John Bourne.

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_A._ - Quite the contrary. If to permit a greater speed of the engine the
floats be diminished in area instead of being raised out of the water, no
appreciable accession to the speed of the vessel will be obtained; whereas
there will be an increased speed of vessel if the accelerated speed of the
engine be caused by diminishing the diameter of the wheels. In vessels
intended to be fast, therefore, it is expedient to make the wheels small,
so as to enable the engine to work with a high velocity; and it is
expedient to make such wheels of the feathering kind, to obviate loss of
power from oblique action. In no wheel must the rolling circle fall below
the water line, else the entering and emerging floats will carry masses of
water before them. The slip is usually equal to about one-fourth of the
velocity of the centre of pressure in well proportioned wheels; but it is
desirable to have the slip as small as is possible consistently with the
observance of other necessary conditions. The speed of the engine and also
the speed of the vessel being fixed, the diameter of the rolling circle
becomes at once ascertainable, and adding to this the slip, we have the
diameter of the wheel.



CONFIGURATION AND ACTION OF THE SCREW.

562. _Q._ - Will you describe more in detail than you have yet done, the
configuration and mode of action of the screw propeller?

_A._ - The ordinary form of screw propeller is represented in figs. 46 and
47; fig. 46 being a perspective view, and fig. 47 an end view, or view such
as is seen when looking upon the end of the shaft. The screw here
represented is one with two arms or blades. Some screws have three arms,
some four and some six; but the screw with two arms is the most usual, and
screws with more than three arms are not now much employed in this country.
The screw on being put into revolution by the engine, preserves a spiral
path in the water, in which it draws itself forward in the same way as a
screw nail does when turned round in a piece of wood, whereas the paddle
wheel more resembles the action of a cog wheel working in a rack.

[Illustration: Fig. 46. Fig. 47. ORDINARY FORM OF SCREW PROPELLER.]

563. _Q._ - But the screw of a steam vessel has no resemblance to a screw
nail?

_A._ - It has in fact a very close resemblance if you suppose only a very
short piece of the screw nail to be employed, and if you suppose, moreover,
the thread of the screw to be cut nearly into the centre to prevent the
wood from stripping. The original screw propellers were made with several
convolutions of screw, but it was found advantageous to shorten them, until
they are now only made one-sixth of a convolution in length.

564. _Q._ - And the pitch you have already explained to be the distance in
the line of the shaft from one convolution to the next, supposing the screw
to consist of two or more convolutions?

_A._ - Yes, that is what is meant by the pitch. If a thread be wound upon a
cylinder with an equal distance between the convolutions, it will trace a
screw of a uniform pitch; and if the thread be wound upon the cylinder with
an increasing distance between each convolution, it will trace a screw of
an increasing pitch. But two or more threads may be wound upon the cylinder
at the same time, instead of a single thread. If two threads be wound upon
it they will trace a double-threaded screw; if three threads be wound upon
it they will trace a treble-threaded screw; and so of any other number. Now
if the thread be supposed to be raised up into a very deep and thin spiral
feather, and the cylinder be supposed to become very small, like the newel
of a spiral stair, then a screw will be obtained of the kind proper for
propelling vessels, except that only a very short piece of such screw must
be employed. Whatever be the number of threads wound upon a cylinder, if
the cylinder be cut across all the threads will be cut. A slice cut out of
the cylinder will therefore contain a piece of each thread. But the
threads, in the case of a screw propeller, answer to the arms, so that in
every screw propeller the number of threads entering into the composition
of the screw will be the same as the number of arms. An ordinary screw with
two blades is a short piece of a screw of two threads.

565. _Q._ - In what part of the ship is the screw usually placed?

[Illustration: Fig. 48]

_A._ - In that part of the run of the ship called the dead wood, which is a
thin and unused part of the vessel just in advance of the rudder. The usual
arrangement is shown in fig. 48, which represents the application to a
vessel of a species of screw which has the arms bent backwards, to
counteract the centrifugal motion given to the water when there is a
considerable amount of slip.

566. _Q._ - How is the slip in a screw vessel determined?

_A._ - By comparing the actual speed of the vessel with the speed due to the
pitch and number of revolutions of the screw, or, what is the same thing,
the speed which the vessel would attain if the screw worked in a solid nut.
The difference between the actual speed and this hypothetical speed, is the
slip.

