AN IMPROVED SCREW PROPELLER
Year: 1878

page_text: LETTERS PATENT to John Thomson Stocks, of Kirkcaldy, in the County of
Fife, North Britain, for the Invention of "AN IMPROVED SCREW PROPELLER,"
Sealed the 18th April 1878 and dated the 7th Fahun
1970
LETTERS PATENT to John Thomson Stocks, of Kirkcaldy, in the County of
Fife, North Britain, for the Invention of "AN IMPROVED SCREW PROPELLER."
Sealed the 18th April 1878, and dated the 7th February 1878.
*
PROVISIONAL SPECIFICATION left by the said John Thomson Stocks at the
Office of the Commissioners of Patents on the 7th February 1878.
JOHN THOMSON STOCKS, of Kirkcaldy, in the County of Fife, North Britain.
AN IMPROVED SCREW PROPELLER."
This Invention, which consists of an improved screw propeller, has for its object
to prevent as far as possible what is known by engineers as the "racing" of the
engines at those times when the stern of the ship in passing through waves is more
or less raised out of the water.
.
The foregoing object is effected by constructing the screw propeller with a
quicker pitch of screw towards its central parts than towards its circumference, the
pitch gradually merging from the quick portion at the boss to the slower portion at
the periphery. By means of this construction those portions of the screw propeller
at or near the boss strike the water at a less angle than those portions at or near to
the periphery, and thus a greater resistance to the rotation of the screw is afforded
by those portions near the boss than by those at the periphery.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said John Thomson Stocks in the Great Seal Patent Office on the 3rd
August 1878.
JOHN THOMSON Srocks, of Kirkcaldy, in the County of Fife, North Britain.
"AN IMPROVED SCREW PROPELLER."
"
My said Invention consists of an improved screw propeller, and has for its object
to lessen in a great degree, or to entirely prevent what is known by engineers as the
"racing of the engines, which at present occurs when the ship pitches, and the
stern of the ship in passing through the waves is so raised that the propeller is
more or less out of the water.
I effect the foregoing' object by constructing the blades of the screw propeller of
such a form that they have a quicker helical curve or pitch at their roots next the
boss than the curve or pitch which they have at their outer extremities or periphery,
that is to say, the propeller has a faster pitch of screw towards the central part
than towards its circumference, the pitch gradually merging from the quick portion
at the boss to the slower portion at the periphery.
In practice I have found that if the pitch at the periphery is one-third of the
pitch at the boss good results are obtained.
And in order that my said Invention may be properly understood, I now proceed
more particularly to set forth the system, mode, or manner, in or under which the
same is or may be used or practically carried into effect, reference being had to the
annexed Sheet of Drawings, and to the letters marked thereon, that is to say,-
Figure 1 of the accompany Drawing is a front elevation; and Figure 2 is a
side elevation of a screw propeller constructed with blades formed in accordance
with my Invention, the quick portion at the boss being marked a, and the slower
portion near the circumference being marked b.
By means of the aforesaid construction those portions of the screw propeller at
or near the boss strike the water at a less angle than those portions at or near the
periphery, and thus a greater resistance to the rotation of the screw is afforded by
those portions near the boss, so that when the screw propeller is partly out of the
water racing" is prevented, or to a great extent prevented.
$6
Having now described and particularly ascertained the nature of my said Inven-
tion, and the system, mode, or manner, in or under which the same is or may be
used, or practically carried into effect, I would observe in conclusion that I wish it
to be understood that I do not limit myself to any precise relative degrees of
helical twist of the root and peripherical portions of the blades of the propeller, as
shewn in the accompanying Drawings; but what I consider to be novel and
original, and therefore claim as the Invention secured to me by the herein-before in
part recited Letters Patent is, the improved screw propeller constructed or formed
essentially as herein-before described and shown on the accompanying Drawings,
that is to say, a screw propeller whose blades have a quicker helical twist at their
roots or the central part of the propeller than the helical twist which they have at
their outer extremities or parts at the periphery of the propeller, and wherein the
pitch at the roots gradually merges into the pitch at the periphery.
In witness whereof, I, the said John Thomson Stocks, have hereunto set my
hand and seal, this First day of August, in the year of our Lord One
thousand eight hundred and seventy-eight.