Copyright
Osborne Reynolds.

Papers on mechanical and physical subjects (Volume 1) online

. (page 15 of 40)
Online LibraryOsborne ReynoldsPapers on mechanical and physical subjects (Volume 1) → online text (page 15 of 40)
Font size
QR-code for this ebook


enough to overcome the effect of the reversal of the screw ; but this was not
the case with the models.

The effect of the screw to turn Hie boat independently of tlie rudder.
It seems to be supposed by some that a screw necessarily tends to force
the stern of the boat in a direction opposite to that in which the tips of its
lower blades arc moving. This is undoubtedly the case when the screw is
racing or acting in broken water (i.e. water mixed with air), also when the
screw is not completely covered with water. When, however, the screw is
properly immersed and is working in unbroken or continuous water, and is
not affected by dead water, it has not the least tendency to move itself
laterally, whatever it may have on the ship. Under these circumstances the
screw-shaft can exert no lateral pressure on its bearings ; and in ships with
fine runs this is the case.

Owing to the effect of the dead water, however, it may happen that even
when the screw is properly immersed it will tend to move laterally. If the
water be following the ship faster above than below (which it often is), the
upper blades of the screw will have more work to do than the lower, and
consequently they will have to meet with greater lateral resistance ; and
hence upper and lower resistances will not balance, but there will be a lateral
thrust transmitted to the bearings.

Besides the lateral pressure which may be transmitted through the
bearings, the screw may also tend to turn the ship by the lateral motion
which it imparts to the water, which is again communicated to the ship or
the rudder. If the form of the ship and the rudder were symmetrical above
and below the screw-shaft, then the effect of the lateral motion which the
screw imparts to the water below would exactly balance the effect above the
serew-shaft ; but owing to the fact that the surface both of the ship and the
rudder is in general much greater above than below, the water which is
driven laterally by the upper blades has much more surface to act upon than
that which is driven in the contrary direction by the lower blades, and



138 ON THE STEERING OF SCREW-STEAMERS. [19

therefore drives the stern of the ship laterally, or tends to turn the ship.
This effect is in the opposite direction to that which arises from the unequal
rate at which the water is following the ship, as long as both the ship and
the screw are going ahead ; and consequently these two effects tend to
counteract each other. When, however, the screw is reversed, and the vessel
is still moving forwards, the two effects are in conjunction ; and consequently
they are more likely to become apparent and important. This was the
case in the experiments with the spring model. When screwing ahead
she went straight enough, but when towed ahead with the screw reversed
she turned to the left. In this case the effect was small ; and I imagine
that it must always be so, particularly when the ship has a fine run. In the
steam model, of which the run is very fine, the screw-way very large, and
the screw small (being only three inches while the boat draws five), the
effect of the screw to turn the boat when not racing was altogether im-
perceptible. I conclude, therefore, that these effects may be left out of
consideration with reference to steering ; and in opposition to a popular notion
I derive law 4.

4. That when not breaking the surface the screw has no considerable
tendency to turn the ship so long as the rudder is straight.

The effect of racing. Although the direct effect of the screw is insig-
nificant when it is not racing or breaking the surface, this is not the case
when it is racing. It then exerts a very decided and important effect ; and
it is doubtless experience of this which has given rise to the popular notion
above referred to.

In the experiments with the spring model when the screw was drawing
air down, the stern always showed a tendency to move in the opposite
direction to that in which the tips of the lower blades were moving, even
when the boat was going ahead at full speed and the quantity of air very
small ; and when the screw regularly raced, frothing the water, its effect to
turn the stern of the boat was very great.

The screw of the steam model was so deeply immersed that it would not
race ; but if the stern of the boat was raised by a string it then raced, and
the effect of the screw to turn the stern of the boat was the same as with the
spring model.

The screw of the spring model showed a much greater tendency to draw
air when reversed (the boat being towed) than when it was driving the
boat ahead; but its greatest tendency to race was when the boat was
stationary, or nearly so. This latter tendency I have observed in large
steamers; in fact I have never seen a large steamer start or reverse her
screw when moving but slowly without frothing the water. It appears,



19] ON THE STEERING OF SCREW- STEAMERS. 139

therefore, that the effect of racing on the steering may be stated in the
following laws :

5. That when the screw is frothing the water, or only partially immersed,
it will have a tendency to turn the stern in the opposite direction to that in
which the tips of the lower blades are moving.

