IMPROVEMENTS IN SCREW PROPELLERS FOR PROPELLING VESSELS, APPLICABLE ALSO FOR RAISING AND FORCING FLUIDS
Year: 1879

page_text: LETTERS PATENT to George John Stevens and John Samuel Smith, both of
87, Blackman Street, Borough, in the County of Surrey, for the Invention of
"IMPROVEMENTS IN SCREW PROPELLERS FOR PROPELLING VESSELS, APPLICABLE
ALSO FOR RAISING AND FORCING FLUIDS."
Scaled the 5th December 1879, and dated the 9th June 1879.
PROVISIONAL SPECIFICATION left by the said George John Stevens and
John Samuel Smith at the Office of the Commissioners of Patents on the 9th
Juno 1879.
.
GEORGE JOHN STEVENS and JOHN SAMUEL SMITH, both of 87, Blackman Street,
Borough, in the County of Surrey. "IMPROVEMENTS IN SCREW PROPELLERS FOR
PROPELLING VESSELS, APPLICABLE ALSO FOR RAISING AND FORCING FLUIDS."
According to this Invention in order to generate the screw surface of a screw
propeller or screw to be used for pumping or forcing fluids we take a tractrix curve,
that is, a curve the length of whose tangents to a given straight line is constant,
and cause this curve to revolve uniformly around the given straight line, which is
an asymptote to the curve, and at the same time traverse along it at an uniform
rate. A curved screw surface is thus generated having the properties that any
circles described in planes at right angles to the axis, and having their centres on
the axis will meet the screw surface at a constant angle, and if such screw surface
is cut by a plane at right angles to the axis the trace of the curved screw surface
will be a spiral which meets the axis at an angle equal to the angle of pitch at the
periphery of the screw.
For distinction we will call the tractrix curve the primary curve, and the trace
of the screw surface when cut by a plane at right angles to the axis the secondary
curve.
By the aid of this secondary curve we generate the screw surface of a screw
propeller or screw for forcing or raising fluids.
If we take the secondary curve and let it rotate about and traverse along the
axis, keeping the amount of rotation and of traverse the same as in the former case,
the original screw surface is produced; but if the traverse be more or less, then the
angle of surface to circles in planes at right angles to the axis will vary continuously
Prine 71

page_text: from the periphery to the axis. The angle at the periphery will be the angle of
pitch, whilst the angle when meeting the axis will remain the same as in the former
case.
It will thus be seen that we are enabled to construct a screw surface of any
desired angle, irrespective of the pitch of the said screw, in any of the following
ways:-
1st. Constant angle of surface from periphery to root or axis,-
A. Equal to the pitched angle of screw; that is, when the whole of the primary
curve is used.
B. Equal to any angle less than the pitched angle of screw; that is, when only
part of the primary curve is used, the outer extremity of this curve being cut.
2nd. Angle to vary from periphery to root or axis, produced by giving greater or
less traverse to the secondary curve,--
A. From greater to less.
B. From less to greater.
There are other spiral curves which are very similar to the secondary curve
generated as above described from a tractrix curve, but which have not precisely
the same properties, still it will be seen that they may be used in a similar manner
to generate screw surfaces that in their action will be little, if anything, inferior
to those generated by the tractrix curve.
The form of the blade of the propeller will depend entirely upon the form or curve
of the back surface, which should be so shaped as to give the requisite strength of
blade, and at the same time gives its entering and leaving edges the finest lines
consistent with the length of the blade.

page_text: The dotted lines marked a, a, shew what we term the secondary curve or spiral,
and the dotted lines b, b, shew the sections of the blade when cut by the plane of
the axis.
Figure 4, Sheet 2, shews our Invention applied to the surface of a screw suitable
for screw-cutting lathes, dividing engines, and other machines and apparatus whore
a regular and uniform wear of surface is required.
There are other spiral curves which are very similar to the secondary curve
generated as above described from a tractrix curve, but which have not precisely
the same properties, still it will be seen that they may be used in a similar manner
to generate screw surfaces that in their action will be little, if anything, inferior to
those generated by the tractrix curve.
The form of the blade of the propeller will depend entirely upon the form or
curve of the back surface which should be so shaped as to give the requisite strength
of blade and at the same time gives its entering and leaving edges the finest lines
consistent with the length of the blade.
Having thus described our Invention, and the means of performing the same,
what we claim and desire to protect by the herein in part recited Letters Patent
is,―
Firstly. The manufacture of screw propellers for propelling vessels having their
surfaces formed or generated in the manner substantially as herein described and
illustrated in the Drawings annexed.
Secondly. The manufacture of screws capable of being used for raising or forcing
fluids, and for other like purposes, by forming or generating their surfaces substan-
tially as hereinbefore described and illustrated in the accompanying Drawings.
In witness whereof, we, the said George John Stevens and John Samuel
Smith, have hereto set our hands and seals, this Eighth day of December,
in the year of our Lord One thousand eight hundred and seventy nine.
Witness,
G. F. REDFERN,
Patent Agent,
4, South Street,
Finsbury.
GEORGE JOHN STEVENS. (L.S.)
JOHN SAMUEL SMITH.
(L.S.)