[Footnote: A paper recently read before the Institution of Naval
Architects, London.]
By E.A. LINNINGTON.
One of the most interesting and valuable features in the development of
naval construction in recent years is the great advance which has been made
in the speeds of our war ships. This advance has been general, and not
confined to any particular vessel or class of vessel. From the first class
armored fighting ship of about 10,000 tons displacement down to the
comparatively diminutive cruiser of 1,500 tons, the very desirable quality
of a high speed has been provided.
These are all twin screw ships, and each of the twins is driven by its own
set of engines and line of shafting, so that the propelling machinery of
each ship is duplicated throughout. The speeds attained indicate a high
efficiency with the twin screws. In all ships, but more especially in high
speed ships, success depends largely upon the provision of propellers
suited for the work they have to perform, and where a high propulsive
efficiency has been secured, there is no doubt the screws are working with
a high efficiency. The principal purpose of this paper is to record the
particulars of the propellers, and the results of the trials of several of
these high speed twin screw ships. The table gives the leading particulars
of several classes of ships, the particulars of the screws, and the results
obtained on the measured mile trials from a ship of each class, except C.
The vessels whose trials are inserted in the table have not been selected
as showing the highest speeds for the several classes. Excepting C, they
are the ships which have been run on the measured mile at or near the
designed load water line. On light draught trials, speeds have been
attained from half a knot to a knot higher than those here recorded. No
ship of the class C has yet been officially tried on the measured mile, but
as several are in a forward state, perhaps the actual data from one of them
may shortly be obtained. All these measured mile trials were made under the
usual Admiralty conditions, that is to say, the ships' bottoms and the
screws were clean, and the force of the wind and state of the sea were not
such as to make the trials useless for purposes of comparison. On such
trials the i.h.p. is obtained from diagrams taken while the ship is on the
mile, and the revolutions are recorded by ruechanical counters for the time
occupied in running the mile. Not less than four runs are made during a
trial extending over several hours. The i.h.p. in the table is not
necessarily the maximum during the trial, for the average while on the mile
is sometimes a little below the average for the whole of the trial. The
revolutions are the mean for the two sets of engines, and the i.h.p. is the
sum of the powers of the two sets. The pitch of the screw is measured. The
bolt holes in the blade flanges allow an adjustment of pitch, but in each
case the blades were set as nearly as possible at the pitch at which they
were cast. The particulars given in the table may be taken to be as
reliable and accurate as such things can be obtained, and for each ship
there are corresponding data; that is, the powers, speeds, displacements,
revolutions, pitches, and other items existed at the same time. There are a
few points of detail about these propellers which deserve a passing notice.
In Fig. 1 is shown a fore and aft section through the boss. It will be
observed that the flanges of the blades are sunk into the boss, and that
the bolts are sunk into the flanges. The recess for the bolt heads is
covered with a thin plate having the curve of the flange, so that the
flanges and the boss form a section of a sphere. This method of
construction is a little more expensive than exposed flanges and bolts,
which, however, render the boss a huge churn. With the high revolutions at
which these screws work, a spherical boss is extremely desirable, but, of
course, the details need not be exactly as shown in the illustration. The
conical tail is fitted to prevent loss with eddies behind the flat end of
the boss, and is particularly valuable with the screws of high speed ships.
The light hood shown on the stern bracket is for the purpose of preventing
eddies behind the boss of the stern bracket, and to save the resistance of
the flat face of the screw boss. The edges of the blades are cast sharp,
instead of being rounded at the back, with a small radius, as in the usual
practice--the object of the sharp edge being the diminution of the edge
resistance. The driving key extends the whole length of the boss, and the
tapered shaft fits throughout its length.
[Illustration: FIG. 1.]
These points of detail have been features of all Admiralty screws for some
years.
