IMPROVEMENTS IN AND CONNECTED WITH SCREW PROPELLERS FOR NAVIGABLE VESSELS
Year: 1880

page_text: LETTERS PATENT to James Robertson, Engineer, of Govan, in the County of
Renfrew, for an Invention of IMPROVEMENTS IN AND CONNECTED WITH
SCREW PROPELLERS FOR NAVIGABLE VESSELS."
PROVISIONAL SPECIFICATION left by the said James Robertson at the
Office of the Commissioners of Patents on the 4th August 1880.
"IMPROVE-
JAMES ROBERTSON, Engineer, of Govan, in the County of Renfrew.
MENTS IN AND CONNECTED WITH SCREW PROPELLERS FOR NAVIGABLE VESSELS."
My Invention relates to such oblique or curved bladed propellers as are made to
rotate and screw through water for the purpose of propelling vessels or ships; these
propellers are usually termed and known as screw propellers.
My improvements consist mainly and essentially in allowing or causing air or gas
to flow in from the atmosphere to the partial vacuum formed on the forward side
and following edges of the blade or blades of the propeller when in motion. The
free admission of air, by reason of its low specific gravity as compared with water,
and quick flowing qualities, greatly increases the efficiency of propellers.
For the admission of air into the partial vacuum above mentioned, I form the
propeller blade or blades hollow, with an opening or openings out to the forward
side and following edges, at or near to positions where the partial vacuum is mostly
formed, and I form the boss of the propeller also hollow, communicating with a
hollow channel or channels formed in the blades, and by providing openings from
this annular or otherwise formed cavity, out to the end or ends or periphery of the
propeller boss, to which I connect, by means of a stationary adjacent or contiguous
boss or bosses, or annular hollow embracing gland, a pipe or channel led to and
through the skin of the vessel, or up on the outside of the vessel sufficiently above
the water to allow the air to flow in freely into this ascending pipe, and from it
through the propeller boss and to the forward side of the propeller blades. The
air also could be made to flow from the propeller boss to the sternward side of the
propeller blades to serve a like end when the vessel is being driven sternwards, but
this in driving ordinary vessels would be of little importance.
It will generally be most convenient to form the hollow boss communicating with
the hollow boss of the propeller in the stern pipe, which may be conjointly with the
Don 031

page_text: hollow propeller boss of a spigot and faucet form, and may be made watertight by
a stuffing box, but it is not necessary that the joint between the propeller boss and
air channel boss be absolutely tight, as though a small quantity of water flows in
along with the air into the boss and blades it is readily thrown out along with the
air from the air orifices in the blades by the motion of the propeller, and therefore
simple annular or intermittently annular plain contiguous joints without stuffing
boxes, which have little or no tendency to wear or cause loss of power from
friction, will be generally best for forming the air channels with the propeller boss.
Instead of the air communication being made to the propeller by a swivel joint,
formed with the boss of the propeller, the propeller shaft may be formed hollow
and an air channel formed either in or beyond the stern pipe, with openings out to
the periphery of the shaft, or to its extreme end or ends or first coupling, and where
outside the vessel communicating by means similar to that described to an
ascending air channel or pipe; but as a large quantity is required to destroy the
negative pressure or partial vacuum, from the air rapidly getting dispersed through
and up to the surface of the water, and a proportionately large duct or passage
necessary, the forming of the air channel in the propeller shaft would obviously
involve a considerable and inconvenient increase of diameter of the shaft, and
therefore is less easy and convenient than other means of establishing a sufficiently
capacious air communication to the propeller blades.
Any outward form of known or other screw propeller is more or less suitable for
having the air communication under my Invention to the reverse side of the thrust
side of their blades, in the way described, and will generally be advantageous with
an increase of blade surface to that now used. The ordinary Griffith form of cast
metal with blades bolted on to the box is a convenient form. Instead of being
formed of cast metal the hollow blades may be formed of two wrought iron, steel,
or other metal plates, with stud pieces in the centre to keep their central parts
apart, and rivetted at the edges, suitable air orifices being formed on their forward
following or tip edges, which may be of a louvre board fashion, and at the parts
where negative pressure mostly takes place, but ordinary cast blades of iron, brass,
or steel will generally be the most suitable.
