Copyright
Osborne Reynolds.

Papers on mechanical and physical subjects (Volume 1) online

. (page 25 of 40)
Online LibraryOsborne ReynoldsPapers on mechanical and physical subjects (Volume 1) → online text (page 25 of 40)
Font size
QR-code for this ebook


Conclusion.

The difficulty of obtaining a column of mercury thirty inches above the
pressure of the atmosphere does not, I think, prove that the limit of the
cohesive power of the liquid has been arrived at, or even the limit of the
adhesive power of the water for glass and mercury, but simply shows that,
although imperceptible, there are still bubbles of air in the liquid between the
mercury and the glass which will not readily pass out.

It seems to me to be probable that, with sufficient care, or by using
apparatus more suitable to the purpose, much greater heights might be
attained. But however this may be, we have proof that mercury and water

* At the Meeting not only did the mercury remain suspended when the tube was erect, but on
the pressure of the atmosphere being removed with an air-pump it still remained suspended,
although the tension at the top of the tube was nearly equal to two atmospheres.



31] ON THE INTERNAL COHESION OF LIQUIDS. 241

will, by their cohesion, resist a tension of at least one atmosphere, or that the
common pump would, if the water were free from air, raise water by suction
to a height of more than sixty feet. At first sight it cannot but appear
remarkable that such a fact should for so long have escaped notice ; but a
little consideration removes the difficulty.

Water is almost always more or less saturated with air, which separates
into bubbles as soon as the pressure is relieved ; and in the common pump a
single minute bubble would be sufficient to cause the column to break and
prevent it being raised to a greater height than that due to the pressure of
the atmosphere.

In the case of barometers it is the custom to fill the tubes full and boil
the mercury, so as to get rid of the air ; but the column falls to the usual
height not by the rupture of the mercury, but by the separation of the
mercury from the glass, for which it has but little adhesion. Whether the
ordinary method of boiling the mercury really disengages all the air is,
I think, an open question. In vacuum-gauges of small diameter it is not
uncommonly found that the mercury sticks to the glass until the pressure
has fallen considerably below what is represented by the height of the
mercury, so that on the gauge being shaken the mercury falls with a sudden
drop. Although it does not seem to have attracted any special notice, this
phenomenon is clearly due to the same cause as that which I have found
capable of maintaining thirty inches of mercury suspended in a comparatively
large tube.

It would seem then that, although the facts which I now bring before the
Society have little bearing on the practical limits to the height of the column
of mercury in the barometer or the column of water in the common pump,
they show that these limits are owing to the presence of air or some other
minor disturbing cause, and are not, as seems to have been hitherto supposed,
owing to the want of cohesion of the liquid. And it seems to me that the
cohesion now found to exist occupies an important as well as an interesting
place in the properties of liquids.

APPENDIX (26th April). Previous Notices of the Cohesion of Liquids.

Besides the hanging of mercury in small gauges, another phenomenon,
which has long been known, shows a small degree of cohesion in water ; that
is, that water will rise up small tubes by capillary attraction as well in the
receiver of an air-pump as in air at the ordinary pressure. This fact was
shown before the Royal Society by Robert Hooke.

Prof. Maxwell, in his Treatise on the Theory of Heat, p. 259, after com-
menting on the fact that water has been raised to a temperature of 356 F.,
o. R. 16



242 ON THE INTERNAL COHESION OF LIQUIDS. [31

without boiling, remarks : " Hence the cohesion of water must be able to
support 132 Ibs. weight on the square inch," from which it would appear
that he recognizes cohesion as a property of water, and considers that the
possibility of raising the temperature above the boiling-point is evidence of
such cohesion ; but I am not aware that he has anywhere given his reasons
for such a conclusion.

I am indebted to Dr Bottomley for reference to a paper in the Ann. de
Chim. et de Phys. (3) xvi. 167, by M. F. Donny, in which M. Donny gives an
account of experiments in which he found that columns of sulphuric acid
could be suspended in vacuo to a height of 1 '3 metre (about 50 inches),
showing a tension of about 7 inches of mercury, care having been taken first
to remove all the air from the acid. M. Donny further describes experiments
made with water in exhausted tubes, in which he showed the effect of
cohesion by shaking the tube. M. Donny does not, however, appear to have
thought of the plan which I adopted of making mercury adhere to the tubes
by wetting them with sulphuric acid or water. Not being able to use
mercury, the tensions which he obtained were comparatively small; and
although he seems to have considered that greater tensions might be obtained,
he mentions one or two atmospheres as probably possible. It would therefore
appear that he had not conceived the possibility of the cohesion of liquids
being comparable with that of solids.

