Turunmaa class Corvette (1967)

Finnish Fast Gunboats (Turunmaa-luokan tykkivene)

Finnish Day !
TURUNMAA, built in Wartsila and KARJALA at Wartsila were launched in 1967 but ordered in 1965 as ASW gunboats, in service by 1969 and among the most interesting and the largest designs of the cold war Finnish Navy. One quirky detail was their engine exhausts taken astern either side of the quarterdeck discharging as one 50ft astern and providing an IR decoy so that they could manage 12 kts without even using propellers (top speed 37 kts) and still could reach 17kts on diesels alone. They were refitted in 1984-86 at Wartsila in Turku, fitted with new electronics to stay relevant until their late decommission in 2002. Both were preserved at different locations.

Turunmaa in June 1983, before refit.

Development

The Finnish Navy in the cold war reflected the situation of force neutrality of Finland, in the shadow of USSR and terrible memories of WW2, both a subject of pride and weariness. The biggest factor was the limitation of the Finnish Navy through the Paris peace talks in 1947. It was indeed capped at 10,000 tons and 4,500 personnel. This lmeant coastal battleships like those of the interwar were out of the question, as well as submarines. Weaponry even excluded torpedoes, mines and later missiles. They were eased in the 1960s and missiles and mines were allowed again, and the torpedo restriction never applied to the Soviet-sold Riga-class frigates. Plus Finland’s parliament managed to build many “gunboats” that could be quickly converted as torpedo boats in case of war. Torpedoes at last became official again in… 2018. After the breakup of the Soviet Union, thiose limitations of course were all gone, with the exception of tonnage. The latter, despite integration into NATO since the 2020 Russian invasion of Ukraine, which was seen as a way to limit budget spendings, seems gone as well as Finland needs the right assets to do its part in the collective defence.

Profile extract (see below)

But back in the 1960s, with limitations on mines lifted and authorization of torpedoes, Finland, not a part of the Warsaw Pact, was free to either purchase some Soviet vessels, which it did in its 1950s history, was not looking after its own design, thanks to the experience gained for its major shipyards, and if needed, do electronics and machinery procurement (this non-lethal equments) to the West without provoking the wrath of Moskow. This opened a new range of possibilities. No ships better incarnated this than the 1960s Turunmaa class Gunboats. They were the largest military ship built in Finland so far, and a departure over traditional minelayers/minesweepers, sloops and motor torpedo boats, now banned. The concept reflected a moderately sized vessel capable of littoral combat in a maze of islands, with a lsimited draft, a versatile equipment ideal for peacetime, with both traditional dual purpose armament and ASW capabilities, but the performances of motor torpedo boats for a quick rearmament as torpedo boats in case of war.

Development started in 1963 with five hull designs and over thirty propulsion variants looked at along the process. Since Finland had acceess to western equipments, the choice was large. When the technical puzzle was assembled and agreed upon in 1965 they were ordered on 18 February that same year. At the time, their electronics and propulsion system were groundbreaking, state of the art, attracted international attention. Both were built by Wärtsilä at their Hietalahti shipyard in Helsinki, the largest and most experienced shipyard of Finland at the time. Construction time was relatively long for such an ambitious project that was about to “ate” a substantial proportion of authorized tonnage. Both were launched in 1967-68, and commissioned in 1969 with the lead vessel was named after the famous turuma or “archipelago frigates” designed by Federick af Chapman and winners of the Archipelago Fleet against Russian in the 18th and 19th centuries.

Design of the class

Reconstruction of the plans src

Hull and general design

The Turunmaa class Gunboats were designed at the core like giant motor torpedo boats (MBTs) wuth provision for torpedoes, albeit they were never mounted, or missiles, if the electronic suite was modified. The hull was low on water and shallow draft, with a base displacement, light, at 700 tons, rapidly rising to 800 tonens with 1980s modifications and jumping to 1,350 tons full load with all equipments on board at the end of their service in the 2000s. Their hull reached its absolute structural limits by then. 1,350 tonnes is indeed destroyer territory, for an overall lenght of only 74 m (243 ft) and a beam of 7.8 m (26 ft) so about 1:9 ratio preserving speed while keeping stability by the very shape and full lines of the hull, and a draught at 3 m (10 ft) that still enabvled access to most points along the shorelines but hull shapes enabling off-shore operations even in rough Baltic weather.