567. _Q._ - In well formed screw propellers what is the amount of slip found
to be?

_A._ - If the screw be properly proportioned to the resistance that the
vessel has to overcome, the slip will not be more than 10 per cent., but in
some cases it amounts to 30 per cent., or even more than this. In other
cases, however, the slip is nothing at all, and even less than nothing; or,
in other words the vessel passes through the water with a greater velocity
than if the screw were working in a solid nut.

568. _Q._ - Then it must be by the aid of the wind or some other extraneous
force?

_A._ - No; by the action of the screw alone.

569. _Q._ - But how is such a result possible?

_A._ - It appears to be mainly owing to the centrifugal action of the screw,
which interposes a film or wedge of water between the screw itself and the
water on which the screw reacts. This negative slip, as it is called,
chiefly occurs when the pitch of the screw is less than its diameter, and
when, consequently, the velocity of rotation is greater than if a coarser
pitch had been employed. There is, moreover, in all vessels passing through
the water with any considerable velocity, a current of water following the
vessel, in which current, in the case of a screw vessel, the screw will
revolve; and in certain cases the phenomenon of negative slip may be
imputable in part to the existence of this current.

570. _Q._ - Is the screw propeller as effectual an instrument of propulsion
as the radial or feathering paddle?

_A._ - In all cases of deep immersion it appears to be quite as effectual as
the radial paddle, indeed, more so; but it is scarcely as effectual as the
feathering paddle, with any amount of immersion, and scarcely as effectual
as the common paddle in the case of light immersions.



COMPARATIVE ADVANTAGES OF PADDLE AND SCREW VESSELS.

571. _Q._ - Whether do you consider paddle or screw vessels to be on the
whole the most advantageous?

_A._ - That is a large question, and can only receive a qualified answer. In
some cases the use of paddles is indispensable, as, for example, in the
case of river vessels of a limited draught of water, where it would not be
possible to get sufficient depth below the water surface to enable a screw
of a proper diameter to be got in.

572. _Q._ - But how does the matter stand in the case of ocean vessels?

_A._ - In the case of ocean vessels, it is found that paddle vessels fitted
with the ordinary radial wheels, and screw vessels fitted with the ordinary
screw, are about equally efficient in calms and in fair or beam winds with
light and medium immersions. If the vessels are loaded deeply, however, as
vessels starting on a long voyage and carrying much coal must almost
necessarily be, then the screw has an advantage, since the screw acts in
its best manner when deeply immersed, and the paddles in their worst. When
a screw and paddle vessel, however, of the same model and power are set to
encounter head winds, the paddle vessel it is found has in all cases an
advantage, not in speed, but in economy of fuel. For whereas in a paddle
vessel, when her progress is resisted, the speed of the engine diminishes
nearly in the proportion of the diminished speed of ship, it happens that
in a screw vessel this is not so, - at least to an equal extent, - but the
engines work with nearly the same rate of speed as if no increase of
resistance had been encountered by the ship. It follows from this
circumstance, that whereas in paddle vessels the consumption of steam, and
therefore of fuel, per hour is materially diminished when head winds occur,
in screw vessels a similar diminution in the consumption of steam and fuel
does not take place.

573. _Q._ - But perhaps under such circumstances the speed of the screw
vessel will be the greater of the two?

_A._ - No; the speed of the two vessels will be the same, unless the
strength of the head wind be so great as to bring the vessels nearly to a
state of rest, and on that supposition the screw vessel will have the
advantage. Such cases occur very rarely in practice; and in the case of the
ordinary resistances imposed by head winds, the speed of the screw and
paddle vessel will be the same, but the screw vessel will consume most
coals.

574. _Q._ - What is the cause of this peculiarity?

_A._ - The cause is, that when the screw is so proportioned in its length as
to be most suitable for propelling vessels in calms, it is too short to be
suitable for propelling vessels which encounter a very heavy resistance. It
follows, therefore, that if it is prevented from pursuing its spiral course
in the water, it will displace the water to a certain extent laterally, in
the manner it does if the engine be set on when the vessel is at anchor;
and a part of the engine power is thus wasted in producing a useless
disturbance of the water, which in paddle vessels is not expended at all.

575. _Q._ - If a screw and paddle vessel of the same mould and power be tied
stern to stern, will not the screw vessel preponderate and tow the paddle
vessel astern against the whole force of her engines?

