The very singular and simple hydraulic motor which we illustrate herewith
is the invention of a Russian engineer, Mr. Jagn. It is scarcely as yet
known in Western Europe, where, however, something will probably be heard
of it ere long. Its true field would seem to be Egypt, India, or any
country where canals or rivers are used for irrigation, and where it is
desired to draw water from them at particular spots in the simplest and
cheapest manner. At present in nearly all such cases water is raised by
hand or steam power; nevertheless it must be obvious that the current of
the canal itself, slow though it may be, is quite sufficient to raise a
small portion of the discharge to the very moderate height generally
needed to lift it over the banks into the adjoining fields. Why then is it
not employed for the purpose? The answer is obvious, when we consider the
various hydraulic motors at present in use. Of course, motors worked by
water pressure must here be excluded; and we are left with scarcely
anything but the undershot wheel, the turbine, and the screw pump. All
these require expensive buildings and erections to set them to work,
present but a very small fraction of their surface to the water at any one
time, and must be very large and costly if they are to draw even a very
moderate amount of power from such a source. There is no possibility of
adjusting them readily to suit variations in the speed of the current or
in the quantity of water required, nor of moving them from place to place
should this be convenient.
[Illustration: PARACHUTE HYDRAULIC MOTOR.]
The motor of Mr. Jagn is on a totally different principle. Its essential
features consist, as shown, of an endless rope made of hemp or aloe fiber,
which takes a turn or two round a pair of drums mounted on a barge or
pontoon, and then passes down the channel to return over a pulley hung
from a floating punt, at such a depth that the whole of the rope is
immersed in the water. Along this rope are suspended at equal intervals a
number of parachutes made of sail cloth. The rope passes through the
center of each of these, and to it are attached a series of strings, the
other ends of which are connected to the outside edge of the parachute.
Thus they act like the spokes of an umbrella to prevent the parachute from
opening too far under the pressure of the current. The parachutes must be
placed so far apart that the current may act fairly on each, and the sum
of the pressures forms the force which draws the rope through the water.
The moment, however, that any parachute has passed round the return
pulley, the current acts upon it in the opposite direction. It then shuts
up like an umbrella, and assumes a volume so small that its resistance on
the return journey is insignificant. After passing round the drum at the
upper end, it at once opens afresh of its own accord, and once more
becomes part of the moving power of the whole system. The parachutes are
formed by first cutting out a complete circle of cloth, and then taking
from this a sector equal to one-fifth or one-sixth of the total area. Such
parachutes are found to keep their form when stretched by the water better
than a surface originally spherical, although the latter would be
theoretically more correct. The motion of the drum is transmitted by spur,
gear, or otherwise as may be required, to give the requisite speed.
It will be seen that the advantages of the system are as follows: First,
the facility it offers for obtaining a large working area, which may be
increased or diminished at will, according to the requirements of the
moment, by lengthening or shortening the rope. Secondly, the ease with
which it is erected and set to work. Thirdly, the small part of the river
section which it occupies, so as to present no obstacle to navigation.
Fourthly, the ease with which it can be mounted on a barge of any kind,
and carried wherever it may be needed. Fifthly, it is not stopped, like
all other hydraulic motors, by the appearance of ice--it has, in fact,
already been worked under ice in the Neva. At the same time, winds and
waves have no influence upon it.
The principle of the apparatus is not altogether new. In 1872 there was
tried on the Ohio River an arrangement termed the Brooks motor. It was
composed of two drums, placed horizontally and parallel to each other.
Round these there passed endless chains at equal spaces apart on the
length of the drums, and to these chains were fixed wooden blades or arms
of a curved form, and so jointed to the frames that they opened when
moving in one direction, and closed down on the chain when moving in the
other. In this machine the weight of the chains was a serious obstacle to
obtaining any large amount of power. The whole apparatus was mounted on a
heavy wooden scaffold, which proved an impediment to the flow of the
river. Again, the resistance due to the surface of the returning blades
and to their stiffness was found to be far from insignificant.
In the present system Mr. Jagn has found, after many experiments, that the
best effect was obtained when the parachutes were spaced apart at twice
their diameter, and when the rope made an angle of 8 degrees to 10 degrees
with the current. It is found that when open and in motion the parachutes
never touch the bottom. This was the case with a rope containing 180
parachutes of 4 feet diameter, and working in a depth of only 6 feet. This
is easily explained by the fact that the velocity of a current always
diminishes as it approaches the bottom. Hence the pressure on the lower
part of the parachute will be less than that on the upper part; but the
former pressure tends to draw the parachute downward, while the latter
tends to raise it to the top of the water. Thus, the latter being the
larger, the parachute will always have a tendency to rise. In fact, it is
necessary to sink the return pulley sufficiently deep to make sure that
the parachutes will not emerge from the surface. For the same reason no
intermediate supports are needed over the driving span; if any are needed
it is for the return span, on which the parachutes are closed. Of course,
if metal were used instead of hemp, the case would be entirely different,
and intermediate supports would have to be used for anything but very
moderate lengths.
