IMPROVEMENTS IN SCREW PROPELLERS
Year: 1882

page_text: PROVISIONAL SPECIFICATION left by the said Robert Duncan at the Office
of the Commissioners of Patents on the 20th July 1882.
ROBERT DUNCAN, of Glasgow, in the county of Lanark, North Britain;
"IMPROVEMENTS IN SCREW PROPELLERS," :
This Invention which relates to improvements in screw propellers for steam
ships, has for its object the formation or construction of the blades of such
propellers in such a manner as to obtain increased efficiency or propelling effect.
According to one improvement of the present Invention each blade of the
propeller is formed for a considerable part of its length outwards from the root or
boss by an approximately straight radial line; and towards the point or outer end
the blade is formed with a quick forward curve, that is a curve whose radius
rapidly decreases as it approaches the extreme point, and which is concave to the
driving side. The blade is so formed for the purpose of giving increased pressure or
effect at the point of the blade, and to cause it, in passing through the, water, to
force or throw back the water in an approximately straight line parallel to and
abaft the keel of the ship.
By a further improvement this formation or construction of the propeller blade
with a forward curved point is combined with a blade of varying pitch from front
to back or from the boss to the tip, the pitch of the screw being by preference
made to increase from front to back.
In each case the improved form of blades may be cast in a piece with the
usual central boss, or separately cast or formed, and bolted or otherwise fixed to
the boss.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Robert Duncan in the Great Seal Patent Office on the 17th January
1883.
ROBERT DUNCAN, of Glasgow, in the County of Lanark, North Britain
"IMPROVEMENTS IN SCREW PROPELLERS.'
"
My said Invention, which relates to improvements in screw propellers for steam
ships, has for its object the formation or construction of the blades of such pro-
pellers in such a manner as to obtain increased efficiency or propelling effect.
According to one improvement of the present Invention, each blade of the pro-
peller is formed for a considerable part of its length outwards from the root or
boss by an approximately straight radial line; and towards the point or outer end.
the blade is formed with a quick curve. This curve may be approximately uniform,
as the arc of a comparatively small circle, or be described by a radius which rapidly
decreases as it approaches the extreme point, and is, in either case, concave to the
driving side. The blade is so formed for the purpose of giving increased pressure
or effect at the point of the blade, and to cause it in passing through the water, to
force or throw back the water in appoximately straight lines parallel to and abaft
the keel of the ship.
•
By a further improvement this formation. or construction of the propeller blade
with a curved point, is combined with a blade of varying pitch from front to back,
or from the boss to the tip, the pitch of the screw being by preference made to
increase from front to back.
My said Invention is illustrated by the accompanying Drawings, which show
several modifications of screw propellers constructed in accordance with my
improvements. Figures 1 and 2 are respectively a longitudinal vertical section and
an end elevation of one modification of the improved screw propeller. In this
propeller the blades A are made approximately straight from the root or boss B to
near the point where they are formed with a rapid curve A¹. This curve may be
uniform, that is, part of a circle, or it may be approximately so, with the radius of
the curve diminishing as it appoaches the extreme point of the blade. The arrows
show the direction of rotation of the propeller, and of the forward motion of the
vessel to which it is fitted. Figures 3 and 4 are similar views of a propeller in
which the curved part A¹ commences about the mid-length of the blade, and
Figures 5 and 6 show a propeller in which the curved part A¹ commence near the
root of the blade. The curve is described with a diminishing radius towards the
point of the blade. In each of these examples the developing radius is shown
perpendicular to the axis of the propeller shaft C.
Figures 7, 8, 9, 10; and 11, 12, are sections and end elevations of improved
propellers corresponding in regard to the form and extent of curve, respectively to
the propellers illustrated by Figures 1, 2; 3, 4; and 5, 6. In the latter three
examples however the developing radius in each case forms an obtuse angle with
the axis of the propeller shaft at the forward side.
Figure 13 shows the form of developing radius which I, by preference, employ in
forming the propeller blades. The curvature begins at about three fourths of the
length of the blade from the root or boss, and has an extreme departure from the radial
line of about the same per centage of the length of the blade, as the estimated slip of
the vessel to which it is fitted, is of the total speed of propeller without slip. The
curvature is similar in nature to that known as the curve of sines, having its
smallest radius near the point.
Propellers of the improved forms shown may have the blades formed with the
pitch of the screw varying from front to back, or from the boss to the tip, the

page_text: pitch being by preference made to increase from front to back, that is from
the forward edge to the following edge of the blade. The method of giving a
varying pitch to the blades is well understood as being commonly applied to the
ordinary forms of propeller blades, and is not therefore illustrated by the drawings.
The improved form of propeller blades may be cast in a piece with the central
boss, as is indicated in the Drawings, or separately cast or formed, and bolted or
otherwise fixed to the boss; and the propeller may be formed with two, three, or
more blades as usual.
What I claim as my Invention is,―
First. Forming the blades of screw propellers to extend outwards from the boss
in an approximately radial line and terminating in a rapid curve concave to the
driving side, substantially as and for the purpose herein set forth.
Second. Forming screw propeller blades so as to combine a radial part and rapid
curved point with a varying pitch of screw substantially as herein set forth.
In Witness Whereof I the said Robert Duncan have hereunto set my hand
and seal this Sixteenth day of January, in the year of our Lord One
thousand eight hundred and eighty-three.
.Witness
0
WALLACE FAIRWEATHER C.E.
of the firm of
W. R. M. Thomson & Co
Patent Agents
Glasgow.
ROBERT DUNCAN. (L.S.)