About the general layout, it maximize what can be done of a limited hull. The mai gun was quite large compared to the hull size and low profile (it was flush deck) and a large, almost beam wide structure occupied half of the hull aft amidship, with another structure above for rew access to inferior level. There was a single low derrick mast supporting basic electronics (navigation and air warning) and the absence of enclosed bridge was a singularity. Like for motor torpedo boats, the bridge was indeed open, with a forward lip and roof line. The two Bofors were placed in superfiring positions making for the best possible fire arc and the 23 mm AA amidship on an elevated platform. The upper structure deck was devoid of any obstacle or system installed in oprder to late fit transverse torpedo tubes in wartime. There were for their crew of 70 two utility boats and a small crane after amidship on deck. The rear structure was mostly a low exhaust rack with ventes on both sides.

Powerplant

Performances counted on a clever mix of Diesels, four trusted German MTU models, and a gaz turbine from Britain. This was a 3-shaft CODOG arrangement, with CP propellers and a large pump jet aft to exhaust gas surbine power without using propellers at 17 knots. The Rolls-Royce Olympus TM1a gas turbine at the center shaft, with gears to be linked on all three shafts, was rated for a substantial 16,000 kW (21,000 hp) for this tonnage, making for a top speed of 37 knots. It was only used for speed runs, whereas the four MTU 2,200 kW (3,000 hp) diesels were the primary drive for the propeller shafts. The steam turbine could be disconnected from its pump jet and/or connected to the three shafts if needed, making for a combination of possibilities depending on the captain’s choices between noise and speed. Max range was calculated at 5,000 nautical miles at economic speed using only diesels, or 9,300 km, opening the entire Baltic Sea east to west up to Norway through the Skagerrak and back.

Armament

As gunboats, these ships were indeed armed, “in the default configuration” with a big cannon as it should forward, completed by a two-tier arrangement of AA guns. This was completd by an old school; still mildy effective ASW suite. These ships had been intended from the start to carry extra torpedo tubes but the detail is unknown. The crew had a surprising proportions of officers, 30, for 40 enlisted sailors, reflecting on its degree of technicity.

Bofors 120mm/L46 gun

Like TAK 120 of which only six were ever manufactured, aside the FAK 120 (fältautomatkanon 120) land gun. Mass is 28 t (62,000 lb) with a 5,520 mm (217 in) full barrel and breech, 1,950 mm (77 in) height. It fired a 120 × 615 mm R Bofors 33.7 kg (74 lb) shell or HE 21 kg (46 lb) with a 5.5 kg (12 lb) propellant. The barrel was liquid cooled, with a 47 cm (19 in) nominal recoil, -10°/+80° ay 32°/s, acc 40°/s squared elevation and 360° traverse at 40°/s, acc 50°/s squared. Its rate of fire is 75–80 rounds/min with a muzzle velocity of 810 m/s (2,700 ft/s) and effective firing range of 8 seconds to 5,000 m and 21.7 seconds to 10,000 m. The maximum firing range was 18,500 m at 45°. It had a feed system with 52 rounds. The Bofors 120 mm Automatic Gun L/46 naval gun) was design,ed in 1963-67 and installed on various ships, some still used by the Indonesian Navy. The prototype was designated 12 cm luftvärnsautomatkanon försöksmodell 1 (12 cm lvakan fm 1), still usable for AA fire. There were six 103mm RFL (flare launchers), three per side of the turret as well.

Bofors 40mm/L70 guns

A continuation of the legendary L60, WW2 ordance. Updated with a semi-turret SAK 40L/70-350 and modern radar-guided fire control system. Of course the new L70 developed from 1946 are replacement has a loger barrel, betetr range and muzzle velocity, a bit quicker rate of fire while keeping the essentials of the WW2 model. Extremely popular (5000+ made) and well exported and seeing plenty of action in the cold war. Many ships still have it.
Mass: 2,400 kg (5,300 lb) for the barrel, 4,800 kg (10,600 lb) with the field carriage and 4 m (13 ft 1 in) for most variants with a barrel 2,800 mm (9 ft 2 in) alone, 3,245 mm (10 ft 7.8 in) with breech & flash hider.
It fires a Cartridge 0.96 kg (2.1 lb) 40 × 365 mm R. Rate of fire is up to 330 rounds/min cyclic in later variants, for a muzzle velocity of c1,000 m/s (3,300 ft/s) and max range of 12,500 m (41,000 ft), much more than the original. The Feed system now uses 16–26 round hopper for longer burst and the loading could be automated.