The frictional resistance of screw propellers is always a fruitful source
of inefficiency. With a given screw, the loss due to friction may be taken
to vary approximately as the square of the speed. This is not to say that
the frictional resistance is greater in proportion to the thrust at high
than at low speeds. The blades of screws for any speed should be as smooth
and clean as possible, but for high speed screws the absolute saving of
friction may be considerable with an improvement of the surface. There is
no permanent advantage in polishing the blades. No doubt there is some
advantage for a little time, and, probably, better results may thereby be
secured on trial, but the blades soon become rough, and shell fish and weed
appear to grow as rapidly on recently polished blades as on an ordinary
surface. These screws are of gun metal. They were fitted to the ships in
the condition in which they left the foundry. It appears that within
certain limits mere shape of blade does not affect the efficiency of the
screw, but, with a given number of blades and a given disk, the possible
variations in the form or distribution of a given area are such that
different results may be realized. The shapes of the blades of these
propellers are shown in Figs. 2, 3, and 4. It will be seen the shapes are
not exactly the same for all the screws, but the differences do not call
for much remark.
[Illustration: FIG. 2., FIG. 3. & FIG. 4.]
Fig. 2 shows the blades for the A screw. C and D have the same form. Fig. 3
shows in full lines the blades of the B screw, and, though very narrow at
the tips, they, like A, are after the Griffith pattern. The blades of E and
F are of a similar shape, as shown in Fig. 4, and approach an oval form
rather than the Griffith pattern. The particulars of these propellers would
be considered incomplete without some reference to their positions with
respect to the hulls. When deciding the positions of twin screws, there is
room for variation, vertically, longitudinally, and transversely. For these
screws, the immersions inserted in the table give the vertical positions.
The immersion in A is 9 ft., showing what may be done in a deep draught
ship with a small screw. Whatever the value of deep immersion may be in
smooth water, there can be no question that it is much enhanced in a
seaway. The longitudinal positions are such that the center of the screw is
about one-fifth of the diameter forward of the aft side of the rudder post.
The positions may, perhaps, differ somewhat from this rule without
appreciably affecting the performance, but, if any alteration be made, it
would probably be better to put the screws a little farther aft rather than
forward. The forward edges of the blades are from 2 ft. to 3 ft. clear of
the legs of the bracket which carries the after bearing. The transverse
positions are decided, to some extent, by the distance between the center
lines of the engines. As regards propulsive efficiency, it would appear
that the nearer the screws are to the middle line, the less is the
resistance due to the shaft tubes and brackets, and the greater is the gain
from the wake in the screw efficiency, but, on the other hand, the greater
is the augment of the ship's resistance, due to the action of the screws.
Further, the nearer the screws are to the hull, the less are they exposed.
But experience is not wanting to show that the vibration may be troublesome
when the blades come within a few inches of the hull. The average of the
clearances between the tips of the blades and the respective hulls is about
one-eighth of the diameter of the screw.
An interesting and noteworthy fact in connection with these propellers is
the wide differences in the pitches and revolutions, though the products of
the two do not greatly vary. Such differences are extremely rare in the
mercantile marine for similar speeds, but in war ships they are inseparable
from the conditions of the engine design. As a general rule, with
(revolutions × pitch) a constant, an increase of revolutions and the
consequent decrease of pitch allow a diminution of disk and of blade
area--other modifying conditions, such as the thrust, slip, number, and
pattern of blades, being the same. The screws for E and F are interesting,
because, with practically the same speeds and slips, there is a
considerable difference in the revolutions. It will be observed that F is a
vessel of finer form and a little less displacement than E, and, therefore,
has less resistance. Although E has the greater resistance and the screw
the smaller pitch/diameter, the higher revolutions permit the use of a
smaller screw. But from this example the influence of the high revolutions
in diminishing the size of screw does not appear so great as some empirical
rules would indicate. The screws for A and B are also worthy of attention.
Although the ship A has a much greater resistance than B, the screw of the
former is much the smaller, both in the blade area and the disk. A's
screws, however, in addition to 22 per cent. more revolutions than B, have
a much larger slip, and the blades have rather a fuller form at the tips.