6. That when the boat is going ahead its effect will be easily counter-
acted by the rudder ; but when starting suddenly, either forward or backward,
at first the effect of the screw will be greater than that of the rudder, and the
ship will turn accordingly.

7. That if when the boat is going fast ahead the screw is reversed, at first
it almost destroys the action of the rudder, what little effect it has being in
the reverse direction to that in which it usually acts. If, then, the screw
draws air or breaks the surface, it will exert a powerful influence to turn
the ship.

In accounts of collisions it may be frequently noticed that there is con-
trary evidence given of the steering of one or both of the ships (if they both
happen to be steamers). In the instance of the collision between the
'Ville du Havre ' and the ' Loch Earn ' the captain of the ' Loch Earn ' stated
that the steamer altered her course almost at the last moment, thus
rendering the collision inevitable. The officers of the steamer asserted that
such was not the case; they state, however, that the screw was reversed just
before the collision. In this case, therefore, the evidence is to show that the
reversal of the screw caused the steamer to change her course, either by its
direct effect or by its action on the rudder. The latter effect would be
sufficient to explain the facts ; and my experiments leave no doubt but that
this must have taken place. With regard to the former I have no evidence ;
although, considering that the ship was moving rapidly at the time, it seems
probable that the screw may have raced on being reversed, and added its
direct effect to turn the ship to its effect on her rudder. In this case,
therefore, the reports of what took place are strictly in accordance with what
was to be expected from my experiments ; and I think that from the light
these throw upon the subject in many cases, the accounts may be less con-
tradictory than they have hitherto appeared ; and I am in hopes that in the
future these experiments may assist not only in the discovery of the causes
of accidents, but, as these become recognized, in the prevention of the
accidents themselves.

As an illustration of how important a clear conception of the whole cir-
cumstances of the effect of the screw on the rudder may be, I will read an
account with which I have been kindly furnished by Mr Henry Deacon ;
from which account it appears that a ship was saved by a combination of
accidents, which led to her being handled in the very manner in which she



140 ON THE STEERING OF SCREW-STEAMERS. [19

would have been had the conduct of the officer in charge been governed by
the laws laid down in this paper.

Mr Deacon says :

"I have been reading your communication to the Engineer of the 4th
inst. about the ' Bessemer's ' steering, and think the following narrative may
have some interest for you. A friend of mine came from Philadelphia, U.S.,
early in May to Liverpool in the S.S. 'Ohio.' To avoid ice the vessel went
out of her course 1GO or 170 miles, and encountered very bad weather. The
captain spent one or two days without taking off his clothes ; and whilst
lying down one day, leaving the chief officer in command of the deck,
amongst fogs and rain, an iceberg was sighted right ahead and quite close
when seen. The officer stopped and reversed the engines, and put the helm
hard round. The cessation of motion awoke the captain, who rushed up
the bridge. The excitement had spread, the officer's orders had been strictly
obeyed. The captain took all in at a glance, put the engines on ahead at
full speed, and the 'Ohio/ breaking through the thin ice always skirting
the icebergs, passed so close to the solid mass, that my American friend, who
is fond of horses and was on deck, says he could have struck the ice from the
ship with a tandem whip. The captain afterwards explained the matter
thus : the steering-gear was the now usual parallel screws, i.e. exerting
the least force when the rudder is most moved, but of course retaining the
rudder in any position with little or no effort. To put the rudder hard
round when the ship is under full way and the engines working is an almost
physical impossibility; but to put it hard round when the engines are
stopped, and especially to put it round when they are reversed, is com-
paratively easy. The chief officer's order, therefore, enabled the rudder to be
put round to the utmost ; he both stopped and reversed the engines. The
captain's arrival and comprehension completed the manosuvre. The ' way '
was but slightly interrupted, but the helm was put hard round and the ship
turned from her course in the shortest possible distance.

