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affected when air was allowed to descend to the blades, he was trying
what influence air would have on the action of a simple oblique vane, when
a very singular phenomenon presented itself. The air, instead of rising in
bubbles to the surface, ranged itself in two long horizontal columns behind
the vane. There was evidence of rotational motion about these air lines.
It was evident, in fact, that they were the central lines of two systematic
eddies.

That there should be eddies was not surprising, but eddies had always
been looked upon as a necessary evil which besets fluid motion as sources of



188 ON VORTEX MOTION. [25

disturbance, whereas here they appeared to be the very means of systematic
motion.

Here then was the explanation of the nature of the motion caused by
the oblique vane, a cylindrical band of vortices continually produced at the
front of the plate, and falling away behind it in an oblique direction.

The recognition of the vortex action caused behind the oblique vane,
suggested that there might be similar vortices behind a disc moving flatwise
through the water, such as are the eddies caused by a teaspoon.

There was one consideration, however, which at first seemed to render
this improbable. It was obvious that the resistance of the oblique vane
was caused in producing the vortices at its forward part ; so that if a vortex
were formed behind a flat plate, as this vortex would remain permanently
behind, and not have to be continually elongated, the resistance should
diminish after the plate was once set in motion; whereas experience ap-
peared to show that this was by no means the case. It appeared probable,
therefore, that from some disturbing cause the vortex would not form, or
would only form imperfectly, behind the plate.

This view was strengthened when, on trying the resistance of a flat plate,
it did not appear to diminish after the plate had been started.

Accidentally, however, it was found that if the float to which the plate
was attached was started suddenly and then released, the float and plate
would move on apparently without any resistance. And more than this,
for if the float were suddenly arrested and released, it would take up its
motion again, showing that it was the water behind that was carrying it on.

There was evidence therefore of a vortex behind the disc, In the hope
of rendering this motion visible, coloured water was injected in the neigh-
bourhood of the disc, and then a beautiful vortex ring, exactly resembling
the smoke ring, was seen to form behind the disc. If the float were released
in time, this ring would carry the disc on with it ; but if the speed of the
disc were maintained uniform, the ring gradually dropped behind and
broke up. Here then was another part played by the vortex previously
undreamt of.

That the vortex takes a systematic part in almost every form of fluid
motion was now evident. Any irregular solid moving through the water
must from its angles send off lines of vortices such as those behind the
oblique vane. As we move about we must be continually causing vortex
rings and vortex bands in the air. Most of these will probably be irregular,
and resemble more the curls in a smoke cloud than systematic rings. But
from our mouths as we talk we must produce numberless rings.



25] ON VOKTEX MOTION. 189

One way in which rings are produced in perhaps as great numbers as
from our mouths is by drops falling into the sea. If we colour the surface
of a glass vessel full of water, and then let drops fall into it, rings are
produced, which descend sometimes as much as two or three feet.

But the most striking rings are those produced in water, in a manner
similar to that in which the smoke rings are produced, using coloured water
instead of smoky air.

These rings are much more definite than smoke rings, and although they
cannot move with higher velocities, since that of the smoke ring is unlimited,
the speed at which they move is much more surprising.

In the air we are accustomed to see objects in rapid motion, and so far
as our own notions are concerned, we are unaware of any resistance ; but this
is quite otherwise in water. Every swimmer knows what resistance water
offers to his motions, so that when we see these rings flash through the
water we cannot but be surprised. Yet a still more striking spectacle may
be shown, if, instead of coloured water, a few bubbles of air be injected into
the box from which the puff is sent ; a beautiful ring of air is seen to shoot
along through the water, showing, like the lines of air behind the oblique
vane, little or no tendency to rise to the surface.

