Improvements in or relating to Screw-propellers.
Year: 1897

page_text: his Print is in accordance with the Original Specification as it stands after
Amendment (under Section 18 of the Patents, &c. Act, 1883) as allowed by
the decision of the Comptroller General of the 18th day of November 1898.
PROVISIONAL SPECIFICATION.
Improvements in or relating to Screw-propellers.
I, THOMAS POUNDS, of 8, Lockyer Road, Mannamead, Plymouth, in the County
of Devon, Captain in the Royal Navy (Retired), do hereby declare the nature of
this invention to be as follows:-
This invention has for object to obviate as much as possible in a screw-propeller the
doubly wasteful negative pressure, due to the thickness of the blades, which, in the
case of a screw-propeller of ordinary construction, is exerted by the relative streams
or currents upon the forward faces of the blades near their leading edges, and which
calculation shows to be very considerable. For this purpose according thereto, the
after or propelling face of each blade is made, at each cylindrical section, not of
uniform pitch from the after edge to the leading edge like an ordinary blade, but of
uniform pitch in its after half and of gradually increasing pitch from there to the
leading edge, and the forward face of the blade is made convex transversely but of
less transverse convexity in its forward half than in its after half, or even straight in
cross section in its forward half, so that not only will there be less negative pressure
on the forward half of the forward face than usual, but what negative pressure
there may be will be exerted at a less detrimental angle than usual.
:
In Fig. 1 of the accompanying illustrative drawings A C B D represents a developed
cylindrical section of an ordinary blade, A representing the leading edge, A B the
after or propelling face, and B the after edge of the blade, a¹ CBD, a² CBD,
a3 CB D, and a C B D represent corresponding sections of blades according to this
invention, in which the increase of pitch in the forward half of the after face is
different in each case, the distances A a¹, A a², A a³, and A at being respectively
equal to t,,, and the whole of the thickness C'D of the blade where the section is
taken. The arrows indicate the direction of flow of a relative stream produced by
the velocity of the blade, the velocity of the ship, the slip, and the inclination of the
actual stream to the axis E F of the propeller. It will be seen that the after face of
such a blade, instead of being represented simply by a straight line A B as in an
ordinary screw-propeller, is represented, as regards the after half of the face, by a
straight line B C, but, as regards the leading half of the face, by a curve C a¹, C a²,
Cas, or C at tangential to the line BC, whereas the forward face of the blade is
represented, as regards its after half, by a curve B D as in an ordinary propeller, and,
as regards its forward half, by a curve Da¹, D a², or D a³, or by a straight line Dat
tangential to the curve B D, according to the distance of the leading edge a¹, a², a³,
or a from the continuation CA of the straight line B C representing the after half.
of the after face of the blade.

page_text: Fig. 2 is a side elevation of a propeller-blade according to this invention secured
to a boss, half only of which is shown; and Figs. 3, 4, and 5 show developed
cylindrical sections thereof corresponding to the points 3, 4, and 5 respectively along
the axial line G J, whilst Fig. 6 shows what would be the developed
cylindrical section corresponding to the point 6 if the blade were continued to that
point.
In the example shown in Figs. 2 to 6 inclusive, the axial line G J of the blade is
straight and perpendicular to the axis E F of the propeller-shaft, and the developed
outline of the blade G H I is approximately semi-elliptical, as indicated in dotted
lines. The several sections Figs. 3, 4, 5, and are all similar as regards the
ratio of the distance A 2 of the leading edge a² from the continuation CA of the
after half B C of the after face, to the thickness C D of the blade, this ratio
being 12, which is the one that will, it is expected, be the most generally useful. It
will be seen from Figs. 3, 4, 5, and that as the axis E F is approached, although the
negative pressure is to a great extent obviated, the pitch-angle near the leading edge
increases so that at the point 6 it has become nearly infinite and therefore the forward
part of the after face would be at a bad angle for propulsion, whilst if the blade were
prolonged further towards the axis, negative pressure would actually be produced on
the after face of the blade. It is therefore very desirable to provide a propeller
according to this invention with a large boss, which should in diameter be at least
30 per cent., of the extreme diameter of the propeller and which has its greatest
diameter at the leading edges of the blades, at I, whence it preferably tapers, as
shown in Fig. 2, but may be cylindrical to and a little beyond H to K, where it has as
a continuation a tapering bonnet K L M. At its fore end N 0) there preferably
adjoins the boss a fixed body P Q R S, the external surface of which forms a continua
tion of that of the boss K MON. In the case of a single-screw vessel, this surface
is preferably cylindrical and is continued so until it meets the lines or run of the
vessel, but in that of a double-screw vessel it is preferably tapered gently forwards, as
shewn, towards the external surface of the casing T of the propeller-shaft U. V is
the bracket for supporting the propeller-shaft.