" I have all this at second hand from my friend ; but this fact of the
easy movement of the helm, whilst the ship was under way with the engines
reversed, appeared to be one well understood ; and of course if no power be
required to move the helm, no power can be exerted in steering the vessel ;
and the whole tale seems to me so illustrative of your remarks on the
' Bessemer,' that I venture to trouble you with it."

(For continuation see papers 27, 31, 34.)



20.

IMPROVEMENTS IN TURBINES AND CENTRIFUGAL PUMPS.

[From the "Specification of Patent No. 724." 1875.]

WHEN the available pressure of fluid for driving a turbine is very great,
and the quantity of fluid is small, the diameter of the wheel has to be small
and its speed inconveniently great, so that in the best class of turbines one
hundred feet is the practical limit of the fall which can be utilized, and in
raising or forcing fluids by means of centrifugal pumps, it is almost im-
possible, by reason of the great speed required in the ordinary centrifugal
pump, to raise fluids to any considerable height. The object of my invention
is to overcome the difficulties above referred to, and my invention consists in
the construction, combination, and arrangement of apparatus which will utilize
the largest pressures of fluids to the fullest extent in obtaining motive
power, while keeping the speed of rotation within practicable limits, and to
obtain the greatest pressures or lifts by centrifugal machinery while keeping
down the speed of rotation. In order to obtain motive power from the fluid,
it is caused to traverse a passage or passages (which for distinction may be
called fixed passages) so formed that it is discharged from them with a rotary
motion or velocity of " whirl " about a certain axis or shaft. In impressing
this rotary motion on the fluid, part of its pressure is spent, so that it
emerges from the passages at a lower pressure than that at which it entered
them. It is then received into a passage or passages moving round the said
axis or shaft. These passages are so formed that the fluid on leaving them
has as far as practicable no velocity of " whirl," that is, no rotary motion
about the shaft. This modification in the motion of the fluid is effected as
far as possible without shock or friction, by so forming the moving vanes (as
is well understood) that the fluid shall be forced from them in a direction
opposite to that in which they are moving, and with a velocity relative to the
moving vanes at the place it leaves them, equal to that with which they are



142 IMPROVEMENTS IN TURBINES AND CENTRIFUGAL PUMPS. [20

moving. To give the fluid this relative velocity a further portion of its
initial pressure has to be spent, so that the fluid will emerge from the moving
passages with a pressure still lower than that at which it entered them, and
the pressure which has thus forced the fluid back will have produced an equal
effect to urge the moving passages forward, and thus give out the motive
power. So far the apparatus above described is identical with some forms of
what is known as the turbine, and in this respect no improvement is claimed.
The novelty of my invention, however, consists in so arranging the size and
motion of the passages, that on emerging from the moving passages the fluid
shall not, as in the case of the ordinary turbine, have spent the whole or
nearly the whole of its available pressure, but that it shall still have sufficient
pressure to carry it through one or more additional sets of passages similar
to those already described ; that is to say, on emerging from the first moving
passages, it shall again be received into other fixed passages, so that on being
forced through them it shall emerge with a velocity of whirl or rotary motion
round an axis riot necessarily the same as before with a reduced pressure,
and again be received into another similar set of moving passages from which
it may emerge with no velocity of whirl. This may complete the entire cycle
of operation to which the fluid is subjected, but this will depend upon
circumstances (videlicet, the magnitude of the initial pressure and the
character of the motive power required). It may, however, be desirable
so to arrange the size and velocity of the passages, that the fluid shall have
to pass through more than two sets of moving passages before all its available
pressure is spent. In fact there is no limit to the number of such sets of
passages that may be employed when desirable. On emerging from the last
set of passages the fluid will be allowed to flow away into such receptacle,
channel, or tail-race as may be provided. So far then my invention might be
described shortly to consist in using two or more turbines in combination
instead of one, the same fluid being made to pass through them successively,
and spend a portion of its pressure in producing motive power in each. The
pressure necessary to drive the later turbines being transmitted through the
fluid in the earlier turbines, the passages in which have therefore to act the
part of pipes to resist the pressure. In order that this pressure may be
transmitted, it is essential that the fluid should entirely fill the passages
along which it passes, so that those turbines in which the water only partially
fills these passages would not be available, and could not be used in carrying
out my invention. Since the stream of water passing through the several sets of
passages, or the several turbines, is continuous, the size of the passages in the
different turbines or sets must be carefully adjusted so as to prevent undue
loss of pressure, as must also the velocity of the moving passages. For
forcing or raising fluids, the apparatus employed is similar to that already
described for obtaining motive power, taken in the inverse order, that is to
say, the fluid is first taken up by passages carried by a rotating axis, in which