Such is the ease with which these vortex rings in water move, and so
slight is the disturbance which they cause in the water behind them, as to
lead to the conclusion that they experience no resistance whatever, except
perhaps a little caused by slight irregularities in their construction. Their
velocity gradually diminishes ; but this would appear to be accounted for by
their growth in size, for they are thus continually taking up fresh water into
their constitution, with which they have to share their velocity. Careful
experiments have confirmed this view. It is found that the force of the
blow they will strike is nearly independent of the distance of the object
struck from the orifice.

The discovery of the ring behind the disc afforded the opportunity of
observing the characteristics of these rings much better than was afforded
by the smoke rings; and also suggested facts which had previously been
overlooked. The manner of motion of the water which formed the ring,
and of the surrounding water, was very clearly seen. It was at once seen
that the visible ring, whether of coloured water or air, was merely the
central line of the vortex ; that it was surrounded by a mass of coloured
water, bearing something like the same proportion to the visible ring, as a ball
made by wrapping string (in and out) round a curtain ring until the aperture
was entirely filled up. The disc, when it was there, formed the front of this
ball or spheroid of water, but the rest of the surface of the ball had nothing



190 ON VORTEX MOTION. [25

to separate it from the surrounding water but its own integrity. Yet when
the motion was very steady the surface of the ball was definite, and the
entire moving mass might be rendered visible by colour. The water within
the ball was everywhere gyrating round the central ring, as if the coils of
string were each spinning round the curtain ring as an axis, the water
moving forwards through the interior of the ring and backwards round the
outside, the velocity of gyration gradually diminishing as the distance from
the central ring increased.

The way in which the water moves to let the ball pass can also be seen,
either by streaking the water with colour or suspending small balls in it.
In moving to get out of the way and let the ball of water pass, the sur-
rounding water partakes as it were of the gyrating motion of the water
within the ball, the particles moving in a horse-shoe fashion, so that at the
actual surface of the ball the motion of the water outside is identical with
that within, and there was no rubbing at the surface, and consequently no
friction.

The maintenance of the shape of the moving mass of water against the
unequal pressure of the surrounding water, as it is pushed out of the way, is
what renders the internal gyratory motion essential to a mass of fluid moving
through a fluid. The centrifugal force of this gyratory motion is what
balances the excess of pressure of the surrounding water in the front and
rear of the ball, compared with what it is at the sides.

It is impossible to have a ring in which the gyratory motion is great, and
the velocity of progression slow. As the one motion dies out so does the
other, and any attempt to accelerate the velocity of the ring by urging
forward the disc, invariably destroyed it.

The striking ease with which the vortex ring, or the disc with the vortex
ring behind it, moves through the water, naturally raised the question as
to why a solid should experience resistance. Could it be that there was
something in the particular spheroidal shape of these balls of water which
allowed them to move freely. To try this, a solid of the same shape as the
fluid ball was constructed and floated after the same manner as the disc.
But when this was set in motion, it stopped directly it would not move at
all. What was the cause of this resistance ? Here were two objects of the
same shape and weight, the one of which moved freely through the water,
and the other experienced very great resistance. The only difference was
in the nature of the surface. As already explained, there is no friction at
the surface of the water, whereas there must be friction between the water
and the solid. But it could be easily shown that the resistance of the solid
is much greater than what is accounted for by its surface friction or skin
resistance. The only other respect in which these two surfaces differ is



25] ON VORTEX MOTION. 191

that the one is flexible, while the other is rigid, and this seems to be the
cause of the difference in resistance.

If ribbons be attached to the edge of the disc, these ribbons will envelope
the ball of water which follows it, presenting a surface which may be much
greater than that of the solid; and yet this, being a flexible surface, the
resistance of the disc with the vortex behind it is not very much greater than
it would be without the ribbons nothing to be compared to that of the solid.

Colouring the water behind the solid shows, that instead of passing
through the water without disturbing it, there is very great disturbance
in its wake. An interesting question is as to whether this disturbance
originates with the motion of the solid, or only after the solid is in motion.
This is settled by colouring the water immediately in front of the solid
before it is started. Then on starting it the colour is seen to spread out
in a film entirely over the surface of the solid, at first without the least
disturbance, but this follows almost immediately.

