Fortress in Tokyo Bay - NO.2 Sea fort - Comparison of three Sea Forts

A table summarizing the specifications of each of the three fortresses

[ note 1 ] Initial armament : Although armament was strongly influenced and changed by the background of military diplomacy and advance of weapons and changes of strategy, initial construction armaments of each sea fort were enumerated here.

[ note 2] Cost converted into today’s yen : Those 3 forts construction costs were estimated based on the cost of Meiji 14 (1881) as a basement of one and multiplied by the deflator of 4.638 of Heisei 30 (2020).

Start

In Meiji 14 (1881), the construction of the Futtsu Sea Fort (later named the No.1 Sea Fort) was started in the sea at the tip of Futtsu Cape. It took nine years to complete the construction in Meiji 23 (1890). Construction of the No.2 Sea Fort was started in Meiji 22 (1889). Then, in Meiji 25(1892), three years after the start of the construction of the No.2 Sea Fort, the construction of the No.3 Sea Fort started.

Area

Sizes of these 3 sea forts at high tide were 23.000m² of No.1 Sea Fort, 41000m² of No.2 Sea Fort (about 1.8times as wider as No.1) and 26.000m²of No.3 Sea Fort ( almost same as No.1)

Water depth

It is said that the most difficult factor of sea fort construction is water depth. Water depth around No.1 Sea Fort was 1.2m -4.6m and 8m-12m for No.2 and even 39m for No.3. The deeper the water depth is the more difficult its construction becomes. To make the matter worse the fast tide velocity there made their work more difficult.

article		No.1 Sea Fort	No.2 Sea Fort	No.3 Sea Fort

Artificial island	Start date of construction	Meiji 14 (1881) Aug.	Meiji 22 ( 1889 ) July	Meiji 25 (1892)Aug.
Artificial island	Completion date of construction	Meiji 20 (1887) June	Meiji 32 (1899 ) June	Meiji 40 (1907) Oct.
Upper structure	Start date of construction	Unknown	Meiji 33 (1900) 16^th March	Taisho 3 (1914) Sept. and after
Upper structure	Completion date of construction	Meiji 23 (1890) Dec.	Taisho 3 (1914) June	Taisho 10 (1921) March
Construction time		9 years	25 years	29 years
Construction site		in the sea at the tip of Futtsu Cape	2,577m west of No.1 Sea Fort	611m south of No.2 Sea Fort 2,589m north of Hashirimizu tei battery (low)
Armament[ note1]		two 12cm cannon (disappearing gun carriages) four 12cm cannon ( siege gun carriages ) one 19cm cannon fourteen 28cm howitzers	two 27cm cannon ( turrets ) four 27cm cannon ( disappearing gun carriages ) eight 15cm cannon ( turrets )	eight 10cm cannons four 15cm cannons
Water depth		Deepest depth 4m60cm Shallowest depth 1m20cm	Deepest depth 12m Shallowest depth 8m	Deepest depth 39m
Geological structure of the seabed		Sands mixed with crushed shells	Sands mixed with crushed shells	Sands mixed with gravels
Speed of tidal current ( per second )		1m	1.2m	1.5m
Difference between high and low tide		2m	2.2m	2.2m
Area of upper foundation		23,000 m²	41,000 m²	26,000 m²
Construction cost [note2]		378322 yen ( worth 1.75 billion yen in today’s worth)	91,647 yen ( worth 3.67billion yen in today’s worth )	24,93,697yen ( worth 11.57 billion yen in today’s worth )