Battleship Mutsu

Main gun barrel is being exhibited in the Kure maritime museum.


Propeller


(Steel plate of disassembled Mutsu)
Steel produced now contains the radioactive material(ingredient). This ingredient is mixed in a manufacture process. However, steel of Mutsu manufactured by before war days does not have mixing of a radioactive material in the process of manufacture. And, Mutsu was located to a long period seabed. For this reason, Mutsu does not have influence of the radioactivity by a nuclear testing. For this reason, steel of Mutsu is a precious material at the radiation measurement equipment with which mixing of a radioactive material is not desired.(Age determination by carbon 14). The age determination by carbon 14 needs the enclosure which intercepts radiation. For the reason for the above, steel of disassembled Mutsu was distributed to the research institute, that is, a university.
-Complete-

Battleship Mutsu

Breaking up of hull and quarter deck area

Battleship Mutsu

Stern section

Battleship Mutsu

Hull near the stern portion
Steering gear is visible on the 1st 2 photograph.

Battleship Mutsu

Main gun turret (with a gun barrel) is exhibited in old Naval Academy now.



Armoured steel added on the front face of a gun turret (No.4 Main gun).


‘Nagato’ of the same type as ‘Mutsu’（1937）

Battleship Mutsu

Salvage by (1,500 tons) Floating Crane.

Explosion of the mystery took place in the powder magazine under the No. 3 main gun turret. For this reason, the whole main gun turret of No. 3 blew away and had fallen to the seabed.

Battleship Mutsu

Salvaged section showing the vital division compartments for defence.
Also the stem sections and the hull of bow sections.

Battleship Mutsu

June, Showa 18(1943),
Battleship Mutsu sank by explosion of the mystery at the Hashirajima anchorage of the base of the combined fleet.　Battleship Mutsu was completed in October of 1921.

The posts that will follow this are photos and information on Mutsu's condition and subsequent salvage operations.

Man Day Control in Warship Construction

36 yea sold navy technical commander's Ryouji Nishijima took charge of construction of Yamato.
He is the person in charge of hull construction. In the Kure navy arsenal, construction of a battleship was since of Nagato.
There was no construction of a battleship for 20 years in the Kure navy arsenal for the disarmament treaty.

Yamato is a huge battleship which the navy has not experienced until now, and furthermore structure of Yamato is complicated.

For this reason, he needs to estimate the budget and the total man day which construction takes.
His duty is to advance smootgly a construction and to complete a battleship by the date of a schedule.
Then, he calculated the weight of Yamato from the drawing. As a result, the weight of Yamato was twice Nagato.

Nagato was started of the construction in August 1917, Man days was 1,960,000. (First ship)
The start of the construction of Mutsu in1918, Man days was 1,740,000. (Second ship)
From this record, he presumed that about 4 million man day was required for the total man day of construction of Yamato.
(The man day which hull construction takes estimated as 1,600,000 man days).

Generally, construction of a warship decides on a date for every main processes of construction.

For example:
Commencement of construction work,
Loading of Boiler,
Loading of the main machine,
Launching,
The main gun loading,
Rehearsal trial.
Official trial.
Completion.

If it judges that construction is behind, a person in charge increases a workman and will order all-night construction,and a completion date is held.
That is, a naval system gives top priority to a date. Unlike a private factory, the navy considers neither the efficiency of work,nor an economical thing.

In case of the construction of Yamato of the total man day 4 million(estimated) will be completed in four years, a man day becomes an average of 83,333 persons everymonth.
And a daily man day is 2,688 persons. But as for the peak period of construction, about 4,000 per day woekers are expected.

However, this is the number of only a shipbuilding part.
Furthermore, the worker of other sections joins.
(For example, a gun manufacturedepartment steel-manufacture part etc...)

If it does so, the construction persons concerned will become the number of many workers.
By the case, there is a possibility that it may become impossible to take supervisor control of construction.

All workers of a shipbuilding part of those days are 6,000 persons.
(In the whole Kure arsenal in wartime, they were 100,000 employees.)
But all of 6000 workers of a shipbuilding part cannot be engaged in construction of Yamato.

They must be engaged also in construction of other warships.
Therefore, shortage of a worker will become certain if construction of Yamato begins.

Then, Nishijima plans introduction of man day control.
The aim of man day control is to finish construction at a low cost moreover, for a short period of time.
(Man day control is called a 'efficiency curve' or 'Nishijima Curve'.)

