IMPROVEMENTS IN THE CONSTRUCTION OF SHIPS' PROPELLERS AND OTHER LIKE ARTICLES WITH HELICAL SURFACES
Year: 1878

page_text: PROVISIONAL SPECIFICATION left by Robert Henry Armit and Thomas
Holt White at the Office of the Commissioners of Patents on the 9th January
1878.
ROBERT HENRY ARMIT, of Craven Street, in the County of Middlesex, a retired
Lieutenant of Her Majesty's Navy, and THOMAS HOLT WHITE, of Richmond, in the
County of Surrey, and of Oriel College, Oxford, Gentleman. "IMPROVEMENTS IN
THE CONSTRUCTION OF SHIPS' PROPELLERS AND OTHER LIKE ARTICLES WITH
HELICAL SURFACES."
Our Invention relates to the construction of screw propellers, fans, and other
articles, with blades formed in a peculiar manner, and which we term cycloidal, and
is a further development of the principles involved in the construction of the screw
propellers described in the Specification of Letters Patent granted to the said
Robert Henry Armit, dated the 15th September, A.D. 1876, No. 3618.
The manufacture of screw propellers with the blades shaped as described and
shewn in the said former Specification, is attended with certain difficulties which
the present Invention is designed to obviate, as herein-after described.
It is well-known to all familiar with the making of screw propellers, that the
ordinary and most convenient manner of forming the moulds of the propellor blades
previous to casting them is by a process termed "sweeping." But the shape of the
propeller blades as shewn in the said former Specification cannot be produced in
this manner, such blades requiring the use of special patterns which are very costly
and inconvenient. Therefore we have designed the following method of forming
these improved cycloidal propellers, whereby we obviate these difficulties without
sacrificing any of the advantages of the cycloidal form of the blades, and which we
will now proceed to explain.
If the circumference of a circle (herein-after termed the generating circle) be
rolled upon a right line until a point in that circumference in contact with the said
line passes entirely around the circle and comes again in contact with it, that point
will describe a curve termed a cycloid.
The line on which the generating circle rolls, and which may equal in length the
diameter of the screw, is the base of the cycloid which starts from the axis or centre
of the screw; and the diameter of the said circle drawn perpendicularly to this base,

page_text: at the centre or middle point of the latter, is the axis of the cycloid. Taking the
said axis of the screw as a centre, a circle is drawn around the same, whose radius
equals the distance from the said centre of the screw to the middle point of the base
of the cycloid, and this circle will represent the diameter of the propeller.
The width of the blade is the chord of an arc or part of the said circle representing
the diameter of the screw; and this width being determined the shape of the
edges of the blade are delineated by the cycloid in the plane of the face of the
propeller boss, or extending beyond this plane, as may be required. The external
points or corners of the said blades may be curved or rounded.
In the operation of "sweeping" our improved cycloidal propeller, it will be
understood that we commence in the usual manner by using iron or other suitable
plates, and complete the formation of the propeller by means of templates. One
template is placed on a right or straight line in the centre of the blade extending
from the boss to the periphery of the propeller, and vertical to the axis of its
rotation. The templates for the remaining portions of the blades will be developed
by or from the natural curves assumed by the blade, in consequence of its having
been delineated by a cycloid in the plane of the faces of the boss or outside of this
plane.
The position of the straight line on the blade above mentioned may vary
according to circumstances and the form of blade required.
One very important feature of this Invention is the peculiar and novel form we
give to the blade or blades of propellers for auxiliary screw steamers and other
vessels. According to this part of our said Invention we cut away the outer
portions of the blade in such a manner as to give the same somewhat of an almond
shape. The curves of this blade are determined and developed by the cycloidal arc
by the aid of templates, as above described. A propeller with its blade or blades of
this peculiar shape will offer very little resistance to the vessel's passage through the
water when under sail. Moreover this peculiarly formed propeller can be used in
many instances where cycloidal propellers constructed as first above described
could not be advantageously or conveniently employed.
According to another part of our Invention we modify the form of the boss of
the said propellers by making the same conical or conoidal in conformity with the
shape of the shaft on which it is fixed. We are thereby enabled to diminish the
weight of the boss without impairing its strength.
Moreover we wish it understood that we may use any desired number of blades
on our improved propellers, and these blades may be cast or otherwise formed
separately from the boss, and attached thereto by screws or other suitable means, so
that in case of fracture or injury of a blade the same may be readily removed and
replaced by a new blade if necessary.