Among the most striking features of the vortex rings, is their apparent
elasticity. When disturbed they not only recover their shape, but vibrate
about their mean position like an elastic solid. So much so, as to lead
Sir William Thomson to the idea that the elasticity of solid matter must
be due to its being composed of vortex rings.

But apart from such considerations, this vibration is interesting as showing
that the only form of ring which can progress steadily is the circular. Two
parallel bands, such as those which follow the oblique vane, could progress
if they were infinitely long, but if not, they must be continually destroyed
from the ends. Those which follow the oblique vane are continually dying
out at one end, and being formed again at the other.

If an oval ring be formed behind an oval plate, the more sharply curved
parts travel faster than the flatter parts; and hence, unless the plate be
removed, the ring breaks up. It is possible, however, to withdraw the plate,
so as to leave the oval ring, which proceeds wriggling along, each portion
moving in a direction perpendicular to that in which it is curved, and with
a velocity proportional to the sharpness of the curvature. So that not only
does the ring continually change its shape, but one part is continually falling
behind, and then overtaking the other.

These were some of the forms of fluid motion which imagination or
reason had failed to show us, but which had been revealed by the simple
process of colouring the water.

Now that we can see what we are about, mathematics can be most
usefully applied; and it is expected that when these facts come to be
considered by those best able to do so, the theory of fluid motion will be
placed on the same footing as the other branches of applied mechanics.



26.



ON THE INVESTIGATION OF THE STEERING QUALITIES

OF SHIPS.

[From the "British Association Report," 1876.]

THE primary object of using steam power in ships is to enable them to
pass quickly over long distances. Under normal circumstances rapidity and
certainty in manoeuvring are matters of secondary importance ; but circum-
stances do arise under which these powers are of vital importance. Experi-
ence has taught those who go down to the sea in steam-ships that their
greatest danger is that of collision ; and fogs are feared much more than
storms. That there must always be danger when long ships are driven at
full speed through crowded seas in a dense fog cannot be doubted ; but this
danger is obviously increased manyfold when those in command of the ships
are under the impression that a certain motion of the helm will turn the
ship in the opposite direction to that in which it does turn.

The uncertainty which at present exists in the manoeuvring of large ships
is amply proved by the numerous collisions which have occurred between the
ships of our own navy while endeavouring to execute ordinary movements
under the most favourable circumstances, and with no enemy before them.
These accidents may be, and have been, looked upon as indicating imperfec-
tions in the ships or the manner in which they were handled ; but it must
be admitted that the ships are the best and best found in the world, and
that they are commanded by the most skilful and highly trained seamen
alive. And if peaceable ships fail in their manosuvres when simply trying
not to hurt each other, what will be the case of fighting ships when trying
to do all they can to destroy each other ? If the general impression as to
the important part which the ram is to play in the naval combats of the
future is ever realized, then certainty in manoeuvring must not only be of



26] ON THE INVESTIGATION OF THE STEERING QUALITIES OF SHIPS. 193

very great importance (this it has always been in sea fights), but it must
occupy the very first place in the fighting qualities of the ship.

Now the results of the investigation of the effect of reversing the pro-
pellers on the action of the rudder appear to show that, however capricious
the behaviour of ships has hitherto seemed, it is in reality subject to laws;
and that by a series of careful trials the commander of a ship may inform
himself how his ship will behave under all circumstances.