This curve devised in Showa12(1937)
And, it is the prototype of the man day control in the shipbuilding community in present Japan.

Showa 40(1965) in postwar period,
The IHI Kure shipyard(the former navy arsenal shipbuilding part)planned introduction of PC system, in order to plan efficiency of management. And PC-ization of the Nishijima curve was tried.
The person in charge of a program was surprised, that, the Nishijima curve devised before as long as 40 years was smoothly inputted to PC without making fundamental correction.

Hull construction takes a pattern of steel materials by loft, and according to this pattern, steel materials cuts. .
And a hull is assembled one by one..

Nishijima planned taking record of the weight of these steel, and simultaneously, also records a man day.
This record carried out for each work section, for each work kind, and it graph-izes of this data.

The weight of steel material fill out in the horizontal axis of graph.
(that is, the weight of steel materials expresses a work load).
And the man day which hull construction totaled fill out in the vertical axis of graph..

And plots on graoh the man day and weight of steer for every seven days oreveryten days.
If does so, the graph of transition of a work load and a man day will be obtained. (this is 'Nishijima curve')
Recording of each work section and work kind is important for statistics of this graph.

In the navy, the sum of a man day was always recorded from the former.
However, this is the total of the last date. Nishijima judged, that such a total was valueless.
This Nishijima's thinking is epoch-making as an opinion of those days.

The idea of Nishijima, the estimated total man day of construction fill out in the vertical axis of graph.(In the case of Yamato, it is the man day of 4 million)
Next, the gross weight of steel materials fill out in the horizontal axis of graph..
And, connects from the zero point of graph in a straight line to the last process.
(the last process is intersection of the total man day and steel-materials gross weight)
This straight line graph makes for each work section, work kind.

This curve shows a straight line upward slanting to the right. Namely, the graph of this straight line expresses the process of ideal efficiency construction..

In each section and each work kind, N-curve records every moment.
And puts this N-curve on straight line graph..
If the line overlaps with two graph , work will advance ideally with the fixed base .
But, the inclination of graph is unstable, or when separated, a certain problem will have occurred on the spot. That is, it means that the efficiency of work is falling.

In this case, the measure of solution can be taken, and the cause of an efficiency fall can be solved immediately.
As a result, the influence to the following process can be controled to the minimum.

The method of the navy before construction of Yamato,
Balancing finally was important for the budge,. that is, the last result was important.
But, in this case, efficiency was bad in which work kind ? or, was inefficient in which section of woek ?
Moreover, did which section have good efficiency ?
It cannot grasp exactly a cause..

Under construction, it always records the efficiency curve. And this data accumulates for every construction of each ship.
If it does so, it will become the useful reference data when constructing a ship newly.
Furthermore, it can use also for the exact estimate of proper distribution of a worker and a budget.

In this time, the shipyard in the world is inexperienced in block construction ,
But Nisijima determines this block construction and adoption of a precedence fitting construction in order to promote the efficiency of construction of Yamato..

<4>
*Originally the word of 'block construction' was Japanese.　It is the same as that of English of a 'typhoon'.

Generally, a warship builds the same type ship(2 to 4 ships)by the necessity on tactics.
The first ship of a warship was custom build.in the Kure arsenal.
Construction ofthe first ship various problems occur.So that, construction is delayed by failure or redo of construction, etc.
For this reason, a man day and construction cost will increase.
But, about the second ship, third ship, has full knowledge of a procedure, so, failure is not repeated like the first ship.
hence, a man day decreases as a result. This is an obvious fact.
However, the construction which showed the reverse result existed in the past.
The battle cruisers Nachi and Atago of the first ship, they were built in the Kurenavy arsenal.
And Myoko and Takao of the second ship were built in the Yokosuka navy arsenal.
These costs of construction were reverse. Namely, the second ship had more man days than the first ship.
Then, The battle cruiser Mogami was built succeedingly in the Kure arsenal.
It is a Mogami type first ship, and Suzuya of the second ship was builtin the Yokosuka arsenal.
However, as for the result, the second ship had many man days thanthe first ship.

Nishijima investigated about this strange phenomenon these past data,
and he obtained the following results of an investigation.
That, in Yokosuka which takes charge of the second ship, the construction man day until launching has very few with these ships. However, the fitting construction after launching had increased extremely.
Then, Nishijima obtained the following conclusion from this result.
That is, the hull construction before launching is comparatively efficient. therefore, a man day is also efficient.
However, since fitting construction is complicated and various, a man day increases. Efficiency of man day is bad

Therefore, Carries out fitting construction as much as ppossible in the period to launching.
Namely, he decided to use the following method abundantly for construction of Yamato.