Sako 23mm/L87

This twin AA gun is a Finnish licenced modified ZU-23-2. Present in two twin mounts on the aft amidship platform. To not mix with the other Sako M23 which is an assault rifle. Finland developed also the 23 ItK 95, upgrade of the original 23 ItK 61 designed by Instrumentointi Oy and Vammas Oy. It had full gyro-stabilization, APU and laser rangefinder. The SAKO 23 mm/87 was its navalized modernization.

RBU-1200

The ASW armament rests on two assets, the Soviet-origin RBU-1200, unchanged, located likely forward of the bow turre and behind the wave breaker. Reload was manual. Thes ewere the smallest in the family weighting 620 kg (empty) and capable of projecting their 73 kgs rockets (30 kgs payload) up to 1200 m if needed with a sink rate if 6.25 m/s down to 350 meters max. See Soviet references for more. The second asset were their pair of depth charge racks at the poop. What was sorely missing was an helicopter, besirdes acoustic torpedoes (still forbidden at the time).

Sensors

Navigation Radar

Unknown Type

Argo intercept EW (1986)

Electronic Warfare System, no more info (legacy cold war system)

WM22 missile control

Inside the radome close to the bridge. X band, with a peak power of 180 kW up to 1 Mw, and instrumented range of 32 NM (60 km) witn an accuracy of 0.8 mrad. More

Simrad hull sonar (1986)

Civilian grade sonar for sea bottom navigation and anti-collision. Today the company is known as Simrad Yachting, part of Navico (Navico Recreational Marine Division, Norway). This was not equal to USN or other navies’s tailored passive and active search sonar but given the noise of Soviet submarines in the 1960s it seems still judicious.

It seems in 1978 both received the 9LV200 Mk 2 radar. In 1986 they were also given a new sonar (old type unknown) and the Argo ECM suite. The radome WM22 fire control radar was there from the start.

What-if export proposal. src secret-project.uk

Conway’s rendition

⚙ specifications, Wärtsilä Hietalahti Shipyard 1969
Displacement	660t standard, 770t FL
Dimensions	74.1 x 7.8 x 2.8 m (243 x 26 x 10 ft)
Propulsion	3 shafts MTU diesels 3000 hp, RR Olympus TM1 gas turbine 21000 hp, waterjet
Speed	Max 37 knots (69 km/h) on gas turbine
Range	5000 NMI (9300 km)
Armament	Bofors 120mm/L46, 2x Bofors 40mm/L70, 2x Sako 23mm/L87, 2x RBU-1200, 2x DCR
Sensors	Simrad hull sonar
Crew	70 in normal conditions

Career of the Turunmaa Class

FNS Turunmaa

Turunmaa was ordered under the pennant 03 as work 381 at Wärtsilä, Helsinki, laid down on March 1967, launched on 11 July 1967 and completed on 29 August 1968. Her deployments logs are not known (so far).

In 1985-86 Turunmaa was refitted at the same yard she was built, with her entire fire-control and communications systems updated. In 1999 she was stricken. Turunmaa was stripped of armaments and served as a floating machine shop and training ship for Satakunta Polytechnics.

FNS Karjala

Karjala was ordered under the pennant 04, as work 382 at Wärtsilä, also laid down on March 1967. She was launched one month later than her sister on 16 August 1967 and completed also later on 21 October 1968. No logs of her deployments as far.In 1985-86 Karjala was refitted at the same yard she was built, with her entire fire-control and communications systems updated.
Karjala has been berthed since 2002 at the maritime museum Forum Marinum in Turku as a museum ship next to Suomen Joutsen.

Preserved Karjala