.
It has been stated that negative pressure is doubly wasteful, for not only does it
'waste the power required to produce it but it also wastes sufficient power to produce
an amount of forward thrust equal to counteract its mischievous effect; therefore,
since according to this invention negative pressure is obviated to a considerablé
extent, the blade-surface required for a propeller to be substituted for an ordinary
propeller similar to it except as regards its forward half, should be considerably less
than that of the last mentioned propeller. Instead of reducing the width of the
blades to effect this reduction, it is preferred, as regards speed or power, to reduce
the number of the blades by one and to make the blades broader, as by so doing the
angles of the leading edges are rendered more favourable for propulsion than they
would be with narrower blades. As regards vibration, however, a better result would
probably be obtained with a larger number of blades.
Improvements in or relating to Screw-propellers.
I, THOMAS POUNDS, of 8, Lockyer Road, Mannamead, Plymouth, in the County of
Devon, Captain in the Royal Navy (Retired), do hereby declare the nature of this
1

page_text: invention and in what manner the same is to be performed to be particularly
described and ascertained in and by the following statement :—
This invention has for object to obviate as much as possible in a screw-propeller
the negative pressure, due to the thickness of the blades, which, in the case of a
screw-propeller of ordinary construction, is exerted by the relative streams or currents
upon the forward faces of the blades near their leading edges, and which calculation
shows to be very considerable and is, moreover, doubly wasteful, since it wastes not
only the power required to produce it but also sufficient power to produce an amount
of thrust equal to counteracting its mischievous effect. For this purpose, according
thereto, the after or driving face of each blade is made, at each cylindrical section,
not of uniform pitch from the following edge to the leading edge like an ordinary
blade, but of uniform pitch in its following half only and of gradually increasing
pitch from there to the leading edge, and the forward face of the blade is made con-
vex transversely but of less transverse convexity in its leading half than in its follow-
ing balf, or even straight in cross section in its leading half, so that not only will
there be less negative pressure on the leading half of the forward. face than usual;
but what negative pressure there may be will be exerted at a less detrimental angle
than usual.
In Fig. 1 of the accompanying illustrative drawings AC B D represents a developéd
cylindrical section of an ordinary blade (i.e., the development on a plane of a section
thereof formed by a line parallel to and revolving about the axis of the propeller-
shaft), A representing the leading edge, A B the after or driving face, and B the
following edge of the blade. CBD, a² CBD, a³ C B D, and at CBD represent
corresponding sections of blades according to this invention, in which the increase
of pitch in the leading half of the driving face is different in the several cases, the
distances A a¹, A a², A a³, and A at being respectively equal to 1,,, and the whole
of the maximum thickness CD of the blade where the section is taken. The arrow's
indicate approximately the mean direction of flow, towards each section of the blade,
of the relative stream produced by the velocity of the blade, the velocity of the
stream running past the propeller (which is less than the speed of the ship by the
wake), and the inclination of the strean flowing past the propeller to the axis E F
of the propeller. On account of the inclination just inentioned, the direction of the
relative stream varies, during a revolution, to the extent of a few degrees to each
side of that indicated by the arrows; near the tip of the blade the mean direction of
flow of the relative stream makes a rather smaller angle with the line A B than
the arrows show, and the direction varies less during each revolution than it does
near the root. It will be seen that the driving face of such a blade, instead of being
represented simply by a straight line A B, as in an ordinary screw-propeller, is repre-
sented, as regards the following half of the face, by a straight line B C, but, as
regards the leading half of the face, by a curve Ca¹, Ca², Ca³, or Cu tangential
to the line BC; whereas the forward face of the blade is represented, as regards its
following half, by a curve B D, as in an ordinary propeller, and, as regards its leading
half, by a curve D a, D a², or D a³, or a straight line D a, tangential to the
curve B D, according to the distance of the leading edge a¹, a², a³, or a from the
continuation CA of the straight line B C representing the following half of
the driving face of the blade.