20] IMPROVEMENTS IN TURBINES AND CENTRIFUGAL PUMPS. 143

passages it has a velocity of "whirl " impressed upon it, and in receiving this
velocity of " whirl " it is driven against a certain amount of pressure, so that
it will emerge from the moving vanes at a greater pressure than that at
which it entered them ; it then enters the fixed passages or directors, which
are so formed and placed that its entrance may be without shock or friction,
and in which it again loses its velocity of whirl and forces itself against
further pressure. So far, the last apparatus is identical with certain so-called
centrifugal pumps and fans, but in the case of obtaining motive power herein-
before explained, the novelty of my invention consists in repeating the action,
and again causing the fluid to traverse one or more additional sets of moving
passages alternating with fixed passages. In this case, as in that previously
described for obtaining motive power, it is essential that the size of all the
passages should be properly adjusted, as well as the velocity of the moving
passages. In both cases, that is, in obtaining motive power and raising and
forcing fluids, instead of alternate sets of fixed and moving passages, all the
passages may be in motion, but in that case the alternate sets of passages
must move in opposite directions. It is not necessary that the several sets of
moving passages should be connected with or move round the same axis or
shaft, but such an arrangement considerably simplifies the apparatus. My
invention applies to all fluids, liquids, vapours, and gases ; and one important
application is that of producing blasts of air at considerable pressure. In
obtaining motive power by my improvements from fluids, or in forcing fluids
such as gases, where the density of the fluid varies with the pressure, the
passages must be arranged to increase in capacity as the pressure diminishes,
when obtaining motive power, and on the contrary the passages must
diminish in capacity as the pressure increases, when forcing gaseous fluids.
My invention further relates to a mode of constructing apparatus for obtain-
ing motive power from fluids, and is applicable to turbines where the whole
of the power of the fluid is imparted to the turbine by once passing through
one set of moving passages, as well as in carrying out my improvements for
obtaining motive power hereinbefore described, and my invention consists in
attaching flat or curved vanes to radiate from a rotating shaft after the
manner of the vanes of a common blowing fan. The vanes are surrounded
by a fixed casing having its ends perpendicular to the shaft, one or both of
which may have openings round the centre ; the space between the two ends
is rather greater than the width of the vanes to admit of their free rotation.
The circumference of the case is formed by a plate or plates perpendicular to
the two ends, and in the form of a spiral or spirals having an opening or open-
ings into the case, which openings form the fixed or directing passages for the
fluid to enter, the fluid escaping from the casing through the holes or open-
ings in the ends of the casing. The spiral plates may be made moveable, so
that the size of the passages may be adjusted, and the fluid may be intro-
duced between them in any convenient manner. The accompanying sheet of



IMPROVEMENTS IN TURBINES AND CENTRIFUGAL PUMPS.



[20



drawings is intended to explain an application of my invention. Fig. 1 is a
diagram showing two sets of fixed and two sets of moving passages, or two






Fig. 1.

sets of passages moving in one direction and two sets in another. The
passages are for simplicity shown as arranged in straight lines, but really they
have to be placed in circles, round an axis, either side by side as in what is
known as the parallel flow turbine, or one set within the other as in radial
flow turbines. A is a set of fixed and B a set of moving passages ; the fluid
enters through A in the direction of the arrows, and passing through B enters
a second set of fixed passages A', on emerging from which it enters a second
set of moving passages B', or A and A' sets moving to the right, and B and B'




Fig- 2.



