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Class overview
Name	Type F submarines
Builders	Kawaski, Kōbe, Japan
Operators	Imperial Japanese Navy
Preceded by	None
Succeeded by	Type L submarine
Subclasses	Type F1 (Ro-1-class); Type F2 (Ro-3-class);
Built	1917–1922
In commission	1920-1932
Completed	5
Retired	5

The Type F submarines (F型潜水艦, F-gata Sensuikan) were medium Imperial Japanese Navy submarines in commission during the 1920s. They were Japan's first true seagoing submarines and the earliest Japanese submarines classified as "second-class" or "medium" submarines.

Design and description

The Type F submarines were designed by the Italian firm Fiat-Laurenti and built under license by Kawasaki at Kobe, Japan.^[1] The Type F submarines were the Imperial Japanese Navy′s first true seagoing submarines,^[1] and when the Japanese adopted a three-tiered classification system of its submarines as first-class (I), second-class or medium (Ro), and third-class (Ha) on 1 November 1924,^[2] the Type F submarines were the earliest to receive the second-class classification,^[1] as reflected in their low numbers in the Ro series,^[1] and in fact they were the earliest Japanese submarine classified as anything higher than third-class.

As built, Type F submarines had no deck gun, but soon after they were completed each of them had a 76.2 mm (3 in)/40 gun installed on her deck.^[1] The Type F submarines had non-cylindrical hulls intended to provide extra internal space, but the Japanese considered the hulls weak despite the provision of additional scantlings during construction to reinforce them.^[1] Because of their disappointing performance, they did not serve as the basis for any later Japanese submarine classes.^[1]

Class variants

The Type F submarines were divided into two subclasses:

Type F1 (F1型（呂一型）, Ro-ichi-gata, Ro-1-class)
Type F2 (F2型（呂三型）, Ro-san-gata, Ro-3-class)

Type F1 (Ro-1-class)

The F1 subclass was ordered under the 1915–1916 naval program.^[1] Two were constructed between 1917 and 1920:^[1]^[3]^[4]

Name	Laid down	Launched	Completed	Notes, fate
Ro-1 ex-Submarine No. 18	5 January 1917	28 July 1919	31 March 1920	Renamed Ro-1 on 1 November 1924. Stricken 1 April 1932.
Ro-2 ex-Submarine No. 21	1 July 1918	22 November 1919	20 April 1920	Renamed Ro-2 on 1 November 1924. Stricken 1 April 1932.

Type F2 (Ro-3-class)

The F2 subclass (Ro-3-class) was ordered in 1918.^[1] It was an improved version of the F1 subclass with a modified bridge.^[1] The Fiat diesel engines were unreliable and the F2 subclass′s top surface speed of 14 knots (26 km/h; 16 mph) was well below the intended 17 knots (31 km/h; 20 mph).^[1] Additional F2 subclass units planned under the 1919 construction program were cancelled and replaced by the new Kaichū-type and Type L submarines.^[1]

Three submarines of the F2 subclass were constructed between 1919 and 1922:^[1]^[5]^[6]^[7]

Name	Laid down	Launched	Completed	Notes, fate
Ro-3 ex-Submarine No. 31	28 October 1919	10 March 1921	15 July 1922	Renamed Ro-3 1 November 1924. Stricken 1 April 1932.
Ro-4 ex-Submarine No. 32	22 December 1919	22 June 1921	5 May 1922	Renamed Ro-4 1 November 1924. Stricken and hulked 1 April 1932.
Ro-5 ex-Submarine No. 33	1 March 1920	17 September 1921	9 March 1922	Renamed Ro-5 1 November 1924. Stricken and hulked 1 April 1932.

Characteristics

Sources^[1]

Type		F1 (Ro-1)	F2 (Ro-3)
Displacement	Surfaced	689 long tons (700 t)	689 long tons (700 t)
Displacement	Submerged	1,047 long tons (1,064 t)	1,047 long tons (1,064 t)
Length (overall)		65.6 m (215 ft 3 in)	60.5 m (198 ft 6 in)
Beam		6.1 m (20 ft 0 in)	6.1 m (20 ft 0 in)
Draft		4.2 m (13 ft 9 in)	4.1 m (13 ft 5 in)
Power plant and shaft		2 × Fiat diesel engines 2 x Savigliano electric motors 2 shafts	2 × Fiat diesel engines 2 x Savigliano electric motors 2 shafts
Power	Surfaced	2,800 bhp (2,100 kW)	2,600 bhp (1,900 kW)
Power	Submerged	1,200 shp (890 kW)	1,200 shp (890 kW)
Speed	Surfaced	13 kn (24 km/h; 15 mph)	14 kn (26 km/h; 16 mph)
Speed	Submerged	8 kn (15 km/h; 9.2 mph)	8 kn (15 km/h; 9.2 mph)
Range	Surfaced	3,500 nmi (6,500 km) at 10 kn (19 km/h; 12 mph)	3,500 nmi (6,500 km) at 10 kn (19 km/h; 12 mph)
Range	Submerged	75 nmi (139 km) at 4 kn (7.4 km/h; 4.6 mph)	75 nmi (139 km) at 4 kn (7.4 km/h; 4.6 mph)
Test depth		40 m (130 ft)	40 m (130 ft)
Fuel		58.4 tons	58.4 tons
Complement		43	43
Armament		As built: • 5 × 450 mm (18 in) torpedo tubes (3 × bow, 2 × stern) • 8 × Type 44 torpedoes • 1 × 7.7 mm machine gun Added after completion: • 1 x 76.2 mm (3 in) deck gun	• 5 × 450 mm (18 in) torpedo tubes (3 × bow, 2 × stern) • 8 × Type 44 torpedoes • 1 × 7.7 mm machine gun Added after completion: • 1 x 76.2 mm (3 in) deck gun