The driving face of the blade is bent forward from the middle of each section,
where the metal is thickest, as at C, rather than from a point nearer the leading
edge, in order that near the leading edge the driving face may make as small an
angle with the direction of the relative stream as is consistent with adequate strength,
and may consequently siand at the greatest practicable angle to the axis, and therefore
at the best practicable propelling-angle; if the driving face began to bend forward
from any point between C and A, the leading half would not give as great an amount
of thrust for a given pressure, and therefore would be less efficient, than it is when
beginning to bend forward at C. From the fact that near the leading edge the driving
face should make as small an angle as practicable with the direction of the relative

page_text: stream so as to stand at the best practicable propelling-angle, it follows that; where
practicable, broader blades should be used rather than narrower ones.
The following half of the driving face is made such as to be straight in developed
cylindrical cross section for the following reasons:-
(1) If it were made convex as indicated by the dotted line C b₁, by gradually
decreasing the pitch from C to 61, it would do little or no work, for the driving
face of the blade is struck by the relative stream at such a small angle that
nearly the whole of the pressure would fall upon the leading half, which is
represented by a¹ C, a² C, a³ C, or a C, and which is standing at a worse propelling-
angle than the following half in consequence of its bending forward, which, as
will be understood, reduces its propelling-efficiency, and is done only for the
purpose of preventing a greater mischief-namely, the negative pressure, or pressure
on the foremost face, which gives the negative thrust to which the leading half
of an ordinary blade is liable on account of its thickness.
(2) If the following half of the driving face were made concave as indicated
by the dotted line C 62, by gradually increasing the pitch from C to b², its
propelling-efficiency would be reduced thereby, and this would be done without
the necessity which is imposed upon the leading half-namely, that of getting
rid of the negative pressure to which practically only the leading half is subject,
when the ship is steaming ahead.
Fig. 2 is a side elevation of a propeller-blade according to this invention
secured to a boss, half only of which is shown; and Figs. 3, 4, 5, 6, and 7 show
developed cylindrical sections thereof corresponding to the points 3, 4, 5, 6, and 7
respectively of the axial line G J, whilst Fig. 8 shows what would be the developed
cylindrical section corresponding to the point 8 if the blade were continued to that
point. In the example shown in Figs. 2 to 8 inclusive, the axial line G J of
the blade is straight and perpendicular to the axis EF of the propeller shaft,
the developed outline of the blade G H I being approximately semi-elliptical, as
indicated in dotted lines.
It is considered important that the axial line J.G of the blade be both straight
and located in a plane perpendicular to the axis, EF of the propeller-shaft: the
line J G should be in a plane perpendicular to the axis E F because, as pressure
exerts a force at right angles to the surface on which it acts, the further the driving
face of the blade is from being perpendicular to the axis of the shaft the more the
direction of the force exerted will diverge from being parallel to the axis, and there-
fore for a given pressure less forward thrust would result if the blade were inclined--
i.e. if its axial line were not in a plane perpendicular to the axis of the shaft; again,
the axial line J G should also be straight, and not bent either wholly or partially
between the root and tip-as is sometimes done, for the purpose of making the
whole or part of the driving-face longitudinally concave with a view to increasing the
pressure, by preventing the centrifugal escape of the water-for the same amount of
pressure, on surface standing at a better angle, can be obtained by increasing the
blade-area, without curving the blade longitudinally, and, for the reason just given,
any increase of variation from the perpendicularity of the driving surface to the
axis E F is detrimental to efficiency. The several sections, Figs. 3, 5, 6, 7, and 8,
are all similar as regards the ratio of the distance A a² of the leading edge a² from
the continuation CA of the following half, B. C of the driving face, to the
thickness CD of the blade, this ratio being 1:2, which is the one that is expected
to be most generally used. It will be seen from Figs. 3, 4, 5, 6, 7, and 8 that as the
axis E F is approached, although the negative pressure is to a great extent obviated,
the pitch near the leading edge increases so that at the point a² of Fig. 8 it would
have become nearly infinite and therefore the leading part of the driving face would
be at a bad angle for propulsion, whilst if the blade were prolonged further towards
the axis, negative pressure, which it is the special object of the invention to prevent,
would actually be produced on a portion of the driving face of the blade. It is there-
foré very desirable to provide a propeller according to this invention with a large
boss, which should in diameter be about 30 per cent. of the extreme diameter of the

page_text: propeller and which has its greatest diameter at the leading edges of the blades-
i.e., at I-whence it preferably tapers, as shown in Fig. 2, but may be cylindrical to,
and a little beyond, H to K, where it has as a continuation a tapering bonnet K L M;
the boss may, however, be cylindrical from I to R, and taper from there rearwardly
to a point about E. At its fore end NO there should adjoin the boss a fixed
body PQRS, the external surface of which forms a continuation of that of the
boss K M O N. In the case of a single-screw vessel, this surface is preferably
cylindrical and is continued so until it meets the lines or run of the vessel; in that
of a double-screw vessel it may be cylindrical or partially so and be formed by bossing
out the hull of the vessel or otherwise, or it may be tapered gently forwards, as
shewn, towards the external surface of the casing T of the propeller-shaft U. V is
the bracket for supporting the propeller-shaft.