1:Block construction, precedence fitting,

1932(Syowa,7),

The Submarine mother ship Taigei is a warship of block construction. However, various problems occurred.
For this reason, block construction is not carried out until construction of Yamato.

2:Adoption of a mock-up,

In really,
Construction of Yamato made the model of the actual size of a bridge and the armoured vital part in the arsenal, and researche of various kinds was made. Usual, it is thought the increase in a man day to make a mock-up.
However, in the warship of complicated structure, It is very effective to confirm structure in advance by a mock-up.
Especially, efficiency promotes the hull of rivet structure. The armor of the steel of the hull of Yamato has the thickness of 410mm.
Furthermore, armor takes charge of a part of hull intensity.For this reason, the mock-up needed to determine the position correctly of the rivet hole of the jointing portion of armor.

(The armor of a foreign warship is the structure where it does not take charge of hull intensity)

3:Electric welding,

To a man day reduction sake,
The component which is not related to the intensity of a hull uses welding.

The estimates of the man day of the beginning of hull construction were1,600,000.(All man days are 4 million).
However, a man day is falling gradually as construction progresses.

And finally the man day of hull construction showed 990,000.After all,
Construction of Yamato was less than the all total man day 2 million for adoption of the man days control, and the improvement of the construction method.
It was the brilliant result.

As for Musashi of the second ship, The man day is the almost double man day of Yamato by the difference of the construction method.

In addition,
National power of Japan of those days, GNP is 1/13 of the U.S. and the quantity of production of steel was 1/18 of the U.S.

Development of the Imperial Japanese Navy Battle Cruiser (Part 10)

The vessel with insufficient stability has improved by withdrawal of some arms, or adoption of fixed ballast. 'Mogami' is - which equipped with the bulge and increased stable ability.

However,at the end of the same year, the fleet of the navy under training encountered the typhoon.
But, the fleet plunged into the typhoon without taking refuge.
And the accident occurred which cuts the bow of a destroyer.
Moreover, in other destroyers and battle cruisers - which wrinkles generated to the deck.

The battle cruiser 'Nachi' was observing the weather at this time.
That, the wind velocity of a typhoon is 35m per second.
Wavelength is 150m.
Wave height was 10m to 12m.

The ratio of (wavelength / wave height)is 10.
This is a numerical value which is not believed.

The common sense of the shipbuilding community
in the world of those days was setting the limit value of(wavelength/wave height)to 20. Hull is designed ( for this as a ultimate value).

However, the adjacent seas in Japan were the severe conditions exceeding ultimate value. The navy kept to secret this accident .

And the improvement of the thorough hull intensity of a battle cruiser and a destroyer was given.

* Weather conditions which are not believed generate the adjacent seas in Japan of winter.

The Showa50s(the1975s),
Three Japanese Panamax type ore ships(Dwt:70,000t to50,000 tons)caused the cutting accident of a bow in the sea of stormy weather.
(As for the crew member, all were rescued.)

Although the hull had floated for several days, and sank.

Moreover, 2 vessels of the foreign ship cut the bow and also sank in an instant. All crew members of 2 ships are missing.)
In any case, they were the usual weather conditions of winter.*

The warship with insufficient hull intensity,
1: The hull of welding structure with a bow and a stern, dissolve the hull above the water line and reinforces.
2: The ship which used welding for the hull central part gives reinforcement for stopping the stress of sagging of a-hull-to20%.

The shipbuilding community in the world did not get interested in OG of a hull until now.. However, after this, it came to get interested in the numerical value of OG of a hull.

These improvement reconstruction was secretly carried out. so that it might not be known by the foreign navy.
For this reconstructions, construction of new style battle cruiser'Tone' was long overdue.

Completion became the anticipation in Showa12. A disarmament treaty is a year end in Showa11.

Then, the main guns of 'Tone' were changed from15.5cm to20cm.

Construction of 'Tone' class remained in 2 ships.
And the battle cruiser of the last of the imperial navy,they are 2 ships of 'Suzuya' and 'Ibuki'..

'Ibuki' is converted into an aircraft carrier by construction halfway.