M. Donny appears to have been influenced in adopting this limit to his
idea of cohesion by a passage from Laplace, Mecanique Celeste, Supplement
au X e livre, p. 3, which he quotes.

Laplace, who was the first to investigate systematically the phenomena of
capillary attraction, proceeded on the hypothesis that the molecules of a
liquid exercise attraction for each other at insensible distances only; and
from this assumed attraction he deduces the surface-phenomena. The entire
passage quoted by M. Donny is too long to introduce here ; but the gist of it
is comprised in the following extract :

"Son expression analitique est compose'e de deux termes : le premier,
beaucoup plus grand que le second, exprime V action de la masse terminee par
une surface plane; et je pense que de ce terme dependent la suspension du
mercure dans un tube du barometre d une hauteur deux ou trois fois plus
grande que celle qui est d-ue d la pression de I' atmosphere, le pouvoir refringent
de corps diaphanes, la cohesion, et generalement les affinites chimiques."

Laplace here speaks of the suspension of mercury to 60 or 90 inches as
if it were a well-known phenomenon; but I cannot find any reference to
experiments, or, indeed, any further mention of the phenomenon in his
memoir.



31] ON THE INTERNAL COHESION OF LIQUIDS. 243

I did not refer to Laplace in the first instance, although I knew well that
it is to him we are indebted for the theory of surface-tension almost in the
form now accepted, because I wished to avoid all reference to molecular
hypothesis, and particularly the molecular attractions assumed by Laplace,
lest it might in any way appear as if the conclusion that continuous liquids
are as capable of resisting tension as solids (at which I arrived simply from
considering the phenomena of surface-tension) were based on such assumptions.
I was not aware, however, that Laplace had at all inferred or attempted to
apply his theory to prove the ability of liquids to resist great tensions ; nor
do I find, on again reading his memoir, that he anywhere, with the exception
of the almost casual reference quoted above, treats of such a property of
liquids. His purpose appears to have been solely to explain the phenomena
of capillarity. It appears obvious, moreover, that his line of reasoning must
have forced upon his notice the conclusion that, according to his hypothesis,
liquids ought to possess the property of very great cohesion ; so that from the
extremely slight notice which he has accorded to this property, one can only
infer that he was not completely convinced of its existence.



162



32.

ON THE STEERING OF SCREW STEAMERS.

Report of the Committee, consisting of JAMES R. NAPIER, F.R.S., Sir W.
THOMSON, F.R.S., W. FROUDE, F.R.S., J. T. BOTTOMLEY, and OSBORNE
REYNOLDS, F.R.S. (Secretary), appointed to investigate the effect of
Propellers on the Steering of Vessels.

[From the "Report" of the "British Association," 1878.]

SINCE the Meeting of the British Association held in Plymouth last year,
the Committee have had the satisfaction of receiving reports of the trials
of various English and foreign steamers, made by the owners and officers of
the steamers, without any further instigation from the Committee than
that contained in their circulars. These reports all show that those by
whom the trials were made have become convinced of the importance of
the facts which they have observed. And, indeed, the mere fact of the trials
having been undertaken shows that the importance of the effect of the
reversed screw on the steering while the ship is stopping herself is beginning
to be recognised. This is further shown by the fact that one of the trials
was undertaken at the instance of the Court of Mr Stipendiary Yorke, in
order to ascertain if the captain of the s.s. ' Tabor' had been justified in star-
boarding his helm in order to bring his vessel round to starboard after his
screw was reversed.