In consequence of having the leading halves of their driving faces more or less
bent forward as shown in Fig. 1, propeller-blades according to this invention offer
more resistance than those of ordinary screw-propellers equal to them in blade-area
but having root-sections such as A CB D; and the more the driving faces are bent
forward, the more resistance they offer. As, however, in consequence of greatly
getting rid of negative pressure, a given resistance will produce a greater forward
thrust in the case of a propeller according to this invention than in that of such
an ordinary propeller, therefore if there were substituted for such an ordinary
propeller, in a ship of a given horse-power, a propeller according to this invention
and having a blade-area equal to that of the ordinary propeller, the result would
be a reduction in the number of revolutions of the propeller-shaft; but such a
reduction would, with the old speed of ship, and a fortiori with the resulting
increased speed of ship, cause the relative stream to come from more ahead and
therefore to strike the driving faces of the blades at smaller angles, which would
partially counteract the advantage of bending forward the leading halves of the
driving faces. It is therefore considered desirable for a given horse-power to make a
propeller according to this invention of less blade-area than that proper for an
ordinary propeller having a uniform pitch throughout and root-sections like A D B C
of Fig. 1, in the following proportion :-
-
If the boss of the new propeller is to be the same as that of the ordinary propeller
with which comparison is made, then if the blade-area of the ordinary propeller be taken
as 100, the blade-area of a new propeller having blades of the section a¹ C B D should
be about 94, that of a propeller having blades of the section a² CBD should be 88;
that of one having blades of the section a³ CBD should be 81, and that of one
having blades of the section at C B D should be 74.
But if the ordinary propeller has an ordinary spherical boss having a diameter of
about 27% of the screw's diameter, and the new propeller is to have a boss such as
recommended and shown in Fig. 2, having a diameter 30% of the screw's diameter,
then if the blade-area of the ordinary propeller be again 100, the blade-area of a new
propeller having blades of the section a C B D should be about 91, that of a
propeller having blades of the section a² C B D should be 82, that of one having
blades of the section C B D should be 75, and that of one having blades of the
section at C B D should be 69.
As the diameter of a propeller according to this invention should be as large as
that of the corresponding ordinary propeller, and as the blade-area of the former
should be less than that of the latter, it is obvious that if both kinds of propellers
have the same number of blades, the improved blades must be made narrower
than the ordinary blades. Instead of making the blades narrower, however, it is
preferred, as regards speed and power, where practicable, to reduce the number of
blades by one, and to make the fewer blades broader, as by so doing the angles that
the driving faces make with the relative stream near the leading edges are rendered.
more favourable for propulsion than they would be with narrower blades. As
regards vibration, however, a better result would probably be obtained with the
larger number of blades.

page_text: The bosses of the screw-propellers of fast steamers should be long and taper
gradually, to give time for the water, rapidly passing astern, to close in and maintain
pressure on the rear part of the bosses, rather than they should suffer the want of
pressure due to a partial vacuum there, which would reduce the propellers' forward
effort or thrust. The length from O to L (Fig. 2), in the case of a fast steamer,
should be about four times that of O N, or about twice the diameter of the boss, or
rather more than J G.
The spherical boss in common use must be subject to an amount of negative
pressure from which the boss recommended and shown in Fig. 2 is wholly or nearly
exempt, because the fore side of the spherical boss offers more resistance to the stream
running past the propeller than the tapering or cylindrical fixed body forming a
continuation of the tapering boss does, and also because, as the blades extend down
on the fore side of the spherical boss towards the axis of the latter, the adverse
pressure on the fore side of the spherical boss must be increased by the revolution of
the blades.