(End)

Development of the Imperial Japanese Navy Battle Cruiser (Part 9)

Showa9,
The torpedo ship 'Tomoduru' was capsized by the sea of stormy weather during training.

'Tomoduru' is the ship which the Fujimoto shipbuilding official designed.

'Loading weight on a deck can be enlarged, so that the numerical value of the width/draft of a hull is large.'

Fujimoto had established this theory, when young.

This was new discovery on the theory of shipbuilding.

If it says simply, If it designs according to his theory, many arms can be carried by the hull of the same size.
This is 'Flat bottom form' theory.
The imperial navy appointed this theory to be a military secret till end of war.

The early destroyer of high free board built by this theory, and was maintaining stability.
As a result, the upper division of a navy has demanded reinforcement of arms.

And since this request is met, a design side does its best in extension of arms.
And, as a result, draft became deep and the ship of low free board has been done.

The ship of the flat bottom had the large inclination as compared with the conventional hull.
However, it surely restored and was not capsized.

But, there was a blind spot that nobody notices in this theory.
That, when a hull inclines, a wave overcomes a deck. In this case, the stability of a hull is lost rapidly.
A ship will be capsized in an instant.

This is a strange blind spot. The capsize of torpedo ship 'Tomoduru' was this case.
The navy starts check of the stable ability of all naval fleets.

*
The torpedo ship of the imperial navy is the meaning of a small destroyer.
'Tomodoru' was 550t in displacement and there were 111 crew members.

Development of the Imperial Japanese Navy Battle Cruiser (Part 8)

Nachi class construction was started in Showa 6, and the battle cruiser of the imperial navy became 16 ships. There are Furutaka, Aoba, Nachi, Mogami class. (4 ships each).
However, the battle cruiser of 20 ships was required.

The battle cruiser of 20 ships was required for the strategy which intercepts the U.S. navy in the sea near Japan.

Then, Showa 9 (1934),
Starts a new style battle cruiser at construction of 2 ships. it is Tone class.
However, the main $B!! (Bguns for the Washington (London) treaty were restricted to 15.5cm.
(All hevy battle cruisers replaced the main guns for 20cm in after years.)

Mr.Ezaki of the subordinate of Mr.Fujimoto took charge of the design of this new style battle cruiser. The Tone class feature, in order to make small of the pattern of the main guns , 4 sets of all main gun turrets were arranged at the bow.
An intensive arrengement of this main guns obtained the good result.

1:An intensive defense of a powder magazine was attained.
2:Even if a powder magazine explodes, is only the portion of a bow.
3:The hull central part may be safe and may escape sinking.
4:The warship of the imperial navy does not have the hangar of a floatplane. floatplane puts on the deck.

For this reason, when it becomes a bombardment game, all floatplanes have to make start (Otherwise, airplane is destroyed with the blast of bombardment)

Tone class was able to make the hangar in the stern. 8 set loading was possible for the floatplane.

Equipment of the arms of Tone is the same as Mogami class.
However, the pattern of the main guns has obtained good results.
Record of the pattern of a battle cruiser : $B!J (BRecord of Syouwa15 (1940).)

Tone: In front and behind is 355m. Right and left are 73m.
Chikuma: In front and behind is 346m. Right and left are 75m.
Takao: In front and behind is 331m. Right and left are 69m.
Atago: In front and behind is 367m. Right and left are 51m.
The rate of a hit of Tone is recording 16%.
If ,in the case of ten shooting of the main guns was discharged with 80 shells,. And 13 hits the mark.
This figure is 3 times the rate of a hit of 'Nachi' at the time of new construction

Tone class is scheduled to complete 4 ships by the end of Showa 11.
And, the second fleet of the imperial navy completes by construction of this Tone class.

Showa 9,
The Japanese government gave notice of canceling a disarmament treaty in Showa 11 coming ends of the year to the world.

And, Showa 12 or later,
The navy in the world(Britain, U.S. Germany, Italy, Japan) rushed into competition of constructio of an unrestricted warship.

Development of the Imperial Japanese Navy Battle Cruiser (Part 7)

Showa 6 (1931),
The imperial navy started the design of the 'Mogami' class.
Mogami class is the warship called masterpiece of a battle cruiser.
It was the battle cruiser of the heavy arms which concentrated the technology of a design of the imperial navy.
As for Mogami class, a subordinate's Mr.Fukuda took charge of the design by instruction of Mr.Fujimoto.
(Mr.Fukuda is engaged in the design of Battleship Yamato in after years.)