_A._ - Yes, that will be so.

576. _Q._ - And seeing that the vessels are of the same mould and power, so
that neither can derive an advantage from a variation in that condition,
does not the preponderance of the screw vessel show that the screw must be
the most powerful propeller?

_A._ - -No, it does not.

577. _Q._ - Seeing that the vessels are the same in all respects except as
regards the propellers, and that one of them exhibits a superiority, does
not this circumstance show that one propeller must be more powerful than
the other?

_A._ - That does not follow necessarily, nor is it the fact in this
particular case. All steam vessels when set into motion, will force
themselves forward with an amount of thrust which, setting aside the loss
from friction and from other causes, will just balance the pressure on the
pistons. In a paddle vessel, as has already been explained, it is easy to
tell the tractive force exerted at the centre of pressure of the paddle
wheels, when the pressure urging the pistons, the dimensions of the wheels
and the speed of the vessel are known; and that force, whatever be its
amount, must always continue the same with any constant pressure on the
pistons. In a screw vessel the same law applies, so that with any given
pressure on the pistons and discarding the consideration of friction, it
will follow that whatever be the thrust exerted by a paddle or a screw
vessel, it must remain uniform whether the vessel is in motion or at rest,
and whether moving at a high or a low velocity through the water. Now to
achieve an equal speed during calms in two vessels of the same model, there
must be the same amount of propelling thrust in each; and this thrust,
whatever be its amount, cannot afterward vary if a uniform pressure of
steam be maintained. The thrusts, therefore, caused by their respective
propelling instruments, when a screw and paddle vessel are tied stern to
stern, must be the same as at other times; and as at other times those
thrusts are equal, so must they be when the vessels are set in the
antagonism supposed.

578. _Q._ - How comes it then that the screw vessel preponderates?

_A._ - Not by virtue of a larger thrust exerted by the screw in pressing
forward the shaft and with it the vessel, but by the gravitation against
the stern of the wave of water which the screw raises by its rapid
rotation. This wave will only be raised very high when the progress of the
vessel through the water is nearly arrested, at which time the centrifugal
action of the screw is very great; and the vessel under such circumstances
is forced forward partly by the thrust of the screw, and partly by the
hydrostatic pressure of the protuberance of water which the centrifugal
action of the screw raises up at the stern.

579. _Q._ - Can you state any facts in corroboration of this view?

_A._ - The screw vessel will not preponderate if a screw and paddle vessel
be tied bow to bow and the engines of each be then reversed. In, some screw
vessels the amount of thrust actually exerted by the screw under all its
varying circumstances, has been ascertained by the application of a
dynamometer to the end of the shaft. By this instrument - which is formed by
a combination of levers like a weighing machine for carts - a thrust or
pressure of several tons can be measured by the application of a small
weight; and it has been found, by repeated experiment with the dynamometer,
that the thrust of the screw in a screw vessel when towing a paddle vessel
against the whole force of her engines, is just the same as it is when the
two vessels are maintaining an equal speed in calms. The preponderance of
the screw vessel must, therefore, be imputable to some other agency than to
a superior thrust of the screw, which is found by experiment not to exist.

580. _Q._ - Has the dynamometer been applied to paddle vessels?

_A._ - It has not been applied to the vessels themselves, as in the case of
screw vessels, but it has been employed on shore to ascertain the amount of
tractive force that a paddle vessel can exert on a rope.

581. _Q._ - Have any experiments been made to determine the comparative
performances of screw and paddle vessels at sea?

_A._ - Yes, numerous experiments; of which the best known are probably those
made on the screw steamer Rattler and the paddle steamer Alecto, each
vessel of the same model, size, and power, - each vessel being of about 800
tons burden and 200 horses power. Subsequently another set of experiments
with the same object was made with the Niger screw steamer and the Basilisk
paddle steamer, both vessels being of about 1000 tons burden and 400 horses
power. The general results which were obtained in the course of these
experiments are those which have been already recited.

582. _Q._ - Will you recapitulate some of the main incidents of these
trials?