The experiments of the Committee on large ships have completely
established the fact to which it was my principal object last year to direct
attention, namely, that the reversing of the screw of a vessel with full way on
very much diminishes her steering-power, and reverses what little it leaves ;
so that where a collision is imminent, to reverse the screw and use the rudder
as if the ship would answer to it in the usual manner is a certain way of
bringing about the collision. And to judge from the accounts of collisions,
this is precisely what is done in nine cases out of ten. In the paper of
to-day I find the following (August 22, 1876) :

" The Fatal Collision off Ailsa Craig. The Board of Trade inquiry into
the collision between the steamer ' Owl ' and the schooner-yacht ' Madcap '
was continued at Liverpool yesterday. Two passengers by the ' Owl ' were
recalled, and spoke to some of the facts of the collision. The night was not
misty, though some rain had fallen. They saw the green light of the yacht
shining brightly after the collision. William Maher, third officer of the
' Owl,' said it was the chief officer's watch at the time of the collision.
There were five able seamen in the watch. Witness and the chief officer
were on the bridge. One man was on the look-out from the starboard side
of the bridge. His ordinary place was on the forecastle-head, but he was
not placed there that night, as there was a heavy head sea, and the vessel
was shipping water. His attention was called to a light by the look-out
man. It was almost ahead about a mile and a half off. He could not at
first distinguish whether it was red or green, as it was dim; but when he
made it out to be a green light it bore two to three points on the port bow,
and it was only three or four lengths off. He heard no order given to the
man at the wheel when the light was first reported ; but when witness found
that it was a green light he ordered the helm hard aport. If the steamer
had starboarded at this time she would have gone right over the yacht. The
' Owl ' had been going at the rate of six or seven knots ; but when she collided
there was no way on her, the engines having been reversed. After the yacht
went down the captain ordered a boat to be got out, but subsequently counter-
manded the order, on the ground that more lives would be lost, as it was not
fit to go out. At the close of his examination the witness stated that he
would not have gone out in a boat on such a night as that, even if the captain
o. R. 13



194 ON THE INVESTIGATION OF THE STEERING QUALITIES OF SHIPS. [26

had ordered him a remark which appeared to greatly astonish the nautical

assessors."

He ported his helm to bring his ship round to starboard, but he also
reversed his screw ; and as he says nothing about having again starboarded
his helm, it would appear that from the time of reversing the screw until
the collision (time enough to stop the ship), she had moved straight forward
or inclined to port. Had he not reversed his screw, but kept on full speed, it
is clear the collision could not have happened, for at the time the collision
did happen his ship would have been more than her own length away from
the spot where the collision occurred. He admitted himself that to have
starboarded his helm must have brought about the collision, so he ported his
helm and reversed his screw, which, as it had the same effect, did bring about
the collision.

From the Committee's report just read, it appears that a ship will turn
faster, and for an angle of 30, in less room when driving full speed ahead,
than with her engines reversed, even if the rudder is rightly used. Thus
when an obstacle is too near to admit of stopping the ship, then, as was done
in the case of the ' Ohio,' mentioned in my paper last year, the only chance
is to keep the engines on full speed ahead, and so to give the rudder an
opportunity of doing its work.

These general laws are of the greatest importance, but they apply in
different degrees to different ships ; and each commander should determine
for himself how his ship will behave. A ship's ordinary steering-power may
soon be learnt in general use, but not so the effect of stopping; there is
thought to be a certain risk in suddenly reversing the engines, which anyone
in charge of a ship will shrink from, unless he knows it is recognized as part
of his duty.

It is also highly important that the effect of the reversal of the screw
should be generally recognized, particularly in the law courts; for in the
present state of opinion on the subject, there can be no doubt that judgment
would go against any commander who had steamed on ahead, knowing that
by so doing he had the best chance of avoiding a collision, or who had
ported his helm in order to bring his ship's head round to port, with the
screw reversed. It seems to me, therefore, that it would be well if steps
could be taken by this Association to bring the matter prominently before
the Admiralty, the Board of Trade, and those concerned in navigation.

So far as the capabilities of each individual ship are concerned, there is no
insuperable difficulty or risk about the experiments, and to have determined
these will be a great point. When the officers know exactly what can be



26] ON THE INVESTIGATION OF THE STEERING QUALITIES OF SHIPS. 195

done in the way of turning their ships, and how to do it, the chances of
accidents must be greatly reduced.

