IMPROVEMENTS IN SCREW PROPELLERS
Year: 1878

page_text: LETTERS PATENT to James Forster, of Glasgow, in the County of Lanark,
North Britain, Gentleman, for the Invention of "IMPROVEMENTS IN SCREW
PROPELLERS."
Scaled the 21st June 1878, and dated the 6th April 1878,
PROVISIONAL SPECIFICATION left by the said James Forster at the Office
of the Commissioners of Patents on the 6th April 1878.
JAMES FORSTER, of Glasgow, in the County of Lanark, North Britain, Gentleman.
"IMPROVEMENTS IN SCREW PROPELLERS."
This Invention, which relates to improvements in screw propellers, has for its
object to enable ships to be propelled more rapidly than at presont.
The improvements consist in constructing screw propellers with one or more sets
of propeller blades aft the first or ordinary propeller.
The first set of blades is preferably of a higher pitch than what is commonly
used, and the next set of propeller blades aft it has the edges of its blades situated
in the same plane as the edges of the first set of blades, but with cach blade inter-
mediate in its angular position to that of the blades of the first set.
In the case of their being more than one aft set of blades, the blades of each
succeeding set are intermediate with the angular position of the blades of the set
which precedes it.]
Furthermore, each further aft set of blades is of a faster pitch, preferably from
10 to 20 degrees faster than the pitch of the set of blades which immediately
precedes it.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said James Forster in the Great Seal Patent Office on the 5th October
1878.
JAMES FORSTER, of Glasgow, in the County of Lanark, North Britain, Gentleman.
"IMPROVEMENTS IN SCREW PROPELLERS."
My said Invention relates to improvements in screw propellers, and has for its
object to enable ships to be propelled more rapidly than hitherto,
The improvements constituting my said Invention consist in constructing screw
propellers with one set, or more than one set, of propeller blades aft the first set or
ordinary propeller.
The first set of blades may be of a pitch equal to that which is commonly
employed, or it may be of a lower pitch, but it is preferably of a higher pitch than
that which is usually employed.
Both or all of the sets of blades, constituting one propeller, are upon the same
shaft, but the blades of each aft set are in their angular position or plane of radial
projection intermediate to the angular position of the blades of the first or next
preceding set.
Each further aft set of blades is of a faster pitch (preferably from ten to twenty
degrees faster) than the pitch of the set of blades which immediately precedes it.
And in order that my said Invention may be properly understood, I now proceed
moro 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, referenco being had to the
accompanying Drawings and to the letters and figures marked thereon, that is to
Bay :-
Figure 1 represents in side elevation a portion of the stern of a vessel shewing
an arrangemont or construction of propeller according to my Invention, in which
two sets of propeller blades are employed.
Figure 2 is an elevation (as seen from the stern of the vessel) of the two sets of
propeller blades.
The first set of propeller blades is marked A, and it is preferably of a higher
pitch than that which is commonly used, but it may be of the same pitch or of a
lower pitch.
The set of propeller blades B is mounted aft the first set A upon the same
shaft. The set of blades B has a higher pitch, that is, it is a faster travelling one
than the fore propeller A.
Figure 3 is an elevation of an arrangement or construction of propeller, according
to my Invention, in which three sets of propeller blades are employed. The fore
set of blades is marked C, the aft set E, and the intermediate set D.
The set C has preferably a higher pitch than that which is commonly used, but
it may be of the same pitch or of a lower pitch.
The next set D is of a higher pitch (preferably from ten to twenty degrees
higher) than that of the set O, and the set E is of a higher pitch (preferably from
ten to twenty degrees higher) than that of the set D.
Figures 1 and 3 are drawn shewing each of the blades of the several sets in the
sane angular position, that is, in the same plane of radial projection, but it is to
be understood that they are thus represented merely for convenience of shewing
clearly the different angles or relative pitches of the blades of each set.
The angular position or radial projection of the blades of each further aft set
will, however, be intermediate to the angular position or radial projection of the
blades set next before them, as illustrated in Figure 2.
The sets of blades may be upon separate bosses, or they may all be cast or bolted
upon one long boss.
Although I have shewn in the Drawings propellers consisting of but two and
three sets of blades, and with two and four blades in each set, it is to be understood

page_text: that more than three sets may be employed, and that each set may consist of two,
three, four, or other number of blados, but in every case any aft set or sets of blades
are of a higher or faster pitch than that of the set of blades immediately preceding
it, and the blades of each aft set or sets are intermediate in their angular position
or plane of radial projection from their boss to that of the blades of the set next
preceding them.
The sides of the blades may have any desired curved or other form.
In the case of twin screw propellers each twin consists of two or more sets of
blades mounted upon the twin shafts, and (as illustrated in Figure 3) the blades of
cach aftward set may be longer than those of the preceding set, and curved after-
wards towards the ends of the blades, that is, the blades aro diagonal instead of
vertical sections of a helix.
The aftward side of the blades of each set may be corrugated or fluted
longitudinally.
Having now described and particularly ascertained the nature of my said
Invention, 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 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 herein-before described
improvements in screw propellers consisting in constructing the said propellers with
one or more sets of propeller blades aft the first or ordinary set of propeller blades,
each set further aft being of a higher or quicker pitch than that of the set imme-
diately preceding it, and the angular position or plane of radial projection of the
blades of each set further aft being intermediate to that of the blades of the set
immediately preceding them, essentially as herein-before described.
In witness whereof, I, the said James Forster, have hereunto set my hand
and seal, this Thirtieth day of September, in the year of our Lord One
thousand eight hundred and seventy-cight.