Mogami carries the 15 guns with 15.5cm main guns. (3guns per Turret) (However, replaced to the 20cm gun with secretly in after years).

This 15.5cm gun was new-type, is the same range as 20cm gun. Moreover, air defense shooting was also possible.

The warship which carried five sets of the turret on the central line of a hull was only this Mogami class in the world. And, the torpedo equipped 12 sets.

Moreover, port side and stb.side of a chimney were equipped with four guns of 12.7cm anti aircraft guns.

In the naval battle cruiser, it was the beginning. Eight 25mm machine guns were also equipped, furthermore, equips the front of a bridge with two 13mm machine gun.
Equipment of this machine gun was also the beginning in the battle cruiser.
As for the conventional battle cruiser, equipment of a torpedo was given top priority.

A displacement is 9,000t. As for the stern and the portion of a bow, electric welding was used abundantly in order to reduce weight.

The main machine was 152,000H.P. This output is larger than the Battleship Yamato built by in after years.

The speed took out 37 kts.

Development of the Imperial Japanese Navy Battle Cruiser (Part 6)

Fujimoto of a shipbuilding official started investigation.
This result, 'Nachi' class is arranging the heavy gun turrets to the both ends of a long and slender hull.

For this reason, a twist arises in a hull. and a center line of hull shifts for diflection. This was a conclusion.

And the following improvement was added to the 'Nachi' class.
The distance of the gun turret between a bow and a stern was shortened 8.5m.
However, in a battle cruiser, in order to obtain high speed, a boiler division occupies the half of a hull.
For this reason, the structure of a bridge will be located on a boiler room.
The big box was placed on the boiler room like 'Furutaka'. and the bridge was built on this box.

Here is the reason with 'Nachi' class huge bridge.
A huge bridge is a model of a ship which is not looked at the battle cruiser of the navy of a foreign country.

The foreign navy named 'Pagoda of Castle' to the bridge of the warship of Japan.
In addition, the cruising range was reinforced to 8,500 sea miles by 14kts.

Development of the Imperial Japanese Navy Battle Cruiser (Part 5)

The 'Nachi' class defect was the accuracy of the main guns.
System of bombardment from 1910 to the end of World War II was probability shooting system .

All the main guns volley. and all shells fall on a sea surface and the range to which a shell falls on a sea surface is called 'pattern'.
(In the imperial navy, it was called the spraying range of the shell.)
Spraying range of the main guns of a battleship is 200 to 300m pattern.
If it catches and shoots at an enemy ship at the centre of this pattern, a hit will surely be obtained.

In 'Myokoh' class, this pattern was the range of 430m diameter.
An enemy's large vessel is about 200m hit will not be obtained if there is a pattern no less than 200m.
In the 'Aoba' class of loading of the same 20cm gun as 'Myokoh', a pattern is small..
Hence, the rate of a hit of 'Aoba' has obtained 6 to 16%.

'Nachi' class was not able to obtain 7% or more of rate of a hit.

When 'Aoba' class shoots 10 times, the number of shells is 60, and a hit shell will become 9.6 if the rate of a hit is 16%.

But in 'Nachi' class, when it shoots 10 times, the number of shells is 100.

(However, a hit shell is 7.

It became a big problem in the navy.

Development of the Imperial Japanese Navy Battle Cruiser (Part 4)

However -- Soon, Mr. Hiraga was opposed to the naval upper levels by the thing of the arms of a battle cruiser.

As a result, the post of the design section was resigned..
And Mr.Fujimoto of 37 years old inherited it's post.

Henceforth, as for 11 years, Mr.Fujimoto will take charge of the design of a warship.
The duty of the beginning of Mr.Fujimoto was reconstruction of the No. 3 ship and a No. 4 ship of the 'Furutaka' class. It was reinforcement of arms.
And the second duty was change of a design of the 'Myokoh' class.

Furthermore, he started the design of a new style destroyer.

The destroyer which he designed is high freeboard and ship's bottom is flatness..
Now, the type of a flat bottom is natural. However, it was epoch-making discovery those days.

The imperial navy was set as the military secret with this Fujimoto's theory.
Battle curiser'Mogami' class, Yamato class and the VLCC of present has inherited his theory.

Soon,
Official trial operation of 'Myokoh' of designed by Mr.Hiraga was carried out.
However, the serious defect for the main guns was discovered.

