Improvements in Screw Propellers.
Year: 1893

page_text: Date of Application, 21st Nov., 1893
Complete Specification Left, 17th Aug., 1894-Accepted, 22nd Sept., 1894
PROVISIONAL SPECIFICATION.
Improvements in Screw Propellers.
I WALTER CHILD of 35, Alfred Place West, London, Gentleman do hereby'
declare the nature of this invention to be as follows:-
My invention relates to improvements in screw propellers revolving in gas or
liquid which are constructed as hereinafter described. For the purpose of
explanation only, I take a screw propeller of the normal Admiralty or Griffith
pattern with left hand uniform pitch, 4 or 3 elliptical blades with generating line
at right angles to the axis, with axis horizontal and centre line of blade upright.
To the leading and trailing edges of each working screw face I adapt a "gliding
lip" which at each point of the edge is tangential or parallel to the inflowing
column of water at such point. That is to say I make the leading lip "everted
at an angle varying from 0° at the root of the blade to say 20° at the tip whilst
the breadth of this lip decreases uniformly from root to tip and so does the
thickness.
""
The trailing lip of the working face I make inverted at an angle varying
uniformly from say 10° at the root of the blade to say 30° at the tip and I make
the breadth of this lip also to decrease uniformly from root to tip.
As these angles vary with the speed it follows that the shape of the "gliding
lips" varies also to suit the speed.
The arices formed by the junction of the two gliding lips and the working face
of the screw I soften down so that a horizontal section through any point of the
vertical blade shews an easy sinuous line free from sudden bend or angle.
With regard to the back of the leading edge I so form it that the surface at
any point is tangential or parallel to the direction in which that point of the screw
is actually moving under the joint influence of progression and revolution. This
direction if there were no slip would coincide at each point with the angle of the
screw surface at such point.
.
With reference to the accompanying diagram, in the case of a screw of 8 feet
radius and 20 feet pitch I set off a, d, equal to the developed diameter of the
screw, 1 set off de, at right angles to a, d equal to the pitch and on the same
line e, b, equal to the slip. I join a, b, and thus obtain the angle b, a, d equal to 0
which is suitable for the inclination of the back of the blade at the tip. I
divide a,
d into eight equal parts and join these with b and thus obtain the
angles a, ẞ, y, &c., suitable for the inclination of the back of the 'blade at six
feet, four feet, and two feet from the centre respectively.
I then soften off the junction of this curved surface with the back of the blade
and I make the trailing edge as thin as is compatible with strength.
The purpose of these "gliding lips" which in no way assist in propulsion is to
diminish the present heavy loss of power:-
Firstly. By lessening the impact of the leading edge on the water.
Secondly. By lessening vibration.
Thirdly. By gradually (and not suddenly) deflecting and accelerating the
inflowing stream.
Fourthly. By diminishing the drag under the lee of the trailing edge.
Furthermore, I increase the size of the boss which I make so large that its
circumference is equal to the pitch of the screw. The size of the boss is in this..
way a function of the pitch of the screw and each pitch has a corresponding size-of
boss more suitable to it than any other.

page_text: Thus; supposing the pitch to be the same as in the previous example viz. 20 feet,
then in that case I make the circumference of the boss 20 feet and its diameter will
therefore be six feet, four inches, and this I say is the diameter of the circle
suitable for this pitch and therefore I call it the "pitchring."
The inclination of the screw face at its intersection with a coaxial cylinder
whose circumference equals the pitch is 45° and each point of this section describes
an orbit of exactly 10 feet at each revolution of the screw.
Points outside this circle describe larger and larger orbits as the tip of the
blade is approached.
33
On the other hand all points of the screw surface lying within the " pitchring
must describe orbits that are smaller than the pitch. The inclination of the
screw surface at any of these points is less than 45° consequently the sternward
motion of the water caused by one revolution of this part of the screw is less than
the pitch, and thus all these points act as a drag and as a churn and in no wise
aid in propulsion. That means the propulsive action is wholly due to that portion
of the blade lying outside the " pitchring."
"
Since the blades within the "pitchring" do not serve in propulsion, it is
important to reduce the resistance they offer to the progress of the vessel through
the water and this I accomplish by enlarging the boss up to the " "pitchring SO
that the blades emerge from the boss at an angle of 45° with the line of
progression and I give to this increased boss the form of a solid of least resistance.
Lastly I cause the cross section of the boss to increase continuously from before
backwards so that on cutting the boss into a series of parallel slices all
perpendicular to the axis, the section of water enclosed between any two blades
diminishes as you approach the rear end of the boss owing to the increase in the
size of the boss which I have mentioned. I employ no casing or guide blades of
any kind for the screw to react on; notwithstanding this I claim that, by this
arrangement, the stream of water is really accelerated as it passes through the
screw and that thus I attain the object aimed at by a gain in pitch (which last
alone is ineffectual since the same bulk of water must enter the screw as leaves
it and if the cross section is unchanged it must leave it at the same rate and no
other).
By this increase in the size of the boss I obviate to a great extent the rotation of
the race and thereby obtain with the same engine power an increase in the speed.
The speed of the race though not quite uniform (being fastest at about two-thirds
of the radius) is yet much less irregular than with the ordinary boss.
Dated this 21st day of November 1893.
F. G. CASSELL,
Of Cassell & Co.,
22, Glasshouse Street, Regent Street, London, W.,
Agent for the Applicant.
Improvements in Screw Propellers.
I, WALTER CHILD, of 35, Alfred Place West, London, Gentleman, do hereby
declare the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement :-
My invention relates to improvements in screw propellers revolving in gas or
liquid, which are constructed as hereinafter described. For the purpose of
explanation only, I take a screw propeller of the normal "Admiralty" or "Griffith"
pattern with left-hand uniform.pitch, four or three elliptical blades, with generating
line at right-angles to the axis, with axis horizontal and centre line of blade
upright.

