IMPROVEMENTS IN PROPELLING VESSELS
Year: 1854

page_text: LETTERS PATENT to Alexander Mitchell, of Belfast, for the Invention of
"IMPROVEMENTS IN PROPELLING VESSELS."
PROVISIONAL SPECIFICATION left by the said Alexander Mitchell at the
Office of the Commissioners of Patents, with his Petition, on the 10th
January 1854.
I, ALEXANDER MITCHELL, of Belfast, do hereby declare the nature of the
Invention for "IMPROVEMENTS IN PROPELLING VESSELS" to be as follows:-
By my method I propose that no part of the propeller shaft shall be removed
with the propeller, though when the propeller is placed in the dead run or wake
of the vessel it will be necessary to draw it back in the manner herein-after
described.
The extremity of the propeller shaft is turned to a cone, next to which is a
cylindrical portion with a diameter less than that of the shaft, the shaft being
itself increased to the required size by the frustum of a cone or any other form
that may make its connection with the propeller more perfect. Should the
propeller be a right hand one, the cylindrical portion of the shaft above
described must he chased with a left hand screw, or vice versa, with a left hand
propeller. But reversing the screw on the shaft to that of the propeller is only
necessary when it is intended to place the propeller in the run of the vessel or
screws on either side of it.

page_text: A portion of the central opening in the propeller corresponding with the
chased portion on the shaft is chased in such a manner as to make a perfect
junction with it; the remainder of the opening is turned conical so as closely to
fit on the frustrum of the cone above described. The connection thus formed
would be quite sufficient for propelling a vessel forward, but were the engine
to be reversed, which is absolutely necessary at times, it is plain that the pro-
peller would be screwed off from the shaft, -to prevent which an additional
security is required, which is thus effected:-Two rectangular grooves are sunk
at opposite sides of the shaft, and fitted with two long keys or feathers flush
with the shaft. One extremity of these keys forms a portion of the frustum of
he cone (or other form) before mentioned; on the other extremity, which is far
within the vessel, there is a raised snug by which the keyes or feathers can be
driven back or forward. Two key beds are sunk in the conical or hollow
portion of the propeller to receive these keys when driven forward, thereby
strengthening the connection. When the propeller has to be unshipped or
removed, these keys are driven back and the engine reversed, by which the pro-
peller becomes detached, and the shaft drawn within the vessel by means of a
similar screw worked by a corresponding clutch. The description of screw
propeller I propose using I have named my elliptical propeller, for this reason,
that in place of the radial lines being straight from the axis to the periphery
they are curves, and their curvature increases as they depart from the axis to
the periphery, so that if the two arms were divided by a plane, in which the
centre of the shaft would lie, the section would show a portion of an ellipse or
a figure nearly approaching it, the curve increasing as it approaches the
periphery; the point where it crosses the centre of the shaft being a terminus
of its conjugate or lesser diameter.
The propeller may have any number of arms, though four may be found the
most convenient; each arm is connected with those adjacent to it by a flat bar
of malleable iron rivetted through the arms near the periphery. These bars
are for the purpose of "fishing," the propeller, when it is to be removed; and
lowering it to its place when to be attached, being sharpened at either edge to
offer less resistance to the propeller when in motion.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said Alexander Mitchell in the Great Seal Patent Office on the
10th July 1854.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, ALEXANDER
MITCHELL, of Belfast, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Tenth day of January, in the year of our Lord One
thousand eight hundred and fifty-four, in the seventeenth year of Her reign,
did, for Herself, Her heirs and successors, give and grant unto me, the
said Alexander Mitchell, Her special licence that I, the said Alexander
Mitchell, my executors, administrators, and assigns, or such others as I, the
said Alexander Mitchell, my executors, administrators, and assigns, should at
any time agree with, and no others, from time to time and at all times there-
after 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 PRO-
PELLING VESSELS," upon the condition (amongst others) that I, the said Alexander
Mitchell, by an instrument in writing under my hand and seal, should par-
ticularly 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 Alexander Mitchell, 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 (that is to say):-
First, in a certain form given to the blades or arms of the screw propeller, by
means of which the water passes from the propeller when in motion in a dense
cylindrical mass, and is not scattered or broken as is the case with the ordinary
propeller, which by sending off' a certain portion of the water centrifugally at
right angles with the axis gives a violent vibratory motion to the entire mass
of ship and cargo, and thus subtracts materially from the propelling power of
the engines. The blades or arms of my propeller I curve throughout their
entire length, commencing where they spring or radiate from the central axis,
and ending at the outer verge, the said curve not being the segment of a circle,
but increasing or becoming more hooked as it approaches the periphery. Now
as this curve is either the segment of a true ellipse, or a figure closely resembling
it, taking the centre of the hubb as a terminus of the conjugate or lesser axis, I