Instead of establishing the air communication through channels in the blade or
blades of the propeller as described, hollow channels may be bolted on or otherwise
fixed to the propeller blades, or revolving apart from it, communicating with the
propeller hollow boss or a similar separate boss, and communicating also with
similar ascending air channel or channels, and formed with air openings into the
water at or near the portion of or on the propeller blades where the partial
retarding vacuum is formed. Outward or external air channels would not retard
the propelling effect or power of the engines so much as the negative pressure caused
on the propeller blades, but though this would also be advantageous and applicable to
solid and existing propellers, it is nevertheless less complete than the before described
arrangements for attaining the objects of this Invention.
Instead of the air being allowed to flow into the forward side of the propeller
blades by the atmospheric pressure, only the air may be forced through the ducts
or channels formed as described, by fans, steam jets, or pistons, and this would be
an advantage, particularly where the air channels are not large enough to give the
required quantity of air by the pressure of the atmosphere only. I estimate the
advantages of the free admission of air to the forward side of the propeller blades
for obviating or diminishing negative pressure on the propeller itself, and on the
stern of the vessel, together with the increased blade surface obtainable, and
consequent diminution of propeller slip from that now obtained in ordinary
practice, to about equal to one fourth more propelling effect from the engine power
now used.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said James Robertson in the Great Seal Patent Office on the 3rd February
1881.
"IMPROVE-
JAMES ROBERTSON, Engineer, of Govan, in the County of Renfrew.
MENTS IN AND CONNECTED WITH SCREW PROPELLERS FOR NAVIGABLE VESSELS."
My Invention relates to such oblique or curved bladed propellers as are made to
rotate and screw through water for the purpose of propelling vessels or ships, which
are usually termed and known as screw propellers.
My improvements consist mainly and essentially in allowing or causing air or
gas to flow in from the atmosphere to the partial vacuum formed in the forward
side and following edges of the blade or blades of the propeller when in motion,
and in the ways, ducts, channels, and apparatus for forcing, leading, conveying, and
regulating the air onwards to the forward side or sides and following edges of the
propeller blades to reduce or destroy the vacuum at the parts or positions on the
propeller blades where its existence mainly causes retarding action or waste of the
propelling power used. The admission of air to these parts, by reason of its low
specific gravity as compared with water and quick flowing qualities of air, and by
the modifications of the propellers and apparatus for conveying and regulating
forward the same, as hereinafter set forth and illustrated by the Sheet of
Drawings hereunto attached, readily effect this end and greatly increases the
efficiency of propellers.
For the admission of air into the partial vacuum above mentioned I form the
propeller blade or blades with hollow ways or ducts for conveying the air, with
an opening or openings to lead the air out to the forward side and following
edges at or near, or advantageously accessible to, the positions where the retarding
partial vacuum mostly takes place, or as will best destroy the retarding effect of
same. Communicating with a hollow channel or channels formed in the blades
I form the boss of the propeller, also communicating with the hollow air ducts
formed in the blades, and by providing an air opening or openings from this
annular or otherwise formed cavity out to the ends of the propeller boss, to which
I connect, by means of a stationary adjacent or contiguous boss or annular embracing
gland, an air pipe or duct led to same and through the skin of the vessel, or up
on the outside of the vessel, sufficiently above the water to allow the air to flow
into the ascending pipe without water to the propeller boss and blades.
Figure 1 is an end elevation of a four bladed or four threaded propeller outwardly
of an ordinary helical form, but with an increase of surface beyond what is com-
monly used, having two of the blades broken off, and the boss A shown in section,
an annular swivel joint piece B embracing the forward part of the propeller boss,
as shown on it in section, showing the air ducts and branch air pipe connection B¹
formed on it, and air pipe c connected to same, the propeller being shown fixed
upon a shaft D and set in a stern pipe bush E of the ordinary form, which stern
pipe E is placed in a frame F, consisting mainly of an inner stern post F¹ and
outer stern post F2, also all of the usual form; Figure 2 is an elevation of this
propeller from the forward side showing the air ducts A, A1, A2, A¹, leading from
the end of the propeller boss A, the air ducts A leading to the ducts formed
radially in the blades, indicated by the dotted line A, and terminating in the air
openings As which open out to the forward side of the blades, seen in the sectional
elevation of the blades, Figure 3. The air pipe C seen connected to the annular
air channel embracing gland B, by preference made of brass, which is bored out a
working fit and placed on the propeller shaft D between the stern pipe E and the
propeller boss A, seen in section, as before described, in Figure 1, and in end
elevation in Figure 4. In Figure the air pipe C, as shown and described in

page_text: connection with Figure 1, is shown bent round from the sternward side of the
stern post F to the side skin of the vessel, and led up through the skin of the
vessel at the flange C'. This air pipe, which should have a cross sectional area of
about the sixtieth part of a square inch to each indicated horse power of the pro-
pelling engines, is terminated in a standard pipe C² provided at its extreme end
with a cock G for admitting, regulating, or shutting off the air by hand at pleasure
from the propeller, its form to be seen in an enlarged scale drawing of it in
external elevation by Figure 6. This standard pipe C2 is also provided with a
vacuum gauge H, which will indicate the degree of vacuum on the propeller blades
while in motion when the cock G is shut. This standard terminal pipe may be
placed on the bridge to be readily in view of the commander of the vessel, or to the
engine room, and the air pipe C led to same or to any other required position in
the vessel, thus completing the pipe or air ducts for the admission of the air to the
forward side and following edges of the propeller blades.