Compared with the practice in the mercantile marine, the revolutions of
these screws are very high, and from the foregoing remarks it may appear
that much larger screws would be required for a merchant ship than for a
war ship of the same displacement and speed. There would, however, be
several items favorable to the use of small screws. For a given
displacement the resistance would be less in the mercantile ship, and with
the lower revolutions the proportion of blade area to the disk could be
increased without impairing the efficiency. Thus in passing from the war
vessel to a merchant ship of the same displacement, there are the lower
revolutions favorable to a larger screw, but, on the other hand, the
smaller resistance, larger proportion of blade area, and the coarser pitch,
are favorable to a diminution of the screw. The ship B has a very large
screw at 88 revolutions, but the tips are very narrow. If the blade were as
dotted for a diameter of 16 ft., the same work could be done with the same
revolutions, but with a little coarser pitch and a little more slip.
There is something to be said for large screws with a small proportion of
blade area to disk. For instance, two bladed screws have frequently given
better results than four bladed screws of smaller diameter, neglecting, of
course, the question of vibrations. Twin screws, however, should, as a
rule, be made as small as possible in diameter without loss of efficiency.
The advantages of small twin screws are the shorter shaft tubes and stern
brackets, deeper immersion, and less exposure as compared with large
screws. The exposure of the screws is usually considered an objection, but,
perhaps, too much has been made of it, for those well qualified to speak on
the subject consider that careful handling of the ship would, in most
cases, prevent damage to the screws, and that where the exposure is
unusually great, effectual protection by portable protectors presents no
insuperable difficulty.
HIGH SPEED TWIN SCREWS.
---------------------------------------------------------------------
|Ship A.|Ship B.|Ship C.|Ship D.|Ship E.|Ship F.
---------------------------------------------------------------------
Length, ft. | 325 | 315 | 300 | 300 | 220 | 250
Breadth, ft. | 68 | 61 | 56 | 46 | 34 | 32½
| | | | | |
Draught on trial, | 26 ft | 24 ft | | 15 ft | 12 ft | 13 ft
forward. | 2 in | 6 in | .... | 6 in | 10 in | 1 in
| | | | | |
Draught on trial, | 27 ft | 25 ft | | 19 ft | 15 ft | 14 ft
aft. | 3 in | 6 in | .... | 9 in | 2 in | 7 in
Displacement, | | | | | |
tons. | 9,690 | 7,645 | 5,000 | 3,584 | 1,560 | 1,544
I.M.S., sq. ft. | 1,560 | 1,287 | 1,000 | 744 | 438 | 392
Speed of ship, | | | | | |
knots. | 16.92 | 17.21 | 18.75 | 18.18 | 16.91 | 17
I.H.P. |11,610 |10,180 | 8,500 | 6,160 | 3,115 | 3,045
Revolutions per | | | | | |
minute. | 107.2 | 88 | 120 | 122.6 | 150.4 | 132.1
| | | | | |
Pitch of | 19 ft | 22 ft | 18 ft | 17 ft | 12 ft | 14 ft
screw. | 5 in | | 9 in | 6 in | 7½in | 9 in
| | | | | |
Slip. per cent | 17.6 | 10 | ... | 14.2 | 9.7 | 11.4
| | | | | |
Diameter of | 15 ft | 18 ft | 14 ft | 13 ft | 10 ft | 11 ft
screw. | 6 in | | 6 in | | 6 in |
| | | | | |
Diameter of | 4 ft | 4 ft | 3 ft | 3 ft | 2 ft | 2 ft
boss. | 4 in | 11 in | 9 in | 5 in | 9 in | 10 in
Number of blades | 4 | 4 | 3 | 3 | 3 | 3
Blade area of one | | | | | |
screw. | 72 | 87 | 60 | 47 | 24 | 24
Shape of blade. |Fig. 2.|Fig. 3.|Fig. 2.|Fig. 2.|Fig. 4.|Fig. 4
Pitch | | | | | |
---------- | 1.25 | 1.22 | 1.3 | 1.34 | 1.2 | 1.34
Diameter | | | | | |
Disk | | | | | |
-------- | 2.62 | 2.92 | 2.75 | 2.82 | 3.6 | 3.96
Blade area | | | | | |
Immersion of | 9 ft | 5 ft | | 4 ft | 2 ft | 1 ft
screw. | | 3 in | .... | 4 in | 9 in | 10 in
--------------------------------------------------------------------
The slips of these screws vary from 10 to 17½ per cent., which is certainly
not an extensive range, considering the widely different working
conditions. Slip, as an indication of the efficiency of the screw, is not
only an interesting subject, but it is often one of importance. In these
ships, however, there is nothing about the slips which would give rise to
any doubts as to the fitness of the screws for their work.
