water which it drives backward in what may be termed hollow
cylinders or concentric annular volumes. To this end I curve each
blade in the direction of its length in such manner that assuming
the blade to be cut in a plane passing through the axis of the pro-
peller, and through the centre of the blade in a direction parallel to
the said axis, the section of the blade will be convex on its driving.
face, that is, on the outer, or in other words, the after side or surface,
and I make this curvature of the blade such as throughout its longth
to counteract the centrifugal tendency of the water.
In the direction of the screw's diameter I gradually increase the
pitch of the blades from the boss towards the centres of the blades,
at or about which I give them the greatest pitch, and thence I again
gradually diminish the pitch towards the outer ends or extremities of

page_text: .
the blades, and in the direction of the screw's axis I increase the pitch
of the blades towards the after or rearward end of the screw.
My reason for adopting this form of blade is as follows:-The surface
of an ordinary screw propeller at any point tends to produce motion
of the water in a direction which may be resolved into two components,
the one being lateral or at right angles to the axis of the propeller, and
the other parallel to the axis. The former motion is entirely useless in
propelling the vessel, so that the large proportion of power absorbed in
its production is dissipated.
In my screw propeller the propelling surface being inclined towards
the axis of the propeller, the whole or the greater portion of this lateral
motion is prevented, and the water is thrown backwards in concentric
cylindrical volumes, as already stated. Thus the efficiency of the pro-
peller is greatly increased, and the propelling power of the vessel is
economised. This result has been aimed at before, and it has been
proposed to accomplish it in various ways differing from my Invention,
as for example, in the Earl of Dundonald's propeller, wherein the blades
are thrown backward, but are not curved; but I know of no plan by
which the object in view has been successfully attained.
By curving the blade backwards according to my Invention I obviate
the tendency of the water to lateral, motion, which in the ordinary
screw propeller varies at different points in the blade, being greatest
near the axis and least at the tips.
In carrying out my Invention the method by which I determine
the form of each blade of the propeller is as follows, reference being
had to Figs. 1 of the accompanying Drawings which represent (half
the full size) portions of a three-bladed propeller five feet in diameter:
-The diameter of the propeller and the intended speed of the vessel
being given I draw two straight lines A B and CD at right angles
to each other, AB being the axis of the propeller, and the motion of
the vessel being supposed to be in the direction from A to B. From
the point E where these lines intersect I measure E F towards A,
making EF equal to the distance moved by the vessel per revolution
of the propeller divided by 2=6.28 (that is, the ratio of the cir-
cumference of a circle to its radius). I then lay off in ED, EG n,
which I make equal to the semi-diameter of the screw, and I divide

page_text: this distance, as indicated at G, G, G, G³, into a number of equal
parts depending upon the diameter of the propeller, preferring in the
present case about ten equal parts although less are sliewn in the
Drawing to avoid confusion. Through the points G, G¹, G³, G³, G n,
and the point F I draw the lines F G, F G¹; F¯Gº, F G³, F G n, which
lines I produce to the points I, I, I, I³, In; these lines give the
inclination of the several points of the blade at the leading edge';
I then lay off on the line A. B, EL, EL, EL, ELn, from E towards A,
making EL, EL, EL³, &c., equal to the distance moved by the vessel
per revolution of the screw plus the expected slip at the several sections
divided by 26-28, as before. The points L, L, L, &c., projected
to the points K, K³, K², K³, K n, are so arranged that a fair curve may
be drawn through them giving the greatest pitch at about the centre
of the blade, it being understood that K is simply the projection of
the point L, K¹ of L', K² of L³, K³ of L', and Kn of Ln, to the radial
distance to which the corresponding section belongs.
་
:
Lines drawn through LG, L' G¹, L² G², Ln. Gn, give the inclination
of the after edge of the screw blade, the two lines defining the in
clination of the leading and following edges are joined by a curve
which has for tangents these lines at the respective edges of the
blade. These lines now found are developements of a cylindrical
section of the driving face of the blade. They are all most conveniently
found on the line CD as centre line, but when found must be moved:
forward or aft until the part cut by the line C D is cut by the line
MGN, at the same time maintaining the same radial distance and
5 angular position to the axis. To find the line M.G'N I measure off,
from E towards B, EH, making E H equal to three-fourths of E F.. From
HI draw lines HG¹, HG², HG, H Gn, cutting the line CD. The first.
of these lines is a tangent to the curve at the point G³, and the others.
are parallel to tangents to the curve at the points G2, G3, Gn.
In order to form the curve from these line I bisect the interval
G1, G2, at the point O¹, and through that point I draw a line parallel.
to the axis, cutting the tangent in the point P¹; I then draw a line
P¹, P, parallel to the line H G2 cutting I', G³, produced in the point Q².
The point Q is a point in the curve. Similarly I subdivide G², G³,
5 in the point Qs, and through that point I draw a line parallel to the
axis, cutting the line P Q produced in the point P. Through this,

page_text: point I draw another line P³, P³, parallel to H G³, cutting I³, G³,
produced in the point Q. The point Q is another point in the curve.
The remainder of the curve is similarly found. In this way the forms
of the face of the blade at a number of cylindrical sections of the pro-
peller are defined, and these are to be joined by a fair surface, and 5
strengthened by giving the backs an increased curvature, as shewn in
the Drawings, the whole being joined by fair surfaces.
The blades are also strengthened at the root by modifying the curve
MGN between the point G¹ and the root. Thus a form of screw
blade is obtained which I have found to give most satisfactory results. 1
With a propeller of the full size and form shewn. I have propelled a
boat 87 feet long by 12 feet beam (extreme dimensions) on the river
at a mean speed of about 24½ miles per hour.
Figs. 2 represent to half the full size portions of one of my improved
propellers three feet in diameter. I have found this propeller to answer 15
well in a boat of the extreme dimensions of 58 feet long by 7 feet 6
inches beam.
This propeller will be readily understood from the explanation already
given without further description.
By the use of my improved propeller the power of the engines is 20
materially economised, so that with a given consumption of fuel a
greater speed is attained than is the case with any other form of pro-
peller so far as I know, and with my propeller vibration is obviated
considerably diminished as it enables the engines to run smoothly.
or
Havin gdescribed my said Invention of "An Improved Screw Propeller 25
for Vessels," and having explained the manner of carrying the same.
into practical effect, I hereby declare that what I consider to be novel
and original, and therefore claim as the Invention secured to me, by
the herein-before in part recited Letters Patent is, the improved form
of screw propeller hercin-before described and illustrated in the Drawings. 30
I claim particularly the curving each blade in the direction of its length
in such manner as that assuming the blade to be cut in a plane passing
through the axis of the propeller, and through the centre of the blade
in a direction parallel to the said axis, the section of the blade will be
convex on its driving face, and the gradually increasing the pitch of 35
,

page_text: the blades in the direction of the screw's diameter from the boss towards
the centres of the blades, and thence gradually diminishing the pitch
towards the outer ends or extremities of the blades, the pitch of the
blades being increased in the direction of the screw's axis towards the
after or rearward end of the screw.
the blades in the direction of the Screw Sameter from USS Was
the centres of the blades, and thence gradually diminishing the pitch
towards the outer ends or extremities of the blades, the pitch of the
blades being increased in the direction of the screw's axis towards the
after or rearward end of the screw.
In witness whereof, I, the said John Isaac Thornycroft, have here-
unto set my hand and seal, this 29th day of April, in the year of
our Lord One thousand eight hundred and seventy-four.
JOHN ISAAC THORNYCROFT, C.E. (L.S.)