_A._ - I may first state some of the chief dimensions of the vessels. The
Rattler is 176 feet 6 inches long, 32 feet 8-1/2 inches broad, 888 tons
burden, 200 horses power, and has an area of immersed midship section of
380 square feet at a draught of water of 11 feet 5-1/2 inches. The Alecto
is of the same dimensions in every respect, except that she is only of 800
tons burden, the difference in this particular being wholly owing to the
Rattler having been drawn out about 15 feet at the stern, to leave abundant
room for the application of the screw. The Rattler was fitted with a
dynamometer, which enabled the actual propelling thrust of the screw shaft
to be measured; and the amount of this thrust, multiplied by the distance
through which the vessel passed in a given time, would determine the amount
of power actually utilized in propelling the ship. Both vessels were fitted
with indicators applied to the cylinders, so as to determine the amount of
power exerted by the engines.

583. _Q._ - How many trials of the vessels were made on this occasion?

_A._ - Twelve trials in all; but I need not refer to those in which similar
or identical results were only repeated. The first trial was made under
steam only, the weather was calm and the water smooth. At 54 minutes past 4
in the morning both vessels left the Nore, and at 30-1/2 minutes past 2 the
Rattler stopped her engines in Yarmouth Roads, where in 20-1/2 minutes
afterward she was joined by the Alecto. The mean speed achieved by the
Rattler during this trial was 9.2 knots per hour; the mean speed of the
Alecto was 8.8 knots per hour. The slip of the screw was 10.2 per cent. The
actual power exerted by the engines, as shown by the indicator, was in the
case of the Rattler 334.6 horses, and in the case of the Alecto 281.2
horses; being a difference of 53.4 horses in favor of the Rattler. The
forward thrust upon the screw shaft was 3 tons, 17 cwt., 3 qrs., and 14
lbs. The horse power of the shaft - or power actually utilized - ascertained
by multiplying the thrust in pounds by the space passed through by the
vessel in feet per minute, and dividing by 33,000, was 247.8 horses power.
This makes the ratio of the shaft to the engine power as 1 to 1.3, or, in
other words, it shows that the amount of engine power utilized in
propulsion was 77 per cent. In a subsequent trial made with the vessels
running before the wind, but with no sails set and the masts struck, the
speed realized by the Rattler was 10 knots per hour. The slip of the screw
was 11.2 per cent. The actual power exerted by the engines of the Rattler
was 368.8 horses. The actual power exerted by the engines of the Alecto was
291.7 horses. The thrust of the shaft was equal to a weight of 4 tons, 4
cwt., 1 qr., 1 lb. The horse power of the shaft was 290.2 horses, and the
ratio of the shaft to the engine power was 1 to 1.2. Here, therefore, the
amount of the engine power utilized was 84 per cent.

584. _Q._ - If in any screw vessel the power of the engine be diminished by
shutting off the steam or otherwise, you will then have a larger screw
relatively with the power of the engine than before?

_A._ - Yes.

585. _Q._ - Was any experiment made to ascertain the effect of this
modification?

_A._ - There was; but the result was not found to be better than before. The
experiment was made by shutting off the steam from the engines of the
Rattler until the number of strokes was reduced to 17 in the minute. The
actual power was then 126.7 horses; thrust upon the shaft 2 tons, 2 cwt., 3
qrs., 14 lbs; horse power of shaft 88.4 horses; ratio of shaft to engine
power 1 to 1.4; slip of the screw 18.7 per cent. In this experiment the
power utilized was 71 per cent.

586. _Q._ - Was any experiment made to determine the relative performances
in head winds?

_A._ - The trial in which this relation was best determined lasted for seven
hours, and was made against a strong head wind and heavy head sea. The
speed of the Rattler by patent log was 4.2 knots; and at the conclusion of
the trial the Alecto had the advantage by about half a mile. Owing to an
accidental injury to the indicator, the power exerted by the engines of the
Rattler in this trial could not be ascertained; but judging from the power
exerted in other experiments with the same number of revolutions, it
appears probable that the power actually exerted by the Rattler was about
300 horses. The number of strokes per minute made by the engines of the
Rattler was 22, whereas in the Alecto the number of strokes per minute was
only 12; so that while the engines of the Alecto were reduced, by the
resistance occasioned by a strong head wind, to nearly half their usual
speed, the engines of the Rattler were only lessened about one twelfth of
their usual speed. The mean thrust upon the screw shaft during this
experiment, was 4 tons, 7 cwt., 0 qr., 16 lbs. The horse power of the shaft
was 125.9 horses, and the slip of the screw was 56 per cent. Taking the
power actually exerted by the Rattler at 300 horses, the power utilized in
this experiment is only 42 per cent.