But at all events for fighting ships it is desirable that the officers should
have experience beyond the mere turning powers of their own ships. When
two ships are inanoeuvring so as to avoid or bring about a collision, each
commander has to take into account the movements of his opponent. To
enable him to do this with readiness, it would be necessary to have friendly
encounters. A fight between two ships whose captains had never before
fought, would be like a tournament between two novice knights who had
never practised with pointless spears ; and such a contest, although not
unequal, must be decided by chance rather than skill.

Unfortunately sham fights or tournaments between ships with blunt rams
would be about as dangerous as a real fight ; and the chance of an accident
would be far too great for such friendly tournaments, however important,
ever to become an essential part of the training of a naval officer, as they
were of the knights of old. For although, should war arise, the danger from
want of experience may be even greater than the danger of an accident in
gaining such experience by friendly fights, yet, as the chance of war is
always remote, the former risk would be preferred ; and this is not all.

As yet there has been no such thing as a ramming fight between steam-
ships; so that not only are our officers without actual experience, but even
the rules by which they are instructed to act (the rules of naval tactics) are
based entirely on theoretical considerations, and hence are very imperfect.

Now there appears to me to be a means by which experience of the
counter - mano3uvring powers of ships, as well as the manoauvring powers of
single ships, could be ascertained without any of the risk and but little of
the cost attending on the trials of large ships, and which, if not equal to an
actual fight, would be very useful as a means of training the officers.

If small steam-launches were constructed similar to the ships, so that
they represented these ships on a given scale (say one-tenth linear measure),
and their engines were so adjusted that they could only steam at what we
may call the speed corresponding to that of the larger ships, then two
launches would mano3uvre in an exactly similar manner to the large ships,
turning in one-tenth the room ; and the time which the manoeuvres with
the launches would take would only be about half that occupied by similar
manoeuvres with full-sized ships. The only points in which it would be
necessary that the model should represent the ship would be in its shape
under water, and as regards the longitudinal disposition of its weights.
The centre of gravity should occupy the same position amidships, and the
longitudinal radius of gyration of the model should bear the same proportion

132



196 ON THE INVESTIGATION OF THE STEERING QUALITIES OF SHIPS. [26

to that of the ship as the other linear dimensions. In other respects the
model might be made as was most convenient. It might be made of wood, and
so strengthened that two models might run into each other with impunity.

There would not be much difficulty in so strengthening the models, as the
speed of the models would be very small. For instance, if the speed of the
ship were 13 knots, then that of the model would be 4| knots.



The study of the qualities of ships from experiments on their models has
not until recent years led to any important results. But this in great part
was owing to the fact that proper account had not been taken of the effect
of the wave caused by the ship and the consequent resistance. It was not
known that the waves set up by the model bear the same relation to the size
of the model as the waves set up by the ship do to the ship when, and only
when, the speed of the model is to the speed of the ship in the ratio of the
square root of the ratio of their lengths.

Since this fact has been recognized, most important information has been
obtained by experimenting on models. Mr Froude, by recognizing this law,
has been able to bring the comparison of ships by means of their models to
such a degree of perfection, that he can now predict with certainty the com-
parative and actual resistance of ships before they are constructed, and the
great practical value of his results have been recognized by the Admiralty.

What I propose is virtually to extend these experiments on models so as to
make them embrace the steering-powers of ships as well as their resistances.
The manner of experimenting would have to be somewhat altered. Steam-
launches would have to be substituted for dummy models ; but the principle
of the experiments would have to remain the same, and the speed of the
launches must be regulated by the same law as that of the models.

The turning qualities of such launches might be verified by comparing



Online LibraryOsborne ReynoldsPapers on mechanical and physical subjects (Volume 1) → online text (page 20 of 40)