page_text: Observing that the inflowing stream of air or water is always set in rotation by
the revolution of the screw before it meets the said screw and consequently strikes
the shoulder of the usual screw-blade and not the edge of it, I therefore adapt
to the leading and trailing edges of each working screw face a "gliding lip" which
at each point of the edge is to be tangential or parallel to the inflowing or out-
flowing column of air or water at that point; that is to say, I make the leading
lip "everted," viz.: turned away from the direction of rotation, so that in a right-
handed screw the leading lip would be turned "against the clock," at an angle
varying from 0° at the root to say 20° at the tip whilst the breadth of this lip
decreases uniformly from root to tip and so does the thickness.
The arices formed by the junction of the two " gliding lips" and the working
face of the screw I soften down so that a horizontal section through any point of
the vertical blade shows an easy sinuous line free from sudden bend or angle.
With regard to the back of the leading edge, I may so form it that the surface
at any point is tangential or parallel to the direction in which that point of the
screw is actually moving under the joint influence of progression and revolution.
This direction if there were no slip would coincide at each point with the angle of
the screw surface at that point.
In the accompanying drawing,
Fig. 1 is a side elevation of the boss and one blade, with the "gliding lips"
shown in dotted lines and
Fig. 2 is a series of cross sections through the blade with the additional parts
constituting one feature of my invention shown in dotted lines.
With reference to the diagram filed with my Provisional Specification, in the
case of a screw of 8 feet radius and 20 feet pitch, I set off a d equal to the
developed diameter of the screw, I set off de at right-angles to a d equal to
the pitch, and on the same line eb equal to the slip. I join ab and thus obtain the
angle bad
to which is suitable for the inclination of the back of the blade
at the tip.
equal
divide ad into eight equal parts and join these with band thus obtain
the angles a, ẞ, y, etc. suitable for the inclination of the back of the blade at 6 feet,
4 feet, and 2 feet from the centre respectively.
I then soften off the junction of this curved surface with the back of the blade
and I make the trailing edge as thin as is compatible with strength.
It is obvious that with a left-handed propeller all the directions and angles will
be reversed.
The purpose of these "gliding lips" which in no way assist in propulsion is to
diminish the present heavy loss of power :-
Firstly. By lessening the impact of the leading edge on the water.
Secondly. By lessening vibration.
Thirdly. By gradually (and not suddenly) deflecting and accelerating the
inflowing stream.
Fourthly. By diminishing the drag under the lee of the trailing edge.
Furthermore, I increase the size of the boss which I make so large that its
circumference at the medium cross section is equal to the pitch of the screw.
The size of the boss is in this way a function of the pitch of the screw and each
pitch has a corresponding size of boss more suitable to it than any other. Thus
supposing the pitch to be the same as in the previous example, viz.: 20 feet, then
in that case I make the circumference of the boss 20 feet and its diameter will
therefore be 6 feet, 4 inches, and this I say is the diameter of the circle for boss
suitable for this pitch, and therefore I call it the " "pitchring."
The inclination of the screw face (at its intersection with a coaxial cylinder the
circumference of which equals the pitch) is 45°, and each point of this section
describes an orbit of exactly 20 feet at each revolution of the screw. Points
outside this circle describe larger and larger orbits as the tip of the blade is
approached. On the other hand all points of the screw surface lying within the
"pitchring" must describe orbits that are smaller than the pitch. The inclination
of the screw surface at any of these points is less than 45° consequently the

page_text: sternward motion of the water caused by one revolution of this part of the screw
is less than the pitch and thus all these points act as a drag and as a churn and in
no wise aid in propulsion; that means the propulsive action is wholly due to that
portion of the blade lying outside the "pitchring."
Since the blades within the "pitchring" do not serve in propulsion, it is
important to reduce the friction they offer to the progress of the vessel through
the water, and this I accomplish by enlarging the boss up to the "pitchring" so
that the blades emerge from the boss at an angle of 45° with the line of progression
and I give to this increased boss the form of a solid of least resistance."
Lastly I cause the cross section of the boss to increase continuously from before,
backwards, so that upon cutting the boss into a series of parallel slices all
perpendicular to the axis and of equal thickness, the section of water enclosed
between any two blades would diminish as you approach the rear end of the boss
owing to the increase in the size of the boss which I have mentioned.
I employ no casing or guide blades of any kind for the screw to react on,
notwithstanding this I claim by this arrangement, that the stream of water is really
accelerated as it passes through the screw and thus I attain the object aimed at by
a gaining pitch (which last alone is ineffectual since the same bulk of water must
enter the screw as leaves it and if the cross section is unchanged it must leave it
at the same rate and no other).
By this increase in the size of the boss I obviate to a great. extent the rotation
of the race and thereby obtain with the same engine power an increase in the
speed.
The speed of the race though not quite uniform (being fastest at about two
thirds of the radius) is yet much less irregular than with the ordinary boss.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what. I
claim is :-
1. In screw propellers revolving in gas or liquid, the application of gliding lips
substantially as herein described and for the purposes specified.
2. In screw propellers revolving in gas or liquid, the special construction of
boss so that it shall be a function of the pitch, substantially as described and for
the purposes specified.
3. The combination of the "gliding lips" and the special construction of boss
whereby it is made a function of the pitch, with any screw propeller so formed
that the vertical sections of air or water enclosed between equidistant vertical
planes, decrease from fore to aft, substantially as herein described and for the
purposes specified.
4. The improvements in screw propellers, substantially as herein described and
for the purposes specified.
Dated this 16th d
A1001