For this reason, change of a large-scale design is needed of hull and arms.
Mr.Fujimoto investigates the cause of a serious defect.

Development of the Imperial Japanese Navy Battle Cruiser (Part 3)

Furthermore, Taisho 12, (1923).

The imperial navy started the design of a 10,000t class of a battle cruiser based on the shipbuilding theory of Mr.Hiraga.
(A battle cruiser 10,000t or more cannot be built those days because of the Washington Convention)

And the No. 1 ship of a treaty type battle cruiser was named 'Nachi'.

The main guns are 20cm. (two guns in a gun turret)
They are three set turrets to a bow and two set turrets to a stern.have been arranged. Main guns of a total of 10 .
This full equipment captured the spotlight of the navy of other country.

In the case of 'Nachi' ,
Since three turrets of the main guns had been arranged to a bow, hence, the position of the bridge came on the boiler room.
Then, the big box was placed on the smoke pass, the support was stood on this, and the bridge has been arranged.
This is a model of a ship which is not looked by other navies. The speed of 'Nachi' was designed by 35.5 kts.
(quicker 1.0 kts than Furutaka)

However, a cruising range is 7,000 nautical miles (fuel is 2,500tons) in 14kt(s).
It was a battle cruiser with a short cruising range.

This was the fundamental plan of the imperial navy.

Development of the Imperial Japanese Navy Battle Cruiser (Part 2)

Mr.Hiraga added change to the form of the warship which was the conventional common sense. Namely, A forecastle of a bow was abolished. $B!J (Babolition of a Fcl. deck. And a flush deck type was adopted.

However, it differs from the conventional a flush deck type.
The conventional a flush deck was a linear deck. But, as Furutaka Class, A bow makes a sheer and makes freeboard high. and makes a stern low much more.
And connects the deck line from a bow to a stern by a loose curve.

Result, the high speed of 34.4kts was realized without reducing arms.

It became the model of the warship of the imperial navy after this.
(The warship of the navy of present Japan are also inherited.)

And Taisyo 11, (1922),
Construction of 'Furutak' and 'Kako' of a new design was started.
Completion is in March, Taisyo15.
The model with the deck of a loose curve, $B!! (Barrangement of the main guns and the inclined chimney was extolled from the navy country in the world.
(The British navy applies for visit of a Japanese battle cruiser in after years.)

Development of the Imperial Japanese Navy Battle Cruiser (Part 1)

Taisho 9, (1920)
The British navy promoted the construction plan of the battle cruiser which 19cm gun carries. However, the imperial navy(Japanese navy)had already ended the design of the battle cruiser of 20cm gun.

October of the same year, Mr.Hiraga of the design chief of navy shipbuilding-official, he discovered that a high-speed ship could be built by improving the llength/ breadth ratio of a hull. And started the 7,600t type design of 'Furutaka' 'Aoba' 'Kinugasa' of a new style battle cruiser.

The six main guns have been arranged on the central line of a hull.. This is the first model at the battle cruiser in the world. Two sets of guns have been saved compared with arrangement of the conventional main guns.

But, the power of bombardment was the same as that of 8 sets of the main guns.
And carries out various improvements and 'Furutaka' class enlarged the of the length/breadth ratio of a hull with 11.4.

The conventional battle cruiser was 8.7 to 10 of the length/breadth ratio.

'Kuma' is10.7.'
'Yubari' was the ratio of 11.
(This cruiser is the design of Mr.Hiraga and is the experiment warship of a high-speed warship.)

The 'Furutaka' class exceeds this, and it became the most smart hull in the battle cruiser in the world.
However, when a hull is made long and slender, much more longitudinal strength of a hull is needed.
For this reason, improvement for the high speed was not able to do the battle cruiser in the world

However, The imperial navy had already solved this problem.
As the method of reinforcing hull strength, it is the structure of sharing the strength of a hull with some defense armor.

This method is adopted by battle cruiser 'Yubari'.
(The conventional armor was attached in the outside of side shell. Therefore, armor was able to remov.

However, Mr.Hiraga, armor was made into the structure which constitutes a hull.
Therefore, removal of armor cannot do after construction.)

The length of 'Yubari' is 140m. 'Furutaka' has 180m length, this method is insufficient for securing strength of a hull.
But, Furthermore, 'Mr.Hiraga' adopts an epoch-making method.

A Historical Collection

This blog was created to document precious and rare information on the Imperial Japanese Navy provided by an old friend from Japan.