IMPROVEMENTS IN SCREW PROPELLERS
Year: 1880

page_text: LETTERS PATENT to Joseph Henry Thomas, of No. 8, Upper Westbourne
Terrace, in the County of Middlesex, for an Invention of "IMPROVEMENTS IN
SCREW PROPELLERS."
PROVISIONAL SPECIFICATION left by the said Joseph Henry Thomas at
the Office of the Commissioners of Patents on the 13th May 1880.
JOSEPH HENRY THOMAS, of No. 8, Upper Westbourne Terrace, in the County of
Middlesex. "IMPROVEMENTS IN SCREW PROPELLERS."
My Invention relates to the construction of screw propellers for vessels in such
a manner as to obtain great propulsive force with littlo disturbance of the water
in which the propeller revolves, and consequently with comparatively little waste
of power. For this purpose I form each blade, of which there may be two or more,
in the following manner :-
The blade as it starts from the boss has its stern or leaving edge continued
outwards nearly in the plane of the stern end of the boss, and this edge beginning
from the boss in a nearly radial direction bends forwards in the direction of
rotation, the forward end becoming more rapid towards the outer periphery. The
front or entering edge of the blade starts from the boss at a considerable anglo
(which may be from 30° to 60°) forwards, and then after continuing to incline
forwards for more than half the radius of the blade it sweeps round with a curvaturo
leading it backwards to the outer edge or periphery. The pitch at the front or
entering edge of the blade is determined according to the speed of the vessel and
of the propeller, such that this edge cuts the water without displacing it laterally,
and the pitch thereafter increases rapidly backwards to the stern or leaving edge.
Thus the blade presents the appearance of a lobe hollowed in its front face, the
hollow being determined by. the two curvatures, the one. in the radial direction
according to which the blade bends forwards in the direction of rotation, and the
other in the axial direction resulting from the increase of pitch backwards.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Joseph Henry Thomas in the Great Seal Patent Office on the 11th
November 1880.
JOSEPH HENRY THOMAS, of No. 8, Upper Westbourne Terrace, in the County of
Middlesex. "IMPROVEMENTS IN SCREW PROPELLERS."
My Invention relates to the construction of screw propellers for vessels in such a
manner as to obtain great propulsive force with little disturbance of the water in
which the propeller revolves, and consequently with comparatively little waste of
power. For this purpose I form each blade, of which there may be two or more,
in the following manner :-
The blade as it starts from the boss has its stern or leaving edge continued
outwards nearly in the plane of the stern end of the boss, and this edge beginning
from the boss in a nearly radial direction bends forward in the direction of rotation,
the forward bend becoming more rapid towards the outer periphery. The front or
entering edge of the blade starts from the boss at a considerable angle (which may
be from 30° to 60°) forwards, and then after continuing to incline forwards for more
than half the radius of the blade, it sweeps round with a curvature leading it
backwards to the outer edge or periphery. The pitch at the front or entering edge of
the blade is determined according to the speed of the vessel and of the propeller, such
that this edge cuts the water without displacing it laterally, and the pitch thereafter
increases rapidly backwards to the stern or leaving edge. Thus the blade presents
the appearance of a lobe hollowed in its front face, the hollow being determined
by the two curvatures, the one in the radial direction according to which the
blade bends forwards in the direction of rotation, and the other in the axial
direction resulting from the increase of pitch backwards.
DESCRIPTION OF THE DRAWINGS.
The character of
Fig. 1 is an end elevation; Fig. 2, a side elevation; and Fig. 3, a plan of a two
bladed propeller according to my Invention; Fig. 4 is a diagram to assist in
determining the curvature of the propelling face of the blade. That curvature, as
stated above, is compounded of a radial and an axial curvature.