[Illustration: FIG. 5. & FIG. 6.]
The ancient fallacy that small slip meant a high screw efficiency was
supported by the great authority of the late Professor Rankine. Experience
proved that considerable slips and efficient screws were companions. The
late Mr. Froude offered an explanation of this general rule in a paper read
before this Institution in 1878, and gave a curve of efficiency with
varying true slip. In Mr. R E. Froude's paper last year there was a form of
this curve, with an arbitrary abscissa scale for the slip, devised to
illustrate in one diagram the wide conditions covered by his experiments.
In the screws now under consideration, the values of the pitch/diameter
vary only from 1.2 to 1.34, and for these the abscissa values for the same
slips do not differ much. Taking the mean value, and bringing the slips to
a common scale, Fig. 5 is obtained, which would approximately represent the
relation between the efficiency of any one of these screws and its true
slip, if this curve were applicable to full sized screws propelling actual
ships. The slips in Fig. 5 being real or true, are not the slips of
commerce, which are the apparent slips, such as those given in the table.
Let us endeavor to split up these real slips into the apparent slips and
another item, the speed of the wake. We then at once meet with the
difficulty that the wake in which the screw works has not a uniform motion.
Complex, however, as are the motions of the wake, the screw may be assumed
to work in a cylinder of water having such a uniform forward velocity as
will produce the same effect as the actual wake on the thrust of the screw.
It is then readily seen that the real slip is the sum of the apparent slip
and the speed of the hypothetical wake. To make this clear, let V be the
speed of the ship, Vs the speed of the screw, _i.e._, revolutions × pitch,
and V the speed of the wake; then--
Apparent slip = Vs - V.
Real slip = Vs - speed of ship with respect to the wake.
" = Vs - (V - V) = (Vs - V) + Vw.
" = Apparent slip + speed of the wake.
If the apparent slip be zero, the real slip is the speed of the wake, and
if the apparent slip be negative, the real slip is less than the speed of
the wake. The real slip is greater than the apparent slip, and can never be
a negative quantity. From Mr. Froude's model experiments, it appears that
this speed of wake for the A class of ship amounts to about 10 per cent. of
the speed of the A screw. If this value is correct, then the real slip is
(10 + 17.6) per cent., or 27.6 per cent. This is shown in Fig. 6, where O
is the point of no slip, being 17.64 from the point of real slip. Slips to
the right of O are positive apparent slips, slips to the left are negative
apparent slips. The vessel F would certainly have a wake with a speed
considerably less than that of A's wake. From the model experiments, the
wake for F is about one-half that for the A class, or, roughly, 5 per cent.
of the speed of the screw. For the ship F, O is the point of no apparent
slip, and the real slip is (5 + 11.4) or 16.4 per cent. For E, the point of
real slip is approximately the same as for F. For B and D, the positions on
the curve would be about the same. The ship B has a higher speed of wake
than D, but the screw D has the greater apparent slip. The influence of the
number of blades on the scale for the slip has been neglected. If this
efficiency curve were applicable to full sized screws propelling actual
ships, and if the determination of the wakes were beyond question, then we
should have a proof that our screws were at or near the maximum efficiency.
