Special interest, says _Engineering_, attaches to the trials of the
protected cruiser Blake, in view of the assertion frequently made by
Admiralty authorities, from the first lord downward, to the effect
that with her sister ship Blenheim she would surpass anything hitherto
attempted. The condition of steaming continuously for long periods and
over great distances at 20 knots per hour was made a ruling condition
in the design, and with forced draught she was to be able to attain 22
knots when occasion required. But all idea of getting these high
results has been abandoned. Our readers do not need to be reminded of
the frequent failure of boilers in the navy. Although in the newer
ships, profit has been gained by experience, larger boilers being
provided with separate combustion chambers for each furnace; the
Blake's boilers belong to the type of defective design, with the
result that, were they pressed under forced draught, the tubes would
leak. It was, therefore, decided some time ago to be content with
natural draught results, and on Wednesday, Nov. 18, the vessel was
taken out from Portsmouth, and ran for seven hours with satisfactory
results, considerably exceeding the contract power. But the speed was
but 19.12 knots, and 22 knots can never be attained, except, of
course, new boilers be provided, and when an expenditure of 5 or 6 per
cent. of the first cost of the vessel (433,755_l._) would give her new
boilers, it seems a pity to be content with the lesser speed, more
particularly as the vessel is well designed and the engines efficient.
[Illustration: THE NEW BRITISH CRUISER BLAKE.]
Before dealing with the engines and their trials, it may be stated
that the vessel is of 9000 tons displacement at 25 ft. 9 in. mean
draught. Her length is 375 ft. and her beam 65 ft. She was built at
Chatham, and the armament consists of two 92 in. 22-ton breech-loading
guns, ten 6-in. 5-ton guns and sixteen 3-pounder quick-firing, and
eight machine guns, with torpedo launching carriages and tubes. The
propelling engines were manufactured by Messrs. Maudslay Sons & Field,
Lambeth. They were designed to develop 13,000 horses with natural, and
20,000 with forced draught. They consist of four distinct sets of
triple expansion inverted cylinder engines, and occupy with boilers,
etc., nearly two-thirds of the length of the ship. They are placed in
four separate compartments, two sets being coupled together on the
starboard and port sides respectively for driving each screw. There
are four high pressure cylinders, 36 in. in diameter; four
intermediate cylinders, 52 in.; and four low pressure cylinders, 80
in.; with a stroke of 4 ft. Each set of engines has an air pump 33 in.
in diameter and 2 ft. stroke, and a surface condenser having 12,800
tubes and an aggregate surface of 2250 square feet, the length of the
tubes between the tube plates being 9 ft. There is also in each
compartment one centrifugal circulating pump driven by a small
independent engine, of the diameter of 3 ft. 9 in., and capable of
pumping from the bilge as well as the sea. The screw propellers are 18
ft. 3 in. in diameter with a mean pitch of 24 ft. 6 in.
Steam is furnished by six main double-ended boilers, having four
furnaces at each end, and one auxiliary boiler, with a heating surface
of 900 sq. ft., the dimensions of the former being 15 ft. 2 in. by 18
ft., and of the latter 10 ft. by 9 ft. long. The total area of
firegrate surface is 863 sq. ft, and of heating surface 26.936 sq. ft.
Each engine room is kept cool by four 4 ft. 6 in. fans. Forced draught
is produced by twelve 5 ft. 6 in. fans, three being stationed in each
stokehold. The electric lighting machinery consists of three dynamos
of Siemens manufacture driven by a Willans engine, each of which is
capable of producing a current of 400 amperes. The after main engines
can be easily disconnected and worked separately for slow speeds.
The Blake had her steering gear tested on Tuesday, Nov. 17. With both
engines going full power ahead and turning to starboard, with her helm
hard over 35 deg., she completed the circle in 4 min. 40 sec., the
port circle being completed in 5 min. 5 sec. The diameter was
estimated approximately to be about 575 yards. Forty-five seconds were
required to change from engine steering to steering by hand. By manual
gear the helm was moved from midships to hard a-starboard in 40 sec.,
from starboard to hard a-port in 2 min. 10 sec., and from hard a-port
to midships in 2 min. 20 sec. The heavy balanced rudder and the speed
of the ship throwing great labor upon the crew manning the wheels, the
hand gear was afterward disconnected and the connection with the
steering engine completed in 40 sec.
[Illustration: THE NEW BRITISH CRUISER BLAKE]
On Nov. 18, when the vessel went on speed trials, the draught of the
vessel was 24 ft. 8 in. forward and 26 ft. 8 in. aft, which gave her
the mean load immersion provided for in her design. There was a
singular absence of vibration, said to be due to the space over which
the machinery is spread, but perhaps also due, in part at least, to
the number of cranks, as the cylinders deliver six throws throughout
the circle of revolution. The results of each hour's steaming are as
under:
Hours. Revolutions. Steam. Power.
1st hour 86.86 120.6 13,568
2d " 89.26 128.0 15,298
3d " 88.55 125.0 14,251
4th " 89.58 127.6 14,759
5th " 89.40 125.0 14,394
6th " 89.55 125.0 14,512
7th " 89.15 126.0 14,893
The trial was originally intended to continue for eight hours, but at
the end of the seventh, as the light began to fade, and as, moreover,
the engines were working with a smoothness and efficiency that showed
no signs of flagging, it was considered expedient to terminate the
run.
Steam pressure in boilers 125.5 lb.
Air pressure in stoke holds 0.42 in.
Revolutions per minute, starboard 88.41
Revolutions per minute, port 89.39
| Starboard. | Port. |
+---------+--------+--------+--------|
| Forward| Aft | Forward| Aft |
Vacuum in condensers. | 27.85| 27.85| 28.1 | 29.1 |
Mean pressure in cylinders, high | 43.04| 38.95| 42.36| 42.45|
Mean pressure in cylinders, inter.| 31.49| 30.82| 30.17| 28.38|
Mean pressure in cylinders, low | 11.68| 12.4 | 12.85| 12.32|
Indicated horse power each engine | 3631.42| 3589.07| 3721.37| 3583.50|
Total | 7220.39 | 7304.88 |
Collectively | 14525.37 |
As will be seen, the collective power exceeds the contract power under
natural draught by 1,525.37 horses, and was obtained with less than
the Admiralty limit of air pressure. The coal used on the occasion was
Harris' deep navigation, but no account was taken of the amount
consumed. Four runs were made on the measured mile with and against
the tide, the mean of means disclosing a speed of 19.12 knots. The
average speed of the seven hours' steaming, as measured by patent log,
was 19.28 knots. This fell short by over three-quarters of a knot of
what was anticipated in proportion to the power indicated by the
engines. Up to the limit of air pressure used the boilers answered
admirably.
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