IMPROVEMENTS IN LIGHT DRAUGHT SCREW-PROPELLED VESSELS AND IN FACILITATING THEIR PROPULSION IN SHALLOW WATER."
Year: 1874

page_text: LETTERS PATENT to John Isaac Thornycroft, of the Firm of John I.
Thornycroft and Company, of Church Wharf, Chiswick, in the
County of Middlesex, Steam Yacht and Launch Builders, for the
Invention of "IMPROVEMENTS IN LIGHT DRAUGHT SCREW-PROPELLED VESSELS
AND IN FACILITATING THEIR PROPULSION IN SHALLOW WATER."
PROVISIONAL SPECIFICATION left by the said John Isaac Thorncroft
at the Office of the Commissioners of Patents, with his Petition,
on the 29th January 1874.
I, JOHN ISAAC THORNYCROFT, of the Firm of John I. Thornycroft
and Company, of Church Wharf, Chiswick, in the County of Middlesex,
Steam Yacht and Launch Builders, do hereby declare the nature of
the said Invention for "IMPROVEMENTS IN LIGHT DRAUGHT SCREW-PROPELLED
VESSELS, AND IN FACILITATING THEIR PROPULSION IN SHALLOW WATER," to be as
follows:-
TT

page_text: constructed, owing to the want of a sufficient body of water for the
screw propellers to act upon.
Now this Invention has for its object to obviate these difficulties by
providing an increased depth of water in the vicinity of the propeller or
propellers. For this purpose I construct the hull of the vessel with and
place the propeller or propellers within an external recess or recesses so
formed in the under part of the hull that when the vessel is afloat the
mouth or opening or mouths or openings of such recess or recesses shall
be below the surface or level of the water in which the vessel is floating,
and the crown or crowns of the recess or recesses considerably above
the said surface or level. Thus it will be understood that the chamber
or chambers, formed as described for the propeller or propellers to work
in is or are bounded by or its or their lateral parts are composed of
portions of the hull of the vessel protruding into the water, so that the
mouth, opening, or lower part, or mouths, openings, or lower parts of
the chamber or chambers is or are sealed by the water, and access of air
to the interior or interiors of the chamber or chambers is thus prevented.
On exhausting air from within the chamber or chambers thus formed,
additional water is forced into it or them by the action of the pressure
of the atmosphere upon the surface of the water external of the
chamber or chambers, thus giving an additional depth or body or bodies
of water for the propeller or propellers to act upon within the chamber
or chambers. The chamber or chambers is or are so formed as to
facilitate the entrance of water as the vessel proceeds, and to permit the
ready escape of superfluous water forced out by the propeller or
propellers.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said John Isaac Thornycroft in the Great Seal Patent
Office on the 28th July 1874.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN 30
ISAAC THORNYCROFT, of the Firm of John I. Thornycroft and Company,
of Church Wharf, Chiswick, in the County of Middlesex, Steam Yacht
and Launch Builders, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Twenty-ninth day of January. in the 3:

page_text: year of our Lord One thousand eight hundred and seventy-four, in
the thirty-seventh year of Her reign, did, for Herself, Her heirs and
successors, give and grant unto me, the said John Isaac Thornycroft,
Her special license that I, the said John Isaac Thornycroft, my
executors, administrators, and assigns, or such others as I, the said
John Isaac Thornycroft, 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, 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 LIGHT DRAUGHT SCREW-
PROPELLED VESSELS, AND IN FACILITATING THEIR PROPULSION IN SHALLOW
WATER," upon the condition (amongst others) that I, the said John
Isaac Thornycroft, 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 John Isaac Thornycroft, 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, and tho Drawings therein referred to
(that is to say):-
Heretofore great difficulty has been experienced in effecting the pro-
pulsion in shallow water by screw propellers of vessels as ordinarily
constructed, owing to the want of a sufficient body of water for the
screw propeller to act upon.
Now this Invention has for its object to obviate these difficulties by
providing an increased depth of water in the vicinity of the propeller or
propellers. For this purpose I construct the hull of the vessel with and
place the propeller or propellers within an external recess or recesses so
formed in the under part of the hull that when the vessel is afloat the
mouth or opening or mouths or openings of such recess or recesses shall
be below the surface or level of the water in which the vessel is floating,
and the crown or crowns of the recess or recesses considerably above the
said surface or level. Thus it will be understood that the chamber or

page_text: chambers, formed as described, for the propeller or propellers to work
in, is or are bounded by, or its or their lateral parts are composed of,
portions of the hull of the vessel protruding into the water so that the
mouth, opening, or lower part, or mouths, openings, or lower parts of
the chamber or chambers is or are sealed by the water, and access of
air to the interior or interiors of the chamber or chambers is thus
prevented. On exhausting air from within the chamber or chambers
thus formed, additional water is forced into it or them by the action of
the pressure of the atmosphere upon the surface of the water external
of the chamber or chambers, thus giving an additional depth or body or
bodies of water for the propeller or propellers to act upon within the
chamber or chambers. The chamber or chambers is or are so formed
as to facilitate the entrance of water as the vessel proceeds and to permit
the ready escape of the backward current or currents of water produced
by the propeller or propellers.
Such being the nature of my said Invention I now proceed to
describe the manner of carrying the same into practical effect, reference
being had to the accompanying Sheet of Drawings, in which Fig. 1
shews in longitudinal vertical section part of a vessel embodying my
Invention; Fig. 2 is a transverse section in the line A, B, of Fig. 1;
and Fig. 3 is a plan.
The hull a is formed with an external recess b within which the pro-
peller is placed. The curve 61, 62, 63, constituting the fore-and-aft
section of the chamber or recess b, taken through the centre line of the
propeller shaft, as in Fig. 1, should be of such form that if a tangent e,f,
to it drawn where 6¹, 6º, 6³, has the greatest inclination be caused to cut
any horizontal line, the angle e, f, h, thus formed shall not be greater
than 15 degrees. The area of the recess or chamber is somewhat con-
tracted as it approaches the propeller, where it attains an area somewhat
larger than the screw's disc, and this area should be maintained without
alteration up to the point of discharge 6³.
In experiments with a model canal tug, in which a five feet screw
was used in three feet six inches draught of water, the five feet screw
was found to give one and seven-eighths of effect as against one with a
3 ft. 3 in. screw working in the ordinary way in the same draught of
water with the same consumption of power. It was also found that
about the same effect was produced by the five feet screw working in
the chamber partly above the water level, as when the same screw was

page_text: working fully immersed four diameters clear of the vessel's stern. In
this method screws may be used at either the bow or stern or at both.
To ensure success it is only necessary to give the water easy access in
the direction of the vessel's motion, both-to and from the propeller. I
have found that the propeller will itself exhaust the chamber or
chambers of air, but special means such, for example, as an air pump
or exhauster may be provided for this purpose when desired. It is
necessary to place the openings or passages to the propeller below water
sufficiently to exclude air.
A man-hole fitted with a suitable air-tight cover may advantageously
be arranged over the propeller, as shewn. When this is removed the
water will partly fall away from the screw and give great convenience
for repairs or inspection.
Having described my said Invention, and having explained the
manner of carrying the same into practical effect, I hereby declare that
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 arranging and adapting light draught scr. w-propelled
vessels to facilitate propulsion in shallow water, in the manner above
described.
In witness whereof, I, the said John Isaac Thornycroft, have here-
unto set my hand and seal, this 27th day of July, in the year
of our Lord One thousand eight hundred and seventy-four.
JOHN I. THORN YCROFT. (L.S.)