All these trials, without a single exception, confirm the results obtained
in the previous trials made by the Committee. But this is not the most
important purpose which this year's trials serve. For, as regards the general
effect of the reversed screw on the action of the rudder, the trials already
reported, particularly those of the 'Hankow' (see last year's Report, p. 201),
are conclusive, and leave nothing to be desired. But the previous trials
were all made with fast vessels at their full draught, their screws being well



32] ON THE STEERING OF SCREW STEAMERS. 245

covered, and the conditions of the weather being most favourable. The trials
this year, on the other hand, appear, for the most part, to have been made
with vessels in light trim ; and in two instances the wind was blowing with
considerable force. The result of these circumstances on the behaviour of the
vessels is very decided, and coincides remarkably with the effects deduced by
Professor Reynolds from his experiments on models (see Report, 1875, I.
p. 145), viz., that when the screw is not deeply immersed and froths the
water, it exerts, when reversed, considerable influence to turn the vessel
independently of the rudder ; the vessel turning to starboard or port, accord-
ing as the screw is right or left handed, which effect (and this seems to be the
point most generally unknown) nearly disappears when the screw is so deeply
immersed that it does not churn air with the water.

Neither the Admiralty, the Board of Trade, nor the Elder Brethren of
Trinity House have taken any further notice of the results communicated to
them by the Committee.

The Marine Board of South Shields has, however, taken considerable inter-
est in the question, and has invited captains to make trials, and Mr J. Gillie,
the Secretary, was present at the trial of the ' Tabor ' ordered by the Court,
and reported the results to the Committee.

There have been numerous collisions during the year. In almost all cases
the practice of reversing the screw has been adhered to. In many, if not in all
instances where this has been done, the evidence goes to show that the vessel
in which the screw was reversed did not turn in the direction in which those
in charge of her were endeavouring to turn her. In two important cases this
fact was fully apparent even to those in charge of the vessel. And in one
instance the owners and captain of the vessel attributed the failure to steer to
its true cause, namely, the reversal of the screw ; although in both cases those
immediately in charge of the vessels contended that the rudder was not
handled according to their directions.

The first case was that of the ' Menelaus ' and the ' Pilot ' schooner on the
Mersey. The ' Menelaus ' was in charge of a first-class pilot, and this steamer,
in broad daylight, ran into and sank the ' Pilot ' schooner, which was dropping
up the river with the tide. The pilot in charge contended that, owing to the
wheel chains having got jammed, his orders were not attended to. The
jamming of the chains was denied by the owners, and the fact that they
subpoenaed the Secretary of the Committee to give evidence at the trial
may be taken to indicate the cause to which they attributed the collision.
The case, however, was only in part heard, for after the evidence for the plain-
tiffs a compromise was effected, and the pilot withdrew all assertion that
the wheel chains had been jammed, thus admitting that the failure to steer
had been brought about by the reversal of the screw.



246 ON THE STEERING OF SCREW STEAMERS. [32

The other case is the well-known accident to the ' Kiirfurst.' In this it
is admitted that the order was to starboard the helm and reverse the screw
of the ' Kb'nig Wilhelm,' and this order was avowedly given with the view of
bringing the vessel round to port. All the experiments of this Committee,
however, go to prove that with a reversed screw and a starboard helm such a
vessel as the ' Konig Wilhelm ' would have turned to starboard rather than
port. This was what, according to all the evidence, did actually happen, and
was the final cause of the catastrophe. But it appears that those in charge
of the ' Konig Wilhelm ' arrived at the conclusion that the men at the wheel
(and these would be many), although they all aver that they heard the order
and obeyed it, in reality turned the wheel the wrong way. Considering,
therefore, that it was not one man but a number of men at the wheel, and
that the vessel behaved exactly as she would have behaved had the order
been obeyed, as the men say it was, the conclusion of the Court seems to
be most improbable, and, for the sake of future steering, most unfortunate.

The Committee are now of opinion that the work for which they were
originally brought together has been fully accomplished. The importance of
the effect of the reversed screw on the action of the rudder has been fully
established, as well as the nature of its effect completely ascertained. Also
for two years the Committee have urged the results of their work upon the
attention of the Admiralty, and the various marine boards, and although
they regret that as yet they have failed to obtain the general recognition of
the facts brought to light which their vital importance demands, they
consider that this will surely follow, and that as a Committee they can do
no more than publish the reports of the trials, and the conclusions to which
they have been led.

