IMPROVEMENTS IN SCREW PROPELLERS
Year: 1871

page_text: LETTERS PATENT to Hermann Hirsch, of No. 20, Northumberland
Street, Strand, in the County of Middlesex, for the Invention of
"IMPROVEMENTS IN SCREW PROPELLERS,"
Sealed the 3rd November 1871, and dated the 28th August 1871.
PROVISIONAL SPECIFICATION left by the said Hermann Hirsch at
the Office of the Commissioners of Patents, with his Petition, on
the 28th August 1871.
I, HERMANN HIRSCH, of No. 20, Northumberland Street, Strand, in
the County of Middlesex, do hereby declare the nature of the said
Invention for "IMPROVEMENTS IN SCREW PROPELLERS," to be as follows:-
The object of a screw propeller being to convert the force given out in
its rotation into a pressure directed in the line of its axis, the best form
of propeller will be that which from a given rotary power will produce
a maximum amount of longitudinal pressure as useful effect for
propulsion. To obtain such maximum of useful effect I construct my
improved screw propeller, or each blade of the same, in the following
manner, and upon the following principles:-Since the mean direction

page_text: in which the water yields is perpendicular to the surface of the screw
blade, I so curve the latter in the plane of rotation in the leading
direction that from the boss towards the circumference a gradually
increasing angle is formed between the radius and the line developed
by the intersection of the acting surface of the blade with the plane of
rotation, and the line thus formed may be represented either by a
spiral, which I prefer, or by any other suitable curve. By virtue of such
curvature the direction perpendicular to the surface of the blade in
which the water yields as aforesaid will be a direction converging from
the periphery of the blade towards the axial line of the propeller; which
direction will consequently be opposed to the centrifugal action which
the rotary motion of the blades tends to impart to the water. Thus this
curvature of the blade forms that element in the construction of my
improved screw propeller which is intended to counteract the centrifugal
action of the blades, and so utilize for propulsion the power that would
otherwise be wasted by such centrifugal action; which waste of power
is of so frequent occurrence with the screw propeller of ordinary
construction, as indicated to a great extent by the vibration to which
not only the stern framing but also the whole after part of the screw
ship is often subjected.
The effect of the above-described curved form of the blades will be
to drive the water in a cylindrical column aft in an axial direction
relative to the screw, thereby producing the most direct and economical
application of the power applied to the propeller, and at the same time
avoiding the said vibration, and increasing the efficiency of the rudder
by surrounding it with a column of unbroken water, thrown directly
against it with considerable velocity. Since however the water readily
yields in the aforesaid axial direction to the action of the leading part
of the width of the screw blade, the following part of the width of the
blade would be of gradually decreasing usefulness unless it were so
formed as to follow up the yielding water. This latter I effect by
gradually increasing the pitch from the fore edge towards the after edge.
of the blade; and the combination of this increasing pitch with the
before-described curvature in the plane of rotation imparts to the
acting surface of the blades of my improved propeller a concave form
calculated to obtain a firm grip on the water, and to exercise a
concentrating action thereon instead of dispersing it, as is the case with
screw propellers of ordinary construction. But on account of the great

page_text: difference which exists in the velocity and direction of pressure of the
various parts of a screw blade I confine the application of the aforesaid
increasing pitch to the outer or circumferential half of the blade, since
its application near the boss would have the effect of obstructing the
escape of the water at that part of the blade which has little or no
propelling effect, and at which part consequently it is essential that the
water should be made to pass away as easily and as rapidly as possible.
In consideration of the last-named circumstance, instead of increasing
the pitch of this inner or central part of the blade, or even making it of
the same pitch throughout, I actually decrease the pitch of the after
part thereof; the limit and direction of such decrease being determined
by a diagonal line extending from about the middle of the blade surface
near the boss in a direction towards the upper aft (or following) corner
of the blade, and terminating in a curved line towards the after edge
about half way between the boss and the circumference. This decrease
of pitch of the inner or central part of the screw blade, extending about
half way up the blade as described, forms one of the important elements
in the construction of my improved propeller, as by it I avoid that loss
of power which in the ordinary forms of propellers results from the
obstruction offered by the after part of the inner or central portion of
the blades to the free escape of the water, whereby the central body of
water is caused to be carried round by the propeller; thus not only
occasioning a useless expenditure of power, but also producing a
detrimental effect upon the proper action of the outer or circumferential
part of the blades through the centrifugal motion imparted to the
rotating water by the central part, as just mentioned. These advantages
gained by a decreasing pitch in the central part of the blade would
however be counterbalanced by the disadvantages which would ensue
were the decreasing pitch extended (as has been done before) to the
outer or circumferential part of the blade up to the aft (or following)
corner, since the efficiency of this for propulsion most favourable part
of the blade would thereby be considerably reduced, while I, on the
contrary augment the efficiency of this part, as already stated, by an
increasing pitch.