In practice, Mr. Jagn has employed two ropes wound upon the same pair of
drums, which are mounted upon a pontoon. The ropes are spread out from
each other, as in Fig. 1, making an angle of about 10 degrees. The low
specific gravity of the system enables ropes to be employed of as great a
length as 450 yards, each of them carrying 350 parachutes of 17.2 square
feet area. As half of these are in action at the same time, the total
working area for the two cables is 5,860 square feet. This immense area
furnishes a considerable amount of power even in a river of feeble
current. Comparing this with a floating water wheel of the type sometimes
employed, and supposing this to have only 172 square feet of working area,
such a wheel must have a length of 46 feet, a diameter of 23 feet, and
seventy-two floats, each 2½ feet wide. The enormous dimensions thus
required for a comparatively small working area point sufficiently clearly
to the advantage which remains on the side of the parachute motor.
The general arrangement of the system is shown in the engraving. Behind
the return pulleys, D D, are attached cords, A A, with some parachutes
strung upon them. These present their openings to the current and preserve
the tension of the connecting ropes. At the further end of each cord is a
board, B, which is kept in a vertical plane, but lying at a slight angle
to the direction of the current; and this acts to keep the two moving
ropes apart from each other. The two return pulleys are, however,
connected by a line, E, which can be shortened or lengthened from the
pontoon, and in this way the angle of inclination between the two ropes
can be varied if required. A grooved pulley presses upon the trailing span
at the moment before it reaches the circumference of the drum. It is
mounted on a screwed spindle, which is depressed by a nut, and thus makes
the wet rope grip the outside of the drum in a thoroughly efficacious
manner.
The author has made a theoretical investigation of the power which may be
developed by the system, and has worked out tables by which, when the
velocity of the current and the other elements of the problem are known,
the power developed by any given number of parachutes can be at once
determined. We do not reproduce this investigation, which takes account of
the resistance of the returning parachutes and other circumstances, but
will content ourselves with quoting the final equation, which is as
follows: T = 0.328 S V³. Here T is the work done in H.P., S is the total
working area in sq. m., and V is the velocity of the current in m. per
sec. Taking V = 1, and S = 1 sq. m., which is by no means an impracticable
quantity, we have T = 0.328 H.P. per sq. m. We may check this result by
the equation given, in English measures, by Rankine--"Applied Mechanics,"
p. 398--for the pressure of a current upon a solid body immersed in it.
This equation, F = 1.8 m A v² / 2g, where m is the weight of a unit of
volume of the fluid--say 62 lb.--A is the area exposed, and v the relative
velocity of the current. Mr. Jagn finds that the maximum of efficiency is
obtained when the rope moves at one-third the velocity of the stream. If
this velocity be 3 feet per second, we shall have v = 2. and we then get F
= 7 lb. per sq. ft. very nearly. Now 1 sq. meter = 10.76 sq. ft., and a
speed of 1 ft. per second (which is that of the rope) is 60 ft. per
minute. Hence the H.P. realized in the same case as that taken above will
be 7 × 10.76 × 60 / 33,000 = 0.137 H.P. The difference between the two
values is very large, but Rankine, of course, depends entirely on the
value of the constant 1.8, which is quite empirical, and is for a flat
band instead of a hollow parachute. Taking, however, his smaller figure,
and an area of 544 square inches, which Mr. Jagn has actually employed, we
get a gross power of = 0.137 × 544 = 7.43 H.P. Hence it will be seen that
the amount of power which can be realized by the system is far from being
inconsiderable.
Lastly, we may point out that the durability of the apparatus will be
considerable. There is no wear except at the moment when the rope is
passing round the drum, and even then there need be no slipping or
grinding. The apparatus worked in the Neva was in very good condition
after running for four months day and night. After five months about
one-fifth of the parachutes had to be replaced, but after seven months the
hemp rope still showed no signs of wear. We think we have said enough to
show that for certain purposes, and especially, as we have, already
mentioned, for irrigation purposes, the new motor is well worthy of a
careful and extended trial. It may be questioned even whether we have not
here the germ of an idea which may hereafter enable us to solve one of the
most interesting and important of engineering problems, viz., the
utilization of the great store of power provided for us twice daily in the
ebb and flow of the tide.--_The Engineer._
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