IMPROVEMENTS IN SCREW PROPELLERS
Year: 1869

page_text: LETTERS PATENT to Thomas Vernon Trew, of Stratford, in the County
of Essex, Ships' Draftsman, for the Invention of "IMPROVEMENTS IN
SCREW PROPELLERS."
PROVISIONAL SPECIFICATION left by the said Thomas Vernon Trew
at the Office of the Commissioners of Patents, with his Petition,
on the 12th February 1869.
I, THOMAS VERNON TREW, of Stratford, in the County of Essex,
Ships' Draftsman, do hereby declare the nature of the said Invention for
"IMPROVEMENTS IN SCREW PROPELLERS," to be as follows:-
This Invention relates to constructing screw propellers in a form
which consists of a combination of a curved propelling surface and a
spiral leading edge, as hereafter explained. The outside or leading edge

page_text: of this screw is of spiral curved form from boss to tip of blade. The
propelling or after surface of the blade curves up and forward from the
centre line to the leading edge.
The Drawings accompanying this Provisional Specification afford
illustrations of the nature of the Invention.
Fig. 1, elevation of one of the blades set off on a cone (shown in dotted
lines); Fig 2, an expanded sketch shewing the form of the face of the
blade at the different points numbered, and also the method of obtaining
the curve of the face of the blade (in blue lines). The centre part of
this screw should be cut away in order to put à nut on the end of the
shaft.
This Invention is applicable to screws of any suitable number of
blades, and I may remark that rigid adherence to the particular curves
shown by the Drawings is not essential so long as in substance the
Invention as above described is followed.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said Thomas Vernon Trew in the Great Seal Patent
Office on the 11th August 1869.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, THOMAS
VERNON TREW, of Stratford, in the County of Essex, Ships' Draftsman,
send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Twelfth day of February, in the year
of our Lord One thousand eight hundred and sixty-nine, in the thirty-
second year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said Thomas Vernon Trew, Her special
licence that I, the said Thomas Vernon Trew, my executors, adminis-
trators, and assigns, or such others as I, the said Thomas Vernon Trew,
my executors, administrators, and assigns, should at any time agree
with, and no others, from time to time and at all times thereafter during
the term therein expressed, should and lawfully might make, usc,
exercise, and vend, within the United Kingdom of Great Britain and
Ireland, the Channel Islands, and Isle of Man, an Invention for "Ix-

page_text: PROVEMENTS IN SCREW PROPELLERS," upon the condition (amongst others)
that I, the said Thomas Vernon Trew, my executors or administrators,.
by an instrument in writing under my, or their, or one of their hands
and seals, should particularly 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 Thomas Vernon Trew, do hereby
declare the nature of my said Invention, and in what manner the.
same is to be performed, to be particularly described and ascertained
in and by the following statement..
This Invention consists in constructing screw propellers in a form.
which is a combination of a curved face or propelling surface and a
spiral leading edge, as hereafter explained, that is to say, the outside
or leading edge of this screw is. of spiral curved form from boss to
tip of blade. The propelling or after surface of the blade curves up
and forward from the centre line to the leading edge. But I may
state that the centre part of the screw is so far hollowed as to allow
a nut to be put on the end of the shaft, and that my said Inven-
tion is applicable to screws of any convenient number of blades, also
that rigid adherence to the particular curves shewn by the Drawings
is not essential so long as in substance the Invention as above described
is followed.
DESCRIPTION OF THE DRAWINGS..
Fig. 1, elevation of one of the blades (set off on a cone shewn in
dotted lines, but for screws that do not revolve at great speeds a fuller
spiral for the leading edge would be found beneficial, in which case
the screw should be set off on a semicircle instead of the cone); Fig. 2,
an expanded sketch shewing the form of the face of the blade at the
different points numbered, and also the method of obtaining the curve.
of the face of the blade (in dotted lines); Fig. 3, plan of a three-blade
screw.
Explanation of the theory upon which the formation of this screw is
based-In designing this screw my object has been to make all parts of
the blade exert a useful pressure by which I mean the
pressure exerted

page_text: in propelling the vessel and (as nearly as possible) a uniform pressure
upon the water, a result not obtained by screws in common use, it being
generally allowed by engineers that the greatest pressure exerted is at
about two-thirds from the centre.
To accomplish my purpose my endeavour has been to make the
screw of the same form as its motion through the water would produce
in the water, to which end I, in the first place, obtained the exact form.
the water takes at the moment of delivery, and to get at this observed
the number and direction of forces produced by a revolving screw.
I assumed these forces to be three in number, with a backward,
revolving, and radiatory motion. Then, combining these three forces I
obtained the whirlpool shape, thus by turning water round radiatory
motion was obtained, gravitation causing the water to sink in the centre.
By putting in the place of gravitation the backward thrust of the screw
the same effect was produced. If therefore a whirlpool be placed with
its concave part against the face or after end of the screw, and its apex
pointing aft upon the centre line, it will at once become evident why
a screw of the ordinary shape should exert a greater useful pressure
at one portion of the blade than at another, for as it is constantly creating
a vacuum in the centre, and as constantly exerting a force to draw itself
away, the result is a division of the two forces, practice proving that
the whirlpool is cut at about two-thirds from the centre. Taking this
as a fact the advantage gained by the large boss and blades bent
forward (as in Mr. Griffiths' screw) will at once be evident.
The principle on which my improved screw is formed now becomes
clear. The great object is to fill up the hollow in the whirlpool, then,
by going a step further and rather more than filling it up, introducing
at the same time a spiral leading edge, a second propelling surface is
formed by the water being driven in towards the centre, and this
propelling force being derived from a different direction to that of the
pitch does not interfere with it as is the case in a screw with an
increasing pitch, the action on the water thus produced being more
like that of a plough.
Having now particularly described and ascertained the nature of the
'said Invention, and in what manner the same is to be performed or
carried out in practice, I hereby declare that I claim, the "Improve-
ments in Screw Propellers" herein-before described, it being understood

page_text: that I do not claim the use of the constituent details herein described
per se and apart from the purposes of such Invention.
In witness whereof, I, the said Thomas Vernon Trew, have
hereunto set my hand and seal, this Tenth day of August, in
the year of our Lord One thousand eight hundred and sixty-
nine,
THOMAS VERNON TREW. (L.S.)
Witness,
ALEX. P. WRIGHT,
10 Secretary to the Inventors' Patent Right
Association, Limited,
21, Cockspur Street,
Charing Cross, London,
Patentee's Agents.