The propeller is shown in forward motion, as indicated by the darts shown on
the coupling disc D' of the propeller shaft, and as shown by the large dart at the
periphery of the blades in Figure 2, the motion of the air being indicated by the
arrows on the air pipe or ducts and on the propeller blades.
In this modification of propeller the blades A', A5, A6, and A' are on their
forward sides formed into channels As, Aº, by the concentric flanges A10, A¹¹, best
seen in the sectional elevation of the blade, Figure 3. These flanges A10, A", which
form these concentric external channels on the forward sides of the blades are
made to divert the flow of the air from the opening As to the following edges of
the blades, where the retarding effect and waste of propelling power by the partial
vacuum in a large degree takes place. In ordinary screw propellers of swift going
vessels the vacuum on the forward sides and following edges of the propeller blades
is generally of from 7 to 10 lbs. to the square inch, but takes places over a small
area, as is generally indicated by the corrosion and "pitting" of iron and steel
blades, caused apparently from an acid gas being generated under this partial
vacuum, and which the admission of air in the way described will tend to remedy.
The area in ordinary or common screw propellers, and as ordinarily formed and
applied, on which this partial vacuum takes place, as before stated, is small, and
about, as indicated by the dotted curved line a, b, shown on the blade A' in Figure 2,
and the propelling effect thereon proportionally small; but by the admission of air
as described caused to issue out at the openings As, and diverted by the concentric
ribs A, A, and the swift motion of the blades through the water, as indicated by
the arrows, the area of the partial vacuum over the forward side of the blade is
greatly extended in area, as is inindicated approximately by the curved dotted
line c, d, seen on the blade A' in Figure 2, and the admission of air at or near
the root of the blade, as from the air opening A³, prevents the centrifugal
flow of the water towards the extremity of the blades on the forward side, which is
a great cause, if not the greatest cause, of waste of power as ordinarily applied by
the screw propeller. Figure 7 is a plan of this propeller with the blade A* shown
in section across the line e, f, the other blades being broken off, showing the
flange A and the blade in this part in sectional plan, the dart indicating the
motion of the blade through the water, and showing approximately the vacant
space formed in the water by the swift motion of the blade without air being
introduced, indicated by the angle formed by the lines h, i, h, k, the proportional
retarding effect on the following edge of the blade being approximately indicated
by the base line of the angle i, k, of the angle h, i, k. On the introduction of air
regulated by the cock G, as described, and issuing from the openings A³, as indicated
by the arrows, and diverted to the following edges of the blades and by the
flanges A, A, as hereinbefore explained, the vacuum is destroyed on the
following edges where it retards the speed of the engines and forward motion of
the vessel, whilst it is partially retained for forward propulsion on the forward side
of the blade, and to about the full vacuum at the tip of the blade and over a space,
as approximately indicated by the curved line h, l, over the breadth of the blade

page_text: shown in Figure 7, and over the length of the blade approximately indicated, as
before described, by the curved line on the blade A' in Figure 2, the degree of
vacuum being small around the air opening A³ and increasing towards the extreme
points of the propeller blades, by the air being guided and diverted to the following
edges by the concentric flanges A10, A", and by the swift motion of the blade
through the water, and the air being dispersed before it can in large quantities
reach the extreme ends of the blades, securing thereby greater propelling effect
from the power of the engines.
In the following Figures the same letters of reference and numerals refer to like
parts; the darts also indicate the direction of the motion of the propellers, and the
arrows indicate the motion of the currents of air, and from the action of these
propellers and air currents in same being in all the modifications similar to that
described in the fore-going Figures are explained by reference to these, differing
modifications and details mainly being hereinafter accurately verbally described.
