Comparison of three Sea Forts

A table summarizing the specifications of each of the three fortresses


No.1 Sea Fort

No.2 Sea Fort

No.3 Sea Fort

  第一海堡 第二海堡 第三海堡

 Artificial island

Start date
of construction

Meiji 14 (1881) Aug.

Meiji 22 ( 1889 ) July

Meiji 25

Completion date
of construction

Meiji 20 (1887) June

Meiji 32 (1899 ) June

Meiji 40 (1907) Oct.

Upper structure

Start date
of construction


Meiji 33 (1900) 16th March

Taisho 3 (1914) Sept. and after

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

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

378322 yen
( worth 1.75 billion yen
in today’s worth)

91,647 yen
( worth 3.67billion yen
in today’s worth )

( worth 11.57 billion yen
in today’s worth )

[ 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).


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.


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.