Full accounts of the experiments made previously to this year have been
given in the two previous Reports, and those which the Committee have
received this year are given at length at the end of this Report. The follow-
ing is a summary of the conclusions which have been established ; and it is
interesting to notice that the conclusions drawn by Professor Reynolds from
experiments on models have been fully confirmed by the experiments on full-
sized ships :



Summary of the Results of the Trials of the Effect of the Reversed Screw
on the Steering during the time a vessel is stopping herself.

It appears both from the experiments made by the Committee, and from
other evidence, that the distance required by a screw steamer to bring herself
to rest from full speed by the reversal of her screw is independent, or nearly
so, of the power of the engines, but depends on the size and build of the ship,



32] ON THE STEERING OF SCREW STEAMERS. 247

and generally lies between four and six times the ship's length. It is to be
borne in mind that it is to the behaviour of the ship during this interval that
the following remarks apply.

The main point the Committee have had in view has been to ascertain
how far the reversing of the screw in order to stop a ship did or did not
interfere with the action of the rudder during the interval of stopping ; and
it is as regards this point that the most important light has been thrown on
the question of handling ships. It is found an invariable rule that, during
the interval in which a ship is stopping herself by the reversal of her screw,
the rudder produces none of its usual effect to turn the ship, but that, under
these circumstances, the effect of the rudder, such as it is, is to turn the ship
in the opposite direction from that in which she would turn if the screw were
going ahead. The magnitude of this reverse effect of the rudder is always
feeble, and is different for different ships, and even for the same ship under
different conditions of loading.

It also appears from the trials that, owing to the feeble influence of the
rudder over the ship during the interval in which she is stopping, she is then
at the mercy of any other influences that may act upon her. Thus the wind,
which always exerts an influence to turn the stem (or forward end) of the ship
into the wind, but which influence is usually well under control of the rudder,
may, when the screw is reversed, become paramount and cause the ship to
turn in a direction the very opposite of that which is desired. Also the
reversed screw will exercise an influence, which increases as the ship's way is
diminished, to turn the ship to starboard or port according as it is right or left
handed ; this being particularly the case when the ships are in light draught.

These several influences the reversed effect of the rudder, the effort of
the wind, and the action of the screw will determine the course the ship
takes during the interval of stopping. They may balance, in which case the
ship will go straight on, or any one of the three may predominate and so
determine the course of the ship.

The utmost effect of these influences, when they all act in conjunction, as
when the screw is right-handed, the helm starboarded, and the wind on the
starboard side, is small as compared with the influence of the rudder as it
acts when the ship is steaming ahead. In no instance has a ship tried by the
Committee been able to turn with the screw reversed on a circle of less than
double the radius of that on which she would turn when steaming ahead.
So that, even if those in charge could govern the direction in which the ship
will turn while stopping, she turns but slowly ; whereas in point of fact those
in charge have little or no control over this direction, and unless they are
exceptionally well acquainted with the ship, they will be unable even to
predict the direction.



248 ON THE STEERING OF SCREW STEAMERS. [32

It is easy to see, therefore, that if on approaching danger the screw be
reversed, all idea of turning the ship out of the way of the danger must be
abandoned. She may turn a little, and those in charge may know in which
direction she will turn, or may even by using the rudder in an inverse manner
be able to influence this direction, but the amount of turning must be small,
and the direction very uncertain.

The question, therefore, as to the advisability of reversing the screw is
simply a question as to whether the danger may be better avoided by stopping
or by turning ; a ship cannot do both with an}' certainty.

Which of these two courses it is better to follow, must depend on the
particular circumstances of each particular case, but the following considera-
tions would appear to show that when the helm is under sufficient command
there can seldom be any doubt.

A screw steam-ship when at full speed requires five lengths, more or less,
in which to stop herself; whereas by using her rudder and steaming on at full
speed ahead, she should be able to turn herself through a quadrant, without
having advanced five lengths in her original direction. That is to say, a ship
can turn a circle of not greater radius than four lengths more or less (see
'Hankow,' 'Valetta,' 'Barge'); so that, even if running at full speed directly
on to a straight coast, she should be able to save herself by steaming on
ahead and using her rudder after she is too near to save herself by stopping ;
and any obliquity in the direction of approach, or any limit to the breadth of
the object ahead, is all to the advantage of turning, but not at all so to
stopping.