Figure 8 is an elevation of a two bladed propeller from the forward side with
one blade broken off; Figure 9 is a plan of same, and Figure 10 a sectional elevation
of the blade A. In this modification the channels A8, A°, formed by the concentric
ribs A, A, are made successively shorter on the following edge than the body of
the blade for more effectually insulating the vacuum formed on the forward side of
the blade towards its extremity from the area around the air opening A³. The air
is admitted through the centre of the propeller shaft D, as explained hereafter in
connection with Figures 16 and 17, otherwise the action of the blades of this
propeller and action of the air is the same as that described by Figures 1, 2, and 3,
the darts indicating the direction of the motion of the propeller, and the arrows
that of the air, and the letters of reference and numerals referring to like parts,
and does not therefore require other description. Figure 11 is an elevation of a
propeller from the forward side of the blade of a generally similar construction to
that just described in connection with Figs. 8, 9, and 10, but having the insulated
air channels A³, A, elongated and stepped towards the following edges, and the
insulating flanges or ribs A10, A1, bent inwards towards the centre of the propeller
seen in Figure 9. The air is admitted by two channels; in one way through the
centre of the propeller boss, as indicated by the arrows, and to channel A10 by the
air pipe I, which admits air by a second pipe, and it may be at a different degree
of vacuum from that led to the centre of the propeller, to be hereinafter more fully
explained in connection with Figures 19 and 20, otherwise the motion of the
propeller is indicated by the darts and the motion of the air currents indicated by
the arrows, and the action, and letters, and numerals of reference referring to like
parts as explained in connection with Figs. 1, 2, and 3, and therefore is thereby
all explained.
It will generally be more convenient to form a hollow annular air boss communi-
cating with the hollow boss of the propeller, shown in a separate piece by Figure
in the stern pipe, as thereby the air pipe C led up from the outside of the vessel
can be dispensed with, air pipe being connected to the stern pipe wholly inside the
vessel.
Figure 14 is a side elevation, and Figure 15 an end elevation of a stern pipe E,
propeller shaft D, and propeller boss A, with the air channel E², and propeller
boss A, shown in section seen in Figure 14. The bush for the shaft is bored out
eccentrically to the external surface of the stern pipe E, seen in end elevation and
in section, and showing the air duct E formed in it in Figure 15. This air duct E²
is terminated in a branch pipe E¹, to which the air pipe C is attached, and from
thence led and fitted up at the bridge or engine room, as explained in connection
with Figure 6. The form of the annular swivel joint on the end of the stern pipe
next to the propeller boss, and the propeller boss itself, is the same as explained in
connection with Figures 1, 2, and 4, and the same reference letters and numerals
used for like parts, and the action of the same, and the arrows indicating the air
currents, is thereby explained.
Instead of the air communication being made to the propeller by a swivel joint

page_text: formed with the boss of the propeller, as described in connection with Figures 1, 2,
and 4, the propeller shaft may be formed hollow, and the air made to pass along
the hollow in the shaft outwards into the centre of the propeller boss. Figure 16
is a side sectional elevation of a propeller boss A adapted for bolted on blades, with
portion of a propeller shaft D and steam pipe E set in a propeller frame F. The
propeller shaft D' is shown formed hollow inwards from the sternward end to the
centre of the propeller hollow boss A, and with openings D², D³, outwards from the
centre of the shaft for conveying the air from the centre air duct in the shaft to
the centre of the propeller boss. The extreme end of the propeller shaft D has shown
on it a seat bored out larger than the air channel formed in its centre, and a hollow.
air centre swivel piece B with a spigot end inserted into this seat in the propeller
shaft a working fit, and is shewn fixed to the outer stern post F2 in front elevation
in Figure 17; there is no stuffing box used, as a little water entering along with
the air is readily thrown out of the air ducts by the centrifugal action of the
propeller. An air pipe C is shown connected to the central air swivel piece B, and
from thence the air pipe C can be led to where required inside the skin of the
vessel.
Figure 18 is a side sectional elevation of a similar propeller boss to that just
described, with propeller shaft D, stern pipe E, and annular air channel embracing
swivel air gland B, also shewn in section and shown set in a stern post F¹ broken
off short. The air channel formed in the centre of the propeller shaft D extends
inwards beyond the stern pipe E, and opens out to the periphery of the shaft by the
air openings or radial channels D', D2, over which the annular embracing air channel
gland B is placed, to which the air pipe C is attached, and from thence led to where
required inside the vessel. The annular embracing air swivel gland is in this
example formed with stuffing boxes and provided with the stuffing glands B¹, B³,
to keep it air tight.