But, as we know, from the total propulsive efficiencies, that the screws
have high and not widely different efficiencies on these ships, we may
argue the other way, and say that there is good reason to consider that at
least the upper part of the curve agrees with experience obtained from
actual ships. Now take Fig. 6 and consider the general laws there
represented. Take the speed of the wake as 10 per cent. of the speed of the
screw, which is probably an average of widely different conditions,
including many single as well as twin screw ships. Then this curve shows
that considerable negative slips mean inefficient screws; that screws may
have very different positive slips without any appreciable difference in
their efficiencies; and that very large positive slips and inefficient
screws may be companions. For instance, a screw with a large positive slip
in smooth water is frequently inefficient at sea against a head wind, which
increases the resistance, and necessitates an increase of slip. I venture
to say that these statements, taken in a general manner, are not at
variance with experience obtained from the performances of screw ships.
Before it is possible to satisfactorily decide if this curve applies in a
general manner to full sized screws propelling ships, we require the
results of trials of various ships where the screws are working about the
region of no slip. Model experiments teach that the scale for the slip
varies with the design of the screw, and that with a given screw the speed
of the wake (which decides the point of no apparent slip) varies with the
type of ship and with the position of the screw with respect to the hull.
Remembering these disturbances, it is not improbable that it may be
possible to account for or explain what at first sight may appear
departures from the curve. The diameters of the screws in the table are not
compared with the diameters given by the method explained by Mr. Froude in
his paper last year, for there are differences in the slips, the
proportions of blade area to disk, and, to some extent, in the shapes of
the blades, which are not taken into account in that method. Assuming,
however, as Mr. Froude does, a constant proportion of blade area to disk,
and a uniform pattern of blade, the determination of the diameter for a
given set of conditions may, as a rule, be a complete solution of the
problem of the design of a screw, but these assumptions do not cover all
the necessities of actual practice, which make it extremely desirable to
know something about the influence or efficiency of various proportions of
blade area to disk, and of the form or distribution of a given area.
During the discussion which followed, Mr. John said that, both as regarded
the mercantile marine and the Royal Navy, there were few data to work upon,
but few ships having been built with twin screws. Mr. Linnington's
proportions of pitch to diameter of 1.2 to 1.34 was not invariably adhered
to. He mentioned a couple of small twin screw vessels where the proportion
of pitch to diameter came nearly to 1.5, and he remembered a few years ago
the propellers in one of these vessels being changed and the pitch
increased, the result being a very considerable improvement. He believed
they might go with quick running twin screw engines to a larger proportion
of pitch to diameter than they could with a single screw. He might instance
the change in the Iris. She was first engined with the pitch equal to the
diameter, and she gained two knots or thereabout when the diameter was
reduced 2 ft. and the pitch increased 2 ft.
Admiral De Horsey said that he tried experiments with the single screw in
the Aurora. She had a feathering serew, and when the sails were used to
assist, they commonly altered the pitch of the screw according to the
strength of the wind. The screw could be altered while it was revolving,
and as the wind freshened they coarsened the pitch, and when they wanted to
stop the engines they coarsened the pitch so as to bring the screw right
fore and aft, so that they never altered the way of the ship in changing
from steam to sail alone. The reason why twin screws had been adopted in
the navy was that if one was damaged there was the other still available.
But it gave them a still further advantage, as it enabled them to have a
fore and aft bulkhead, which with a single screw was difficult. The
mercantile marine had not as yet looked favorably on twin screws. Their
finest and fastest ships were single screws, probably because, in very bad
weather, the single screw was better.
Mr. Spyer said that in designing propellers for ships of war, they were
obliged to attempt to obtain the highest possible speed, and that was not
necessarily coincident with a propeller of maximum efficiency. On the other
hand, for mercantile purposes, coal consumption was obviously of paramount
importance, and the speed of any particular vessel must be obtained with
the smallest possible amount of indicated horse power, and a propeller of
maximum efficiency. Regarding the position of the propellers in a small
pinnace, the propellers were shifted six or seven inches further out, and
with about ten per cent. less indicated horse power she obtained three
tenths of a knot more speed.