page_text: Now
give to it thus modified the name of the elliptic propeller. As this curvature
of the blades is for the purpose of counteracting the centrifugal motion given
to the water, which tendency increases with an increased pitch, it is clear that
the greater the pitch, the greater also should be the hook of the blades or arms.
every required degree of curvature may be found in an ellipse in which
the transverse or greater axis is double that of the conjugate or lesser, the
curve in no case exceeding a fourth part of the perimeter of such ellipse, com-
mencing with a terminus of the conjugate axis, which point should always be
in the centre of the hubb. It may here be stated generally that for the single
purpose of propelling a vessel forward too great a curve could scarcely be.
given, provided the transverse axis be not passed, but an excessive curve would
subtract from the power of the propeller in backing the vessel, or in making.
what is called asternboard. To lay down a fixed rule shewing the precise
curve to be given to the blades of a propeller, and suited to an endless variety
of cases, circumstances, and conditions, would be quite impossible. I shall
therefore give as an example the propeller which, among many, I have found
to work the best, and from which, provided the principle be adhered to:
and correctly applied, any experienced mechanic may calculate for himself the
changes necessary to suit the particular case before him.
In the "Malvina" steam vessel an elliptic propeller was placed some months
since, one of the ordinary straight armed for being removed for the purpose;
this change was found to increase her speed about a knot and a half per hour,
to render the vibration scarcely perceptible, and to decrease the consumption.
of coal fully twenty-five per cent. The diameter of the propeller is nine feet,
the pitch fifteen feet nine and a half inches, and the curve of the blades of
the quadrant of the ellipse above quoted. The vessels burthen is about four
hundred and fifty tons, and the power of her engines one hundred horse.
Gearing being used, the rotary rate of the propeller at the periphery is, under
ordinary circumstances, two thousand four hundred and sixty feet per minute.
Pitch and rotation, the chief causes of centrifugal slip, are here given, and
neither can produce the effect unless in conjunction with the other. Their
disturbing power may be considered as nearly equal, and therefore in calcula-
tion each convertible into the other; as, for example, in two straight armed
propellers of equal pitch, but one rotating with double the velocity of the other,
the centrifugal action would in the latter be also double, and in the case of
two straight armed propellers the pitch of one being double that of the other
but rotating with equal velocity, the slip of that of the higher pitch would be
also double that of the other. The difference of diameter need scarcely enter
into the calculation, for provided the rapidity of rotation at their periphery be

page_text: equal, the effect produced would be also equal, or nearly so. To my propeller I
have not found it desirable to give an increasing pitch (as it has been named),
each concentric line having a uniform pitch throughout the blades, shewing a
uniform pitch along any section parallel to the axis, but the pitch of each
parallel section increasing as such section approaches the axis, and thus, by
adhering closely to the true principle of the screw, inequality of action between
the parts is prevented, and the whole produces one uniformly beneficial effect.
The above particulars have reference only to the hollow or working face of the
propeller, which is turned aft, while the blades on their other side next the hull
of the vessel are slightly rounded on their cross section, to give the necessary
strength without taking from their sharpness on either verge; and experience
has shown that this does not subtract materially from the backward motion of
the ship, the engines, when reversed, going with increased speed by reason of this
form of blade taking less hold of the water. I do not confine myself to any
particular number of blades or arms for this my propeller, to which any
number can be given that may be thought advisable, and to the hubb I give
neither a spherical nor cylindrical form, but rather a lengthened oval, by
which sufficient strength can be had without materially impeding the progress
of the vessel.
With reference to the Drawing shewing the profile or side view of the pro-
peller, it will be seen that the hubb and cach part of the blades or arms have
precisely the same extension fore and aft, thus in this respect also closely.
adhering to the true principle of the screw.
Secondly, I declare my Invention to consist in a certain method of securing
to or detaching from a vessel while at sea a screw propeller of the form just
described, or of any other form. To assist in clearly understanding this process
reference must be had to the accompanying Drawings, by which it will be seen
that in detaching the propeller it only is removed, the propeller shaft being at
the time drawn within the vessel by the same process that disengages the
propeller. To effect this, the propeller shaft is prepared in two parts, a certain
portion of one at the place of their junction being forged hollow, into which a
portion of the other is accurately fitted, the parts being bored and turned to
render the junction complete, but also capable of sliding one within the other,
and therefore of being extended or contracted by means to be described here-
after. The manner in which the propeller is secured upon the shaft will be
best explained by reference to the accompanying Drawing of the propeller
shaft. A is the conical extremity of the shaft fitting into the after stern post
when the propeller is in use; B, the screw which connects the propeller, a
corresponding hollow screw being chased in the hubb; C, a conical portion of.