Instead of leading the air directly to the centre of the boss of the propeller by
swivel joints, or through the propeller shaft, as in the foregoing examples, it may
be led by a crank formed pipe to the propeller blade or blades, or to one blade, and
from it thence through the central boss to other blades.
Figure 19 is a propeller boss in sectional elevation, the same as that just described
in connection with Figures 16 and 18, fitted on a solid propeller shaft D, showing a
stern pipe E broken off and set in a propeller frame F. A small air joint with
socket piece B is shown fixed on to the outer stern post F*, having a swivel hollow
annular piece fitted in it, all by preference made of brass, and much resembling an
ordinary stop cock, but with the key or swivelling piece B³ formed hollow for the
passage of the air, and is formed with a stuffing box and fitted with a stuffing
gland B³ to keep it air tight, seen in front elevation in Figure 20. The socket
piece B' has attached to it the air pipe C for being led to the inside of the vessel,
and from the rotating hollow piece B³ the air is conveyed to the propeller hollow
blade A by the crank shaped air pipe I jointed to the blade and leading the air
into the air duct in the blade Aº, and from thence through the hollow central
boss A of the propeller to the other blades. This arrangement for leading the air
to the propeller has the advantage of being more easily provided and attached than
the foregoing arrangements for the same purpose described, and is suitable for being
applied to propellers where two air pipes may be used, each regulated to different
degrees of vacuum, shown and explained in connection with Figure 13.
Any outward form of known or other screw propeller is more or less suitable for
having the air communication under my Invention led to their blades, and will
generally be advantageous with an increase of blade surface. The free introduction
of air to the forward side of propeller blades, as ordinarily proportioned by the
arrangements for same hereinbefore described, would generally produce an increase
of speed on the propeller by a like pressure of steam equal to about one third, and
without an increase of blade surface, or by working with a reduced vacuum, which
can be readily adjusted to anything by the air regulating valve; the power thus
relieved requires additional blade surface to prevent slip.

page_text: Figure 21 is an elevation of a four bladed propeller of the common form with blade
area of suitable proportions to work on this plan, with two blades removed, and
the blade A, shown in section, and showing the form and area of the same on the
flat and the blades being formed truly helical; Figure 22 being a section of the
blade through the line on Figure 21, m, n; Figure 23, a section through the
line o, p; Figure 24, a section through the line q, r; Figure 25, a cross section
through the line s, t; Figure 26, a cross section of the blade A' through the
line u, v, showing the air duct in same A³; Figure 25 shows in cross section the
air outlet As, and also an air outlet, in this example to the thrust or after side of
the air opening, fitted with a louver board shaped plate A¹ placed to divert the air
to the following edge of the blade. The darts indicate the direction of the motion
of the propeller and the arrows the direction of the air currents, and the action is
similar to that described in connection with Figures 1, 2, and 3, and like letters of
reference used to like parts.
Figure 27 is an elevation of a curved bladed propeller from the forward side,
and Figure 28 an end elevation of same. The form of the central boss A is the
same as that described in connection with Figures 1, 2, and 3; the air ducts in the
blades A³ are also of the same general shape, but are formed on the forward side by
the thin plates A, A, A, A, having a series of small holes A³ diminishing
in size towards the point of the blade for the egress of the air; the motion of the
propeller is as indicated by the dart, and the flow of the air as indicated by the
arrows; the same letters of reference and numerals refer to like parts, as in
Figures 1, 2, and 3. Figure 29 is an elevation of a propeller from the after or
thrust side; Figure 30, a plan of same with the blade A' shown broken off;
Figure 31, an elevation showing the blade on the flat and the position of the air
duct A; Figure 32, a cross section of the blade through the line u, v, in Figure 31,
like reference letters and numerals being used for like parts, as in Figures 1, 2,
and 3. In this example the air duct A³ leading into the centre of the boss A is led
up the advancing edge of the blade, and is formed by the convex shaped plate A¹,
seen in section in Figure 32. The air is caused to flow out from under this louver
board plate by its swift motion and lateral action through the water, the air
passing over the thrust side of the blade in a thin film to the following edge of the
blade, as indicated by the arrows, and tending on its course to relieve the skin
friction of the water across the thrust surface of the blade.