587. _Q._ - What are the dimensions of the screw in the Rattler?

_A._ - Diameter 10 feet, length 1 foot 3 inches, pitch 11 feet. The
foregoing experiments show that with a larger screw a better average
performance would be obtained. The best result arrived at, was when the
vessel was somewhat assisted by the wind, which is equivalent to a
reduction of the resistance of the hull, or to a smaller hull, which is
only another expression for a larger proportionate screw.

588. _Q._ - When you speak of a larger screw, what increase of dimension do
you mean to express?

_A._ - An increase of the diameter. The amount of reacting power of the
screw upon the water is hot measured by the number of square feet of
surface of the arms, but by the area of the disc or circle in which the
screw revolves. The diameter of the screw of the Rattler being 10 feet, the
area of its disc is 78.5 square feet; and with the amount of thrust already
mentioned as existing in the first experiment, viz. 8722 lbs., the reacting
pressure on each square foot of the screw's disc will be 108-1/2 lbs. The
immersed midship section being 380 square feet, this is equivalent to 23
lbs. per square foot of immersed midship section at a speed of 9.2 knots
per hour.

589. _Q._ - In smaller vessels of similar form, will the resistance per
square foot of midship section be more than this?

_A._ - It will be considerably more. In the Pelican, a vessel of 109-3/4
square feet of midship section, I estimate the resistance per square foot
of midship section at 30 lbs., when the speed of the vessel is 9.7 knots
per hour. In the Minx with an immersed midship section of 82 square feet,
the resistance per square foot of immersed midship section was found by the
dynamometer to be 41 lbs. at a speed of 8-1/2 knots; and in the Dwarf, a
vessel with 60 square feet of midship section, I estimate the resistance
per square foot of midship section at 46 lbs. at a speed of 9 knots per
hour, which is just double the resistance per square foot of the Rattler.
The diameter of the screw of the Minx is 4-1/2 feet, so that the area of
its disc is 15.9 square feet, and the area of immersed midship section is
about 5 times greater than that of the screw's disc. The diameter of the
screw of the Dwarf is 5 feet 8 inches, so that the area of its disc is
25.22 square feet, and the area of immersed midship section is 2.4 times
greater than that of the screw's disc. The pressure per square foot of the
screw's disc is 214 lbs. in the case of the Minx, and 109-1/2 lbs. in the
case of the Dwarf.

590. _Q._ - From the greater proportionate resistance of small vessels, will
not they require larger proportionate screws than large vessels?

_A._ - They will.

591. _Q._ - Is there any ready means of predicting what the amount of thrust
of a screw will be?

_A._ - When we know the amount of pressure on the pistons, and the velocity
of their motion relatively with the velocity of advance made by the screw,
supposing it to work in a solid nut, it is easy to tell what the thrust of
the screw would be if it were cleared of the effects of friction and other
irregular sources of disturbance. The thrust, in fact, would be at once
found by the principle of virtual velocities; and if we take this
theoretical thrust and diminish it by one fourth to compensate for friction
and lateral slip, we shall have a near approximation to the amount of
thrust that will be actually exerted.[1]


[1] See Treatise on the Screw Propeller, by J. Bourne, C. E.




COMPARATIVE ADVANTAGES OF DIFFERENT SCREWS.

592. _Q._ - What species of screw do you consider the best?

_A._ - In cases in which a large diameter of screw can be employed, the
ordinary screw or helix with two blades seems to be as effective as any
other, and it is the most easily constructed. If, however, the screw is
restricted in diameter, or if the vessel is required to tow, or will have
to encounter habitually strong head winds, it will be preferable to employ
a screw with an increasing pitch, and also of such other configuration that
it will recover from the water some portion of the power that has been
expended in slip.

593. _Q._ - How can this be done?

_A._ - There are screws which are intended to accomplish, this object
already in actual use. When there is much slip a centrifugal velocity is
given to the water, and the screw, indeed, if the engine be set on when the
vessel is at rest, acts very much as a centrifugal fan would do if placed



Online LibraryJohn BourneA Catechism of the Steam Engine → online text (page 24 of 34)