page_text: the shaft fitted to a conical opening in the propeller; D, one of two long keys
or slide bolts, the ends of which when driven forward, fit into corresponding
slots in the conical cavity of the propeller; E, a projection on the slide bolt,
by which it is either driven forward or withdrawn; F, a strong collar secured
on the shaft for abutting against a fixture in the vessel when making astern-
board; G, a strong screw chased on the propeller shaft of the same pitch as
that within the propeller; H, the end of that portion of the shaft which fits
within the hollow portion of the other; I, one of two slots sunk in this, as
shewn in the Drawing; K, the cylindrical portion of the shaft fitting on the
part last named; L, a portion of it of greater diameter extending from the
opening about one-third its length, in which two slots are sunk, and feathers
or ribs secured corresponding to and working with the slots in the other
portion of the shaft; M, a strong lever of the second order, the fulcrum being
at one end and hinging on a strong journal; it is greatly strengthened towards
its centre, where it is fitted to the propeller shaft grasping about one-third
the circle. In this cavity a screw is chased, fitting with that on the propeller
shaft, by means of which the shaft is either screwed forward into the propeller
or withdrawn from it. N, a strong key slightly tapered, driven through the
hollow portion for the shaft for the other portion to abut against when drawn
out.
means.
The propeller may either be placed in the run of the vessel or behind the
rudder. The latter position, besides possessing great facility for shipping and
unshipping, has may important advantages, but I shall now show how it
may be placed in the run, the propeller shaft being at the same time drawn
within the vessel, and the clutch lever resting on its corresponding screw. The
propeller may be lowered to its place either through an opening in the after
part of the vessel or slung over one of the quarters by a small crane or other
For this purpose a strong hook is rivetted on the back of each arm
to receive rings on the ends of the chains by which the propeller is lowered
and drawn into its place in the run, the hooks being so placed and the chains
so measured and adjusted, that when thus slung the opening through the hubb
will be horizontal and in the same line with the propeller shaft. The engines
being then slowly turned on, the propeller shaft is advanced into the propeller
by means of the screw and clutch lever described above. When the
screw in the hubb has taken hold, the suspending chains may be lengthened
about a fathom, and when the screw is home and the propeller has made half
a turn, the chains will drop from the hooks and the propeller be freed. The
strong key N is then driven into its place, and the clutch lever raised from
the shaft. The propeller is thus sufficiently secured for propelling the vessel

page_text: forward, but not for backing, for by reversing the engines the propeller would
be disengaged.
To prevent this the slide bolts D are driven into the slots in the conical
portion of the propeller, and thus the connexion is completed. To unship and
hoist on board the propeller, the bight of a strong chain is dropped over one of
the arms and there bound by a heavy link sliding by its weight down the
deuble chains. The slide bolts D being then withdrawn from the propeller,
and the key N from the shaft, the clutch lever M is dropped on the screw G,
and the engines reversed, by which the propeller shaft is gradually drawn
within the vessel, and the propeller thus freed can be hoisted on board. With
a right hand screw propeller the screws on the shaft must be left hand, and
vice versa.
Having thus described the nature of my Invention, and the manner of
performing the same, I would have it understood that I do not confine myself
to the details herein described and shown, nor do I claim the mechanical parts
separately. But what I claim is,―
First, the constructing propellers of the form described; and
Secondly, I claim the combination of apparatus for raising and fixing
propellers to propeller shafts.
In witness whereof, I, the said Alexander Mitchell, have hereunto set
my hand and seal, this Eighth day of July, in the year of our Lord
One thousand eight hundred and fifty-four.
Witness,
THOMAS GARRETT,
of Belfast, Solicitor.
THOS. L'ESTRANGE,
of Belfast, Solicitor.
A. MITCHELL. (L.S.)