Mr. Barnaby asked Mr. Linnington whether, in designing twin screws for a
vessel of 8,000 i.h.p., he would make each screw, which would have to take
4,000 i.h.p., of the same diameter as a screw for a single ship of 4,000
i.h.p., of the same speed. Unfortunately in high speed vessels, from one
point of view, the faster they went for a given power the smaller the
diameter of the screw had to be, and the larger the pitch, so that in very
high speed twin screw vessels the ratio of pitch to diameter would be found
to come out very great indeed. In a twin screw torpedo boat, to be tried
shortly, they had a ratio as high as 1.64. In the case of the Inflexible it
was found, owing possibly to the position of the screw, that the whole of
the plates immediately over the screws were damaged. Mr. Beckett Hill had
been using, during the past three or four years, the twin screw steamers
the Ludgate Hill, Richmond Hill, and Tower Hill. These were all over 4,000
tons register, and indicated, when at work at full speed, 2,500 h.p. Before
he and his friends built these steamers, they built some very large tug
boats on the twin screw principle. At the present moment, four of the
fastest steamers building for the Atlantic service were to have twin
screws. The great obstacle to the extension of the twin screw in the
mercantile navy had been the fear that the projection of these screws would
make the vessels very difficult to handle, but he had found no such
difficulties. He had found it an advantage to put the point of the
propeller as near the deadwood as he could, without actually touching it,
and in the large steamers, as well as in the tugs, the distance was a few
inches. As to the point of safety, he thought it a great advantage to have
twin screws, and on two occasions twin screw vessels had met with accidents
which, but for the twin screws, would have necessitated their putting back
to New York for repairs. The Richmond Hill, on one occasion, met with an
accident to her machinery two days after leaving New York; but she was able
to come on with the second set of engines, and was only one day late in the
passage. No difficulty had been found in the docking and undocking of these
vessels, either in London or Liverpool, and while with single screw vessels
they had sometimes to employ one or two dock boats to dock and undock them,
they never had to do so with the twin screw vessels. These vessels were 400
ft. long, with 48 ft. breadth of beam--a very large size to handle in a
river like the Thames. He noticed in the paper a propeller with a diameter
of 15 ft. 6 in. to indicate 11,110 h.p., so that a great Atlantic steamer,
which should indicate 11,000 or 12,000 h.p., and have a beam of about
65ft., would have her screws very well protected.
Mr. White said that as soon as it was found that with twin screws they lost
nothing in efficiency, ship owners generally were contemplating their
adoption, an admirable example of which had been set in the vessels of the
Hill line. In adopting twin screws, the question whether they should
overlap was one that deserved very serious consideration, and it was
interesting to know, from experience gained by the vessels of the Hill
line, that there was no difficulty in the way of the projection of the
screws. With a moderate power, and with vessels of considerable size, the
screws were well sheltered: but in the large ships which were contemplated,
where there must necessarily be larger screws, this might be different, and
become a difficulty.
Mr. Linnington, in reply, said there was no reason to think that the twin
screw at sea might not be as satisfactory, in comparison with the single
screw, as it appeared in smooth water. As a matter of fact, one of the
great advantages of twin screws was that at sea the condition of weather
which would bring the single screw out of the water, and make it extremely
inefficient, would have no appreciable effect on the twin screws. In
vessels of deep draught especially, they were well immersed, and they were
really more efficient at sea than in smooth water. In ships of full form,
the longitudinal position of the screws was of importance; but in the ships
referred to in this table the run was very fine, and the screws were well
covered by the hull. He did not think, in such a case, any small difference
in longitudinal position would affect the performance. If any alteration
were made, it would probably be better to put the screws farther off. When
the rudder was hard over, the blades of the screw should be about a foot
clear of the rudder.--_Industries_.
* * * * *