Instead of being formed of cast metal the hollow blades may be formed of thin
wrought iron, steel, or other metal plates, with stud pieces in the centre of these
plates to keep the plates apart and riveted at the edges and to the centre, suitable
air orifices being formed on their forward or following edges, which may be of a
louver board fashion.
Figure 33 is an elevation from the forward side of a propeller of this form;
Figure 34, a sectional elevation, with blade A broken off; Figure 35, a cross
section of the blade through the line m, n, shown in Figure 33; Figure 36, a
cross section through the line o, p; Figure 37, a cross section through the line q, r;
and Figure 38, a cross section through the blade at the lino 8, t.
The boss A is cast of steel, and the air admitted through the shaft, as explained
in connection with Figures 16 and 17. The plates A1, A3, forming the hollow and
thin parts of the blades are rivetted on to the central steel boss A, and rivetted
together with intervening stud pieces A¹ to keep the plates apart; the air finds
egress in the holes A³ close to the cast steel centre. The same letters of reference
and numerals are used for like parts as in the other modifications of these
propellers.
Figure 39 is an elevation from the thrust side of a propeller similarly constructed
as that just described, composed also of a cast steel centre and of two thin plates
rivetted to it and to themselves in the same way, but having a louver board
arrangement for exhausting out the air by lateral action from its centre, similar to
that described in connection with Figures 29, 30, 31, and 32, the louver board
plate A being shown rivetted on to its forward edge, and portion of it being shown

page_text: broken off to show the air egress holes in it A³. Figure 40 is a cross section of the
blade A through the hole o, p, in Figure 39, showing the form of the louver board
plate, the arrow showing the air issuing from the opening A³ and passing across
the thrust side of the blade to its following edge and forward side; the same
reference letters and numerals are used for like parts as in the foregoing Figure
explained.
Instead of establishing the air communication through ducts or channels in the
blades of the propellers, as herein before described, hollow channels may be bolted or
otherwise fixed to the propeller blades, or made to revolve apart from it, communi-
cating with the propeller boss and swivel joint, or revolving apart from it on a
separate central swivel joint axis, to which the air can be led, and thence led
through arms opening out at or near the positions required on the propeller
blades.
Figure 41 is an elevation of a two bladed propeller from the after or thrust side
and portion of an outer stern post F2, shown broken off, to which is bolted a swivel
air joint B, similar to that described in connection with Figures 19 and 20, having
two crank shaped air pipes fixed to the revolving air hollow centre B³, and made to
pass through holes in the propeller blades loosely, to convey the air to the forward
side of the blades, the position of these air pipes I being seen in the sectional plan,
Figure 42, of the blade through the line u, v, and thereafter the air is guided and
led where required to any position on the blade by the concave duct A. The
motion of the propeller is indicated by the dart. Two or more propellers may be
placed on one propeller shaft, and each propeller may consist of one or more blades,
and are led to each. Figure 43 shows a sectional elevation of one arrangement for
effecting this by a hollow shaft, similar to that explained in connection with
Figure 18, the letters of reference used being alike for like parts.
Fig. 44 shows another arrangement for the same, a coupling air pipe being used
for connecting one or more propellers, similar to that described in connection with
Figures 19 and 20, the same letters of reference being used for like parts as in
these foregoing Figures, the propeller shaft D in this case being solid, the propellers
having hollow bosses to convey the air to the other propeller blades, as indicated
by the arrows.
Instead of the air being allowed to flow into the blade or blades of the propellers by
the pressure of the atmosphere only, as hereinbefore described, the air may be forced
in by fans, steam jets, or by pistons, or by any convenient machine for forcing air,
but it will be generally of most advantage to form the air ducts to effect the
objects herein before set forth without the use of mechanical air forcing machines or
apparatus.
Having now described the nature of the said Invention, and in what manner
the same is to be performed, I have to state that I do not restrict myself to the
precise details herein described or delineated, but that which I believe to be novel
and original, and claim as the Invention secured to me by the hereinbefore in part
recited Letters Patent is, the introduction or admission of air or gas to the blade
or blades of so-called screw propellers generally, so as to reduce and regulate the
vacuum formed in the water by these when in motion, substantially as herein
set forth.
In witness whereof, I, the said James Robertson, have hereunto set my hand
and seal, this Twenty-eighth day of January, in the year of our Lord One