The outline recommended for a screw-blade according to this invention is, when
developed, approximately that of a semi-ellipse, the minor axis representing the
greatest breadth of the blade, which is situated at the root, adjacent to the boss,
about the position of the point 8 of Fig. 2; the semi-major axis, as 8 G, standing
perpendicularly to it, divides the driving face of the blade longitudinally into two
equal parts, and the leading half of the driving face, at each transverse section,
begins to bend forward from this line toward the leading edge, the following half of
the driving face being of uniform pitch throughout as already mentioned. The
semi-elliptic form is considered preferable because, with it, as the blade increases in
thickness it also increases in width, so that where the blade is thickest the driving
face is at a better angle for propulsion than it would be if the blade were narrower
there as it usually would be with a spherical boss which limits the width of the blade
with which it is provided; moreover the thickness of a blade of this form need not be
so great at the root as it would be if the blade were wide near the tip.
The flanges of the blades of a propeller according to this invention should be
recessed into the boss, and the heads of the securing bolts recessed into the flanges
and covered with plates in a manner well known, in order to give a smooth and
even shaped boss, from which the blades cleanly emerge. In order to enable the
water to run smoothly past the division situated just before the greatest diameter
of the boss, the after end of the fixed part P Q R S may be made to overlap slightly
the fore end of the boss.
Calculation has shown that in the case of a ship with very fine lines and consequently
having a wake and an inclination each of small amount; having two propellers such as
shown in Fig. 2 with blades having sections such as shown in Figs. 3, 4, 5, 6, and 7 and a
uniform pitch of the following halves of their driving faces about 1.45 of the propeller's
diameter; and having a boss similar to that shown in Fig. 2; 50 per cent. more
thrust would be obtained from the same power when going at full speed ahead and
with propellers making 20 per cent. of slip, than would be obtained under the same
conditions with ordinary propellers of the same diameter and having the pitch of the
driving face of each of their blades uniform throughout and the same as that of the
following half of the driving face of each of the blades of the improved propellers just
described.
If the slip of the screws were less than 20 per cent., as it would naturally be if
going at slower speeds, then the superiority of the efficiency of the improved propellers
over that of the ordinary propellers, would be still greater.
Calculation, however, shows, that as a ship's lines become coarser, or as the amounts
of wake, and of inclination increase, the superiority of the efficiency of the improved
propeller, over that of the ordinary propeller, decreases.
From what has been said it seems highly probable that the speed of large Atlantic
steamers could be increased by 2 or 3 knots an hour by exchanging their ordinary
propellers for propellers according to this invention, and that if going at their present
speed a great saving of coal would be effected.

page_text: Having now particularly described and ascertained the nature of the said invention
and in what manner the saine is to be performed, I wish it to be understood that I am
aware that it has been proposed to employ screw-propeller blades in which, at each
cylindrical cross section, whilst the forward face of the blade was of uniform pitch in
its leading half at least, the driving face was practically parallel thereto from the
following edge to a point about one sixth of its width from the leading edge, whence
to the leading edge it was bent forward, but I do not consider such blades to be of
satisfactory form as the bending forward of the driving face commenced too near to
the leading-edge, and I declare that what I claim is :-
1. A screw-propeller blade having, at each cylindrical cross section, its driving face
of uniform pitch in its following half and of gradually increasing pitch from there
to the leading edge, and its forward face transversely convex in its following half and
transversely convex, or straight in cross section, from there to the leading edge, sub-
stantially as herein before described.
2. Screw-propeller blades formed as hereinbefore described with reference to and
shown in the accompanying drawings.
3. A screw-propeller having blades of the kind referred to in Claim 1, and a large
boss tapering rearwardly to a point and having its greatest diameter located at the
leading edges of the blades, substantially as hereinbefore described.
4. Screw-propellers formed substantially as herein before described with reference
to and shown in Fig. 2 of the accompanying drawings and having blades with
cylindrical cross sections all substantially like a C B D, a² C B D, a³ C B D, or
a C B D of Fig. 1 thereof.
5. The combination of a screw-propeller according to Claim 3 and a fixed body
which adjoins the boss and whose external surface forms a continuation of that of the
boss and is either cylindrical and meets the lines of the vessel or tapers gently for-
ward towards the casing of the propeller-shaft, substantially as hereinbefore described
with reference to Fig. 2 of the accompanying drawings.
Dated this 5th day of April 1898;
For the Applicant,
W. LLOYD WISE,
46, Lincoln's Inn Fields, London, W.C., Chartered Patent Agent.