| Notes:
1) The Model A mounting was a simple gunhouse
with a one-deck deep semi-stalk going to a handling room directly below
the mounting. Training was by a 14 hp electro-hydraulic motor while
elevation was via an 8 hp electric motor. As noted above, ammunition
supply was complex and required much manual labor for both shells and powder
bags. Magazines and shell rooms were at the same level below the
armored store deck. Shells and powder bags were moved by hand from
their storage locations to lower hoists which raised them to the handling
room at a rate of six complete rounds per minute. Shells and bags
were then pushed manually through an opening of the ring support tube (barbette)
where the shells were placed on an upper hoist that delivered them to the
gunhouse. Both upper and lower shell hoists lifted the shells horizontally.
Powder bags were passed by hand from the handling room to the gun house.
Loading and ramming of both projectiles and powder bags was by hand.
These single mountings were replaced by twin
mountings during the 1930s rebuilds. The Model A1
single mounts used on the carriers were essentially the same mounting without
the gunhouse.
2) The twin mountings varied in detail
from class to class and even within a class but many features were the
same. They were operated with very noisy electric motor driven oil
hydraulic pumps located in the revolving structure. Damage to rubber
insulated wiring by oil leaks was a common complaint. For all models,
run out was by compressed air, the breech blocks were manually operated
and the fuzes were set on the loading tray.
3) The Model C mounts for the Aoba class
were designed after the Model D mounts for the Myôkô class.
The design of the Model C was basically that of the Model D with the stalk
adapted to fit the magazine and shell room arrangements of the Aoba class,
which were the same as the Furutaka class with the shell and magazine spaces
on the same level. Initial firing trials on the Aoba class showed
vibration problems, primarily as these twin mountings were really too large
for the size of the hull. Accordingly, the hull and deck around No.
3 turret were strengthened. The Model D on the Myôkô
class differed in having the shell rooms located under the magazines and
the turret stalk was accordingly longer. The Aoba and Myôkô
supply arrangements had more mechanical assistance than those for the earlier
Furutaka class, giving them a higher rate of fire. Shells were transported
from storage via a tackle arrangement and then moved manually to handling
room trays located around the barbette which passed them into the barbette.
From the tray projectiles were loaded manually into the shell hoists via
a shell dolly which followed the hoists as they rotated with the turret.
For both Model C and Model D, each gun had its own shell and powder hoist
which could supply five complete rounds per minute. Shell and powder
bag hoists were of a "pusher" type and both projectiles and powder bags
were delivered vertically to the gunhouse. The powder bag pusher
hoists effectively created a continuous powder train between the gun breeches
and the magazines. This design flaw was not corrected until the ships
were rearmed during the 1930s rebuilds. Projectiles were loaded and
rammed hydraulically, while powder bags were loaded by hand into the breech
and then rammed hydraulically. Training for twin mounts was by a
50 hp electric motor which drove hydraulic gear. Elevation was by
rack and pinion. A 75 hp motor drove two hydraulic pumps which provided
power to a common ring. This ring supplied hydraulic actuators for
elevating the guns, ramming and the ammunition hoists. Model D had
236 inch (6 m) rangefinders. Model C and Model D mountings were extensively
modified during the 1930s rebuilds.
4) The Model B mountings used on the carriers
Akagi and Kaga were designed after the Model C and D mountings. The
design of the Model B was nearly the same as the Model E used on the Takao
class heavy cruisers, the major difference being that the Model B used
the 20 cm (7.9") Type 3 No. 1 guns while the Model E used the
20
cm (8") Type 3 No. 2 guns. Like the Model E mounts, Model B mounts
had two shell hoists per gun, while the Model C and D mounts had one shell
hoist per gun. The second hoist was intended to support higher angle
(anti-aircraft) firing by making it easier to change ammunition types,
but they resulted in a larger turret trunk, a larger lower chamber, increased
turret weight and added crewmen. As loading was performed at +5 degrees
and the training and elevation speeds were low, the reality was that these
mounts were of little use for anti-aircraft defense. The maximum
elevation of the Model B was +70 degrees, but the elevating and recoil
mechanisms proved to be fragile and it was found that +55 degrees was the
maximum practical elevation. These twin mountings were removed from
Akagi and Kaga during their reconstruction in the 1930s and after that
time they carried only single 20 cm (7.9") guns in casemate mountings.
5) Model C and Model D twin turrets had
a crew of 19 in the gunhouse, nine in the shell room and ten more in the
powder magazine. Model B had additional crewmen due to their additional
pair of shell hoists with a crew of 23 in the gunhouse and more in the
shell rooms.
6) The twin mount gun axes were 74.8 in
(190 cm) apart.
7) Neither the single Model A1
mountings nor the twin Model B mountings used on carriers could fire cross-deck.
8) All Models had only splinter protection,
25.4 mm (1") of NVNC steel plates on all sides except the bottom, which
was protected by 19 mm (0.75"). Models B, C and D had thin steel
sheeting along the top and sides to provide a 10 cm (3.9") air space around
the turrets. This provided shading for tropical service.
9) The Kosaku Ariga (Jarek) website has
apparently disappeared. At that website, there were many pictures
of a Japanese-built 20 cm (7.9") twin mounting used on the Thailand Coastal
Defense ship HTMS Dhonburi. Jarek sent these to me some years ago
for review and I have now added these pictures to the Additional
Photographs page. On his website, Jarek described this mounting
as being either a Model D mounting similar to those on the Myôkô
class or as a Model B mounting removed from the carrier Kaga. From
my examination of these photographs and others, I believe that the first
of these statements is correct. The turret used on Dhonburi shows
the same sloped-roof and super-imposed rangefinder typical of the earlier
twin mountings such as the Model D used on the Myôkô class.
The Model B used on Kaga (and the very similiar Model
E used on later cruisers) had flat roofs with the rangefinder partially
enclosed within the mounting. I believe that the guns in this Thailand
mounting are the 20 cm (7.9") Type 3 No. 1 guns, as the chase shows a tapering
after the "C" tube that does not appear in photographs of the 20 cm (8")
Type 3 No. 2 guns used on Japanese heavy cruisers during World War II.
As a guess, the guns used for Dhonburi could have been among those removed
from Japanese cruisers when they were upgraded to the larger 20 cm (8")
Type 3 No. 2 guns. |