20]



IMPROVEMENTS IN TURBINES AND CENTRIFUGAL PUMPS.



145



sets moving to the left. For centrifugal pumps and fans, the direction of the fluid
and of the moving passages is reversed, the fluid entering the moving set E'
will be discharged into the fixed set A', and then pass to the moving set B
to be discharged into the fixed set A. Fig. 2 is the end view of a turbine
constructed according to my invention, which may be used where the fluid is
to impart all its power by one passage through the turbine. In the view
shown by Fig. 2 the end part of the casing is supposed to be removed to
show the inner parts, and that side of the turbine is represented by Fig. 2 at
which the fluid leaves the vanes after having acted upon them ; a is the shaft,
6 the boss keyed upon it, to which the vanes c are secured. These vanes are
straight, but a part c of each vane near the axis is of the form shown
and is turned round until it is at an angle of 70 or thereabouts with the face
of the vane and axis of the shaft a. The vanes c revolve between the end
casings d, there being a slight clearance between the edges of the vanes and
the surface of the end casings. The motive fluid may first enter by a pipe e
into a chamber or space / formed at one end, at which end there may be a
stuffing box g round that end of the shaft (see Fig. 3), as the fluid at that end




Fig. 3.



O. B.



10



146



IMPROVEMENTS IN TURBINES AND CENTRIFUGAL PUMPS.



[20



is under pressure. The fluid then passes through holes h in the end plate or
casing, and is directed by guides (fixed or adjustable) upon the vanes c
carried by the shaft a, and then leaves the vanes by passing from them
through a hole in the centre part of the end casing. The guides k, if adjust-
able, have pivots k' upon which they turn, which pivots have bearings in holes
in the end casings, and the guides are adjusted by shafts I having arms
which have links n jointed to them, the links being connected by their other
ends to projections on the guides. This turbine is of a class known as Thompson's
vortex turbines, and with the exception of the moving vanes is similar in con-
struction to these turbines. The vanes, however, are shown as constructed on
my improved plan, being only connected with each other by the boss, which
connects them with the axis, and not connected with two parallel discs as
shown in Fig. 4, which is the manner in which the moving passages of such




Fig. 4.

turbines have hitherto been constructed. The advantages of my method of
arranging the vanes are (1) simplicity of construction, and (2) that such an
arrangement gives rise to less friction than the usual construction. This
method of constructing the moving passages is (as has been previously stated)
applicable to all vortex turbines whether used singly as heretofore, or in
combination on my plan. It is not, however, essential to my plan of com-
bining turbines, in which the moving passages may be constructed as shown
in Fig. 4. Fig. 3 shows the manner of combining several turbines like that
above described and shown by Fig. 2, the sectional part of Fig. 3 being taken



20] IMPROVEMENTS IN TURBINES AND CENTRIFUGAL PUMPS. 147

on the line AB (Fig. 2). Three turbines are shown combined in Fig. 3 upon
the same shaft a, the casings of the turbines being secured to each other
end to end by flanges and bolts. The casings may be constructed in two
halves bolted together as shown in dotted line, or in any other suitable
manner. The fluid first enters at the pipe e, then passes into the chamber /,
then from this chamber through the holes h to the vanes c, then through the
central hole into the next chamber f of the next turbine, and so on, leaving
finally by the pipe p in communication with a chamber /into which the third
turbine discharges, I are shafts which come to the exterior of the casings and
can be turned to adjust the directors k. In turbines and pumps combined on
my plan, (as when used singly) the guides and vanes which form the passages
may have various forms, which will depend to some extent on whether the
direction of flow is parallel to the axis or radial. It is essential, however,
that the lips of the guides or vanes at which the fluid enters between them
should be inclined in the direction of the motion of the fluid relative to
them, so that it may glide in without having the direction of its motion
suddenly altered. It is also essential that the openings between the vanes
and the openings between the guides at their lips should be such that the
fluid is exactly capable of filling them, neither more nor less, and also that the



Online LibraryOsborne ReynoldsPapers on mechanical and physical subjects (Volume 1) → online text (page 15 of 40)