There is one consideration, however, with regard to the question of
stopping or turning which must, according to the present custom, often have
weight, although there can be but one opinion as to the viciousness of the
custom. This consideration is the utter inability of the officers in charge to
make any rapid use of the rudder so long as their engines are kept on ahead.
It is no uncommon thing for the largest ships to be steered by as few as two
men. And the mere fact of the wheel being so arranged that two men have
command of the rudder, renders so many turns of the wheel necessary to
bring the rudder over that, even where ready help is at hand, it takes a long
time to turn the wheel round and round so as to put a large angle on the
rudder.

The result is that it is often one or two minutes after the order is heard
before there is any large angle on the rudder, and of course under these
circumstances it is absurd to talk of making use of the turning qualities of a
ship in case of emergency. The power available to turn the rudder should
be proportional to the tonnage of the vessel, and there is no mechanical
reason why the rudder of the largest vessel should not be brought hard over



32] ON THE STEERING OF SCREW STEAMERS. 249

in less than fifteen seconds from the time the order is given. Had those in
charge of steam-ships sufficient control over the rudder, it is probable that
much less would be heard of the reversing of the engines in cases of imminent
danger.

REPORTS OF THE TRIALS OF THIS YEAR.

" S.s. ' North-Western,'

February 7, 1878.

" Right-handed screw. Speed of ship 13 knots. Signalled to engine-
room ' Stop.' ' Full speed astern ' 20 seconds after first order. Engines
moving astern. Helm put hard a-starboard. Head commenced moving to
starboard and went from N. 20 E. to N. 50 E. in \\ minute. The vessel had
by this time stopped going through the water. We then got up full speed
ahead, stopped, put the helm hard a-port, and reversed full speed. The
vessel had stopped going ahead in 1^ minute, and the head had gone to star-
board from N. 30 E. to N. 50 E. At 2| minutes the head stopped going to
starboard, and at 2^ minutes the ship's head was going to port. The vessel
was going astern through the water before her head stopped going to star-
board.

"The draught of water was 9 feet 2 inches and 12 feet 10 inches. The
centre of propeller is 7 feet 1 inch above bottom of keel, and the propeller
is 13 feet in diameter, so that the top of the blade was 9 inches out of the

water.

" W. BOTTOMLEY, Jun."

Remarks by the Committee.

The screw of this vessel being right-handed, its tendency when reversed
would be to bring the vessel's head to starboard, and, owing to the screw
being partially out of water, this tendency would be considerable. Accord-
ingly we find that the direct effect of the screw prevailed over the influence
of the rudder, and when the screw was reversed the vessel turned to star-
board for all positions of the helm. The reversed effect of the rudder was,
however, very apparent, for the vessel went to starboard while stopping much
faster with the helm starboarded than with the helm ported.

The same phenomena exactly will be seen in the trials of the next four

vessels.

Kongl. Gieenska Norsk General Consulatet i Stettin.

"STETTIN, May 11, 1878.

"SiR, Being a subscriber to the Navy I perused an article in No. 124,
vol. V., of that journal (Oct. 7, 1876) regarding experiments on the turning of
screw steamers.



250 ON THE STEERING OF SCREW STEAMERS. [32

" The same inspired me with great interest in the matter it treats of, and
caused me to instruct the captains of my three steamers, ' Martha,' ' Marietta,'
' Susanne ' (of which I subjoin the necessary particulars at foot), to make the
experiments in question. This has been done, and the results obtained
communicated to the Nautical Associations here and at other German ports.
Being indebted to you, as the promoter of these experiments, for the idea, I
consider it my duty to acquaint you with the results of the experiments made
by my captains, and venture to enclose a translation of the report on same.
I need not state that any comments you might favour me with, or a few lines
stating whether the conclusions arrived at correspond to your own, would be

most highly esteemed.

" I am, Sir, your most obedient,

"T. IVERS."
" Professor Reynolds, Manchester."



" On the Steering of Steamships with Right-handed Screws, when the vessel



Online LibraryOsborne ReynoldsPapers on mechanical and physical subjects (Volume 1) → online text (page 25 of 40)