page_text: working surface at each part approximately inversely proportionate to
the angular velocity thereof. I prefer, so to proportion this decrease
of area, that the width of the blade at the circumference shall be from
one-half to two-thirds of its width at the boss. I do not however limit
myself to these proportions.
It will thus be seen from the foregoing, that my improved con-
struction of screw propellers consists,―
Firstly. In the triple combination of a curved form of the blades in
the plane of rotation with an increasing pitch in the outer or circum-
ferential half and a decreasing pitch in the inner half of the blades.
And, secondly. In the further combination of the above-named
features with the decreasing width of the blades from the boss towards
the circumference.
SPECIFICATION in pursuance of the conditions of the Letters Patent,.
filed by the said Hermann Hirsch in the Great Scal Patent Office.]
on the 28th February 1872.
.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, HERMANN
HIRSCH, of No. 20, Northumberland Street, Strand, in the County of
Middlesex, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her 2
Letters Patent, bearing date the Twenty-eighth day of August, in the.
year of our Lord One thousand eight hundred and seventy-one, in the
thirty-fifth year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said Hermann Hirsch, Her special licence:
that I, the said Hermann Hirsch, my executors, administrators, and 2
assigns, or such others as I, the said Hermann Hirsch, my executors,.
administrators, and assigns, should at any 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 Isle of Man, an Invention for "IMPROVEMENTS
IN SCREW PROPELLERS," upon the condition (amongst others) that I,...
the said Hermann Hirsch, my executors or administrators, by an
instrument in writing under my, or their, or one of their hands and.
.

page_text: 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 Seal Patent Office within six
calendar months next and immediately after the date of the said
Letters Patent.
NOW KNOW YE, that I, the said Hermann Hirsch, 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, reference being had to the accom-
paning Sheet of Drawings, and to the letters marked thereon, that
is to say:-
The object of a screw propeller being to convert the force given out
in its rotation into a pressure directed in the line of its axis, the best
form of propeller will be that which from a given rotary power will
produce a maximum amount of longitudinal pressure as useful effect for
propulsion. To obtain such maximum of useful effect I construct my
improved screw propeller, or each blade of the same, in the following
manner, and upon the following principles:-Since the mean direction
in which the water yields is perpendicular to the surface of the screw
blade, I so curve the latter in the plane of rotation in the leading
direction that from the boss towards the circumference a gradually
increasing angle is formed between the radius and the line developed by
the intersection of the acting surface of the blade with the plane of
rotation; and the line thus formed may be represented either by a spiral,
which I prefer, or by any other suitable curve. By virtue of such
curvature the direction perpendicular to the surface of the blade in
which the water yields as aforesaid will be a direction converging from
the periphery of the blade towards the axial line of the propeller, which
direction will consequently be opposed to the centrifugal action which
the rotary motion of the blades tends to impart to the water. Thus the
curvature of the blade forms that element in the construction of my
improved screw propeller which is intended to counteract the centri-
fugal action of the blades, and so utilize for propulsion the power that
would otherwise be wasted by such centrifugal action; which waste of
power is of so frequent occurence with the screw propellers of ordinary
construction, as indicated to a great extent by the vibration to which
not only the stern framing but also the whole after part of the screw
ship is often subjected.

page_text: The effect of the above-described curved form of the blades will be to
drive the water in a cylindrical column aft in an axial direction relative
to the screw, thereby producing the most direct and economical
application of the power applied to the propeller, and at the same time
avoiding the said vibration, and increasing the efficiency of the rudder by
surrounding it with a column of unbroken water, thrown directly against
it with considerable velocity. Since however the water readily yields in
the aforesaid axial direction to the action of the leading part of the
width of the screw blade, the following part of the width of the blade
would be of gradually decreasing usefulness unless it were so formed as
to follow up the yielding water. This latter I effect by gradually
increasing the pitch from the fore edge towards the after edge of the
blade; and the combination of this increasing pitch with the before-
described curvature in the plane of rotation imparts to the acting
surface of the blades of my improved propeller a concave form
calculated to obtain a firm grip on the water, and to exercise a
concentrating action thereon instead of dispersing it, as in the case of
screw propellers of ordinary construction. But on account of the great
difference which exists in the velocity and direction of pressure of the
various parts of a screw blade I confine the application of the aforesaid
increasing pitch to the outer or circumferential half of the blade, since
its application near the boss would have the effect of obstructing the
escape of the water at that part of the blade which has little or no
propelling effect, and at which part consequently it is essential that tho
water should be made to pass away as easily and as rapidly as possible.
In consideration of the last-named circumstance, instead of increasing
the pitch of this inner or central part of the blade, or even making it of
the same pitch throughout, I actually decrease the pitch of the after
part thereof, the limit and direction of such decrease being determined
by a diagonal line extending from about the middle of the blade surface
near the boss in a direction towards the upper aft (or following) corner
of the blade, and terminating in a curved line towards the after edge
about half way between the boss and the circumference. This decrease
of pitch of the inner or central part of the screw blade, extending about
half way up the blade as described, forms one of the important
elements in the construction of my improved propeller, as by it I avoid
that loss of power which in the ordinary forms of propellers results from
the obstruction offered by the after part of the inner or central portion
of the blades to the free escape of the water, whereby the central

