The Sun.
London, England, 1865-09-25 00:00:00

Naval and Military Intelligence.

The recent trials of the iron-clad frigate Bellero-
phon, which have been briefly reported in our Naval
Intelligence from day to day, have resulted in one
of the most extraordinary phenomena ever developed
since the introduction of steam propulsion. For three

days in succession this ponderous ship has been
steaming about at the entrance to the Thames and
Medway under circumstances for which all the
science of the day vainly attempts to account, and
which baffles those who have designed, built, and
put engines into the ship, no less than the nau-
tical gentlemen who bad charge of her during her
trials. In technical language the phenomena in ques-
tion is denominated " negative slip," but in common
parlance it is spoken of as a case of the ship overrun-
ning the screw, which in this instance has occurred
to an altogether unexampled degree. We may state
the case in the simplest manner to the general reader
by saying shortly that, although throughout the
trials of this ship, while the screw propeller which
drives her has been itself advancing with a speed
barely, if at all, exceeding 1.21 knots per hour, the
ship herself has been speeding through the water at
a rate of 134 knots. If a phenomenon of this nature
had occurred with a light vessel constructed with
exclusive regard to fleetness it would obviously
have been a singular circumstance, but for it to
happen with an iron ship of war of the stoutest con-
struction, covered with the most ponderous armour
ever yet applied to a seagoing ship, and carrying a
greater weight on a hull only 300 ft. long than the
Warrior carries on a hull nearly 400 ft. long, is a most
unexpected and unaccountable circumstance. This
overrun of the screw by the ship has happened be-
fore in a less degree even in some of our iron clad
ships, but up to the present time people have stead-
fastly refused to believe in it, and have resorted to
many ingenious devices by way of accounting for
it. The first and most obvious explanation was to
attribute it to errors of calculation or measurement;
another was to attribute it to obscure tidal influ-
ences, and this was favoured by the circumstance
that this has been most frequently observed on the
measured mile at Stokes Bay, where two high and
two low waters are said to happen each tide, the one
resulting from that portion of the Atlantic tidal wave
which finds its way through the Needles Channel at
the west end of the Isle of Wight, and the other from
the portion which flows in round the eastern end of
the island. A further explanation was sought for in
the fact that the screw propeller of a ship works in
a wave of water which is following her, and which
thus adds to the real speed of the screw through the
water—an explanation which is untenable because it
would require that screw ships with full after bodies
would give the best results in steaming, whereas it is
well known that they invariably give the worst. But
perhaps the most popular explanation of all is that
of assuming that the blades of the screw propeller
bend under the resistance of the fluid, and thus be-
come for the time being different screws, having a
greater velocity in advance than when at rest. This
explanation appeared to receive an extraordinary
confirmation in the Achilles, the first iron-clad fri-
gate which was found to overrun the screw, for on
docking the ship after her first trials it was found
that the blades had actually bent forward and grazed
the sternpost of the ship. The explanation proved
insufficient, however, for on trying the ship again
with stronger blades, the bending of which was out
of the question, nearly the same overrunning of the
ship was found to happen. In the case of the Mino-
taur also, the trials of which at Stokes Bay were re-
cently reported in great detail in The San, the speed
of the ship somewhat exceeded that of the screw.
The Achilles and the Minotaur are, however, ex-
tremely long ships, with water-lines as sharp as those
of a river steamer, and the excess of the ship's speed
over that of the screw fell in every instance short of
a knot per hour. It is the occurrence of the same
phenomenon in a far greatEr degree in a short ship
like the Bellerophon, and its repetition day after
day under circumstances which set all the
popular explanations at naught, that give the recent
trials so much importance, and impose upon our
shipbuilders and engineers the duty of thoroughly
investigating and accounting for its existence. If,
iii addition to being a short ship the Bellerophon
had not possessed what in nautical parlance is
known as a very " clean run" aft, the old explana-
tion of the following wave might again have been
adduced in this instance ; but the form of the ship's
after body is as tapering as that of the Minotaur,
and is wholly incompatible with this mode of ac-
counting for what has happened. If this subject
possessed only
.an abstract scientific interest we
should not have taken this notice of it, but it exer-
cises so practical and immediate an influence upon
the speed of ships that it is:calculated to excite the
most general interest among engineers and ship-
builders, and throughout the navy ; for it is found
that with screw propellers that thus lag behind the
ships which they are driving a most injurious drag
is put upon the engines. The engines put into the
Bellerophon are designed expressly to revolve
rapidly, and the ship was taken down the river on
trial with the understanding that the screw would
have been turned round about 70 times per minute,
thus developing 6,000 indicated horse power.
Instead of this, to the astonishment and disappoint-
ment of everybody on board, and of no more than
the engine-makers themselves, who are men of the
greatesteminence, the drag of the four-bladed screw
was found to be so great that not even 60 revolu-
tions could be secured, even when all idea of using
the steam expansively was abandoned and it was al-

lowed to rush with full force into and through the
cylinders. A great waste of steam was thus of

course occasioned, and consequently scarcely 5,000
horse power, instead of 6,000 was developed.
The wonder is that under such circumstances
the high speed of 134 knots was attained, and the
fact that it was the best possible guarantee that
a speed of more than 14 knots will be secured in
this remarkable ship when the full power of her
engines has been developed with a different screw.
The peculiarity of the screw at present applied to
the ship is not limited to the number of its blades.
Each of these four blades is formed with two faces
standing at an angle of inclination to each other, in
order that each half of it may impart a different
velocity to the water, somewhat upon the principle
of the differential screw propeller invented many
years ago by Professor Bennett Woodcroft, of the
Great Seal Patent-office. The Bellerophon's screw
really has eight blades, in fact, arranged in four pairs,
and as the diameter of the whole is no less than
23ft. 6in., the drag which it puts upon the engines
must be truly enormous. So far as we can learn,
neither the designer of. the Bellerophon nor the
Admiralty approve such a propeller being applied
to the ship, and after the trials of last week it is hard
to believe that the engineers will care to insist upon
its use. It is understood, however, that one other
trial is to be made of it, with its blades set to a dif-
ferent pitch, as soon as possible, and if the result
then secured be not satisfactory, a two-bladed
Griffiths screw, like that of the Warrior, will be
fitted to the ship. It is understood that this is what
Mr. Reed would prefer, not only on account of the
steaming qualities of the ship, but because of the
serious obstruction to the speed of the ship under
canvas which the four-bladed screw must necessarily
present. The Bellerophon will be a full-rigged
sailing ship as well as a high-speed steamer ; but
it is not to be expected that any amount of sail-
power would suffice to propel her rapidly through
the water if subjected to the drag of a screw which
lags so much behind the ship, even when it is itself
the propelling agent. The extreme handiness of
the Bellerophon under the action of her novel form
o f rudder affords the strongest possible induce-
meet to disembarrass her as much as possible from
the hindrance of an improper form of screw. Her'
rudder is on what is known as the " balanced
principle," being pivoted near the middle instead of
its forward end, as usual. It is coming into use
rapidly in America, and was tried in the Great Bri-
tain steamship in this country many years ago.
For some cause or other, however, it was prema-
turely abandoned, like many other useful things.
With the exception of an experimental trial in a
small gunboat at Devonport, under the direction of
Capt. Astley Cooper Key, R.N., C. 8., it has never
before been applied to a British ship of war, and it
certainly was a very bold experiment to apply it at
once in so large and powerful a ship as the Beller°.