the radial curvature is shewn in Fig. 1 by the curved lines a, b, c, &c., which are the
profiles of sections made by planes a, b, c, &c., Fig. 3, cutting the blade perpendicularly
to the axis of the propeller. The character of the axial curvature is shown in Fig. 3 by
the curved lines A, B, C, &c., which are the projections of the profiles formed by
equidistant cylindrical surfaces A, B, C, &c., Fig. 1, cutting the blade concentrically
with the axis. As I find practically that the blade should have its greatest width at
a distance from the axis of about 4/ths of the total radius, I take the curvature at
this distance as the determining curvature for that of the rest of the blade, and
proceed as follows:-Dividing the radius into 5 equal parts, I draw, in Fig. 1, the
concentric circles A, B, C, &c., and a radius Oc at an angle of 45° to the horizontal
line OQ. From c I let fall a perpendicular c R on OQ, and from R as a centre
with radius Rc I describe a circular arc O Sc cutting the concentric circles in the
points S₁, S., &c. This are gives the fundamental radial curvature, being the
profile of the section made by the plane Oc, Fig. 3, which I may term the
fundamental plane. From the points S₁, S., &c., in Fig. 1, I project on Oc, Fig. 3,
corresponding points S₁, S., &c., which I may term contact points. Proceeding now
to Fig. 4, I draw OT equal to one fourth of the distance advanced by the vessel
and propeller during one revolution of the latter, and a perpendicular O, equal to
the radius, dividing it into 5 equal parts, and joining the points of division to T by
the diagonal lines T₁, T₂, &c. Taking these diagonal lines respectively as major

page_text: semiaxes, and the corresponding radial lengths O₁, O., &c., as minor semiaxes, I
describe portions of ellipses, as shewn in Fig. 4. In these ellipses I find contact
points Si, S., &c., by drawing tangents inclined to O T at angles of 55°, 11¹º, and
so on successively for the successive ellipses. Recurring now to Fig. 3, through
the contact points S₁, S., &c., I draw lines inclined to the axis at angles of 11/0,
22, &c., these lines representing the tangents in Fig. 4, and to each of them I
apply a portion of the corresponding ellipse from Fig. 4 so as to be tangential at
the contact point. Thus the curve VS W in Fig. 3 is a copy of the elliptical
arc V SW in Fig. 4, and so with the other curves in Fig. 3, which give the
projections of the sections of the blades made by the cylindrical surfaces A, B, C, &c.,
of Fig. 1. Projecting now on to the circles in Fig. 1 the points in Fig. 3 where the
curves A, B, C, &c., proper to each circle cut the several plane lines a, b, c, &c., and
tracing through the projections in Fig. 1 the curves a, b, c, &c., I obtain the profiles
of the sections of the blade made by the planes a, b, c, &c., of Fig. 3. The width
of the blade is necessarily limited by considerations as to bulk and weight. I find
practically that a width such as will give good efficiency in a two bladed propeller
inay be determined in the following manner:-On each side of the radius Oc in
Fig. 1 I draw a radius OV at an angle of 333, and OW at an angle of 783/° to
the horizontal line OQ, and I project on the curve V SW in Fig. 3 the points V
and W where these radii cut the circle D in Fig. 1. The points V W in Fig. 3
determine the effective width of the blade for forward propulsion. From the front
point V I round off the edge of the blade to the boss and to the circumference so
as to avoid abrupt angles and corners. The back edge might in a similar manner
be rounded off from W, but it is of advantage to extend the back edge of the blade
a little beyond W, bending the extended portion forwards with a curvature in the
direction opposed to that of the general curvature of the blade, as by this means
the efficiency of the propeller for moving the vessel astern is considerably increased
without materially affecting its efficiency for propulsion ahead. In the above
description I have assumed that the propeller is arranged to advance a distance
equal to its radius in one quarter of a revolution, this giving a general pitch of
blade which I have found practically useful, and it is on this assumption that I
have based the inclinations of the elliptical tangents which I have stated above.
The pitch however might be increased or diminished, in which case the angles
should be proportionally increased or diminished. Again I have based the radial
curvature of the blade on the fundamental quadrant are O Sc in Fig. 1, drawn as
described. This curve might however be an arc of a somewhat greater or smaller
circle without materially affecting the general character of the curvature.
width of blade might also be reduced when more than two blades are employed,
and the re-curved portion behind W might be made larger in cases where rapid
and effective reversal of the vessel's movement is desirable.
The
Having thus described the nature of my Invention, and in what manner the
same is to be performed, I claim,—
The construction of propeller, each blade of which has its propelling face hollowed
by a radial and an axial curvature, the radial curvature being a bending of the
blade forwards in the direction of its rotation, and the axial curvature being an
increase of the pitch backwards, and both curvatures being determined substantially
as herein described.
In witness whereof, I, the said Joseph Henry Thomas, have hereunto set
my hand and seal, this Eighth day of November, in the year of our Lord
One thousand eight hundred and eighty.
J. HENRY THOMAS. (L.S.)