page_text: body of water is caused to be carried round by the propeller; thus not
only occasioning a useless expenditure of power, but also producing a
detrimental effect upon the proper action of the outer or circumferential
part of the blades through the centrifugal motion imparted to the
rotating water by the central part, as just mentioned. These advantages
gained by a decreasing pitch in the central part of the blade would
however be counterbalanced by the disadvantages which would ensue
were the decreasing pitch extended (as has been done before) to the
outer or circumferential part of the blade up to the aft (or following)
corner, since the efficiency of this for propulsion most favorable part
of the blade would thereby be considerably reduced, while I on the
contrary augment the efficiency of this part, as already stated, by an
increasing pitch.
In my above-described improved construction of screw propellers: I
furthermore gradually diminish the area of the blades from the boss
towards the circumference, in order to render the amount of friction at
every part of the blade as nearly as possible equal by making the
working surface at each part approximately inversely proportionaté to
the velocity thereof. I prefer so to proportion this decrease of area that
the width of the blade at the circumference shall be from one-half to
two-thirds of its width at the boss. I do not however limit. myself
to these proportions.

page_text: points c', c², c³. The curve A, c³, c², c', B, is drawn through these points.
Any other curve resembling it, as for example a circular arc, might be
adopted without materially affecting the efficiency of the propeller.
The profile of the back of the blade B, D, is determined by the thickness
of metal necessary for strength.
The sections of the blade shown in Figure 2 are determined as
follows-Referring to one of these sections at the point F, then
assuming the radius E, F, to represent to any scale the velocity of the
point F as determined by its distance from the centre E and its angular
velocity, a line E, G, is measured along the axis of such length as to
represent to the same scale the velocity of the vessel. To this line is
added a portion G, H, which may be from one-tenth to one-fourth of the
length of E, G; and lines G,, F, H, F,ƒª, are drawn from G and H
respectively through F. The curved line f¹, F, f, is then drawn
tangential to the lines G, f, and H,ƒ³, at the edges f¹ and ƒ³, and this
curve is the profile of the propelling surface of the blade at F taken at
right angles to the radius. The forward or entering part of the blado
being thus in the direction of the resultant of the circumferential and
advancing velocities, the blade glides into the still water without shock,
and the rest of its width having a gradually increasing pitch accelerates
the water backwards, the reaction of the water so accelerated giving
propulsive forco forwards. The back of the blade f¹, K, ƒ³, is
determined by the thickness of metal necessary for strength, the
material being eased off at both edges so as to enable the blade to pass
through the water with as little resistance as possible. In determining
the sections of the blade nearer to the axis, as for example the section
at L, a construction similar to that above described, for the section at F
would give A, L, P, as the face of the blade, tangential at land to the
lines G, A, and H, P, respectively. But instead of continuing the
curvature , L, out to P, I curve back the hinder part of the blado
at L, m, so as to give a free escape for the water that had been engaged
with the face of the blade from A to L. The extent of blade face which
is thus curved back, or made with gradually diminishing pitch, is
greatest near the axis, and is gradually diminished in width till about
midway between the axis and the outer periphery where this curving
back ceases. The dotted line P, N, drawn from the middle of the boss
towards the outer back corner of the blade, and curved off about the
'middle of the blade at N, indicates the extent to which this curving back

page_text: may advantageously be carried. Thus all the surface of blado behind
the line P, N, is curved back or has a diminishing pitch as above
described, while all the surface in front of P, N, is of the profile with
increasing pitch, determined as described with reference to the
section at F.
•
It will be seen on reference to Figures 1 and 2 that the area of the
blade is gradually diminished from the boss outwards, the width at any
distance from the axis being made approximately inversely pro-
portional to the velocity with which that width travels through the
water. I find that in practice the width at the outer circumference
may be from one-half to two-thirds of the width next the boss, but I do
not limit myself to these proportions.
Although in the Figures I have represented a propeller with four
blades, it is to be understood that two, three, or any other convenient
number of blades may be employed; the construction of each blade
being determined on the principles above described.
Having thus described the nature of my. Invention, and in what
manner it is to be performed, I would have it understood that I do
not claim the combination in the blade of a screw propeller of an
increasing pitch with a forward curvature in the plane of rotation, but
what I do claim is,―
First. The triple combination in the blade of a screw propeller of a
curvature forward in the plane of rotation with an increasing pitch on
the front and outer part of the blade, and with a decreasing pitch on
the back and inner part of the blade, substantially as herein described.
Second. The combination of the features above named with a decrease
in the width of the blade from the boss towards the circumference,
substantially as herein described.
In witness whereof, I, the said Hermann Hirsch, have hereunto set
my hand and scal, this Twenty-third day of February in the