In the early 1970s there was a push from several nations around the Baltic Sea to develop larger corvettes as flotilla leaders for their fleets of motor torpedo boats (which would soon also include fast attack craft armed with anti-ship missiles such as Exocet or Harpoon).
Following the cancellation of the Fregatte 70 programme in 1971, the Federal Republic of Germany's Bundesmarine wanted a 1,000 ton corvette armed with Exocets as a command ship for combined FAC/helicopter forces. This operating concept was later abandoned before it could be implemented. The drawing can be found here: viewtopic.php?f=13&t=10710
In December 1972 the Danish Navy began the Korvet-72 (aka KV-72) project, a small corvette of 1,300 tons to replace the four Triton
-class corvettes. In January 1973 Orlogsværftet completed the first sketch design, B.3. This had a standard displacement of 1,050 tons, full load displacement of 1,200 tons and overall length was 82m. Max speed was 20kt on diesels and 30kt on gas turbines with the possibility of achieving 32kt.
Design F.3 had a standard displacement 1,220 tons, full load displacement1,375 tons. The length was 90m, crew of 85. The further refined F.4 was completed in March, standard displacement was 1,175 tonnes, full load 1,325 tonnes. Overall length reduced to 85 m, crew increased to 90. F.5 was 5m longer, standard displacement 1,250 tons and full load 1,400 tons. Crew of 95.
During the summer 1973 the specifications were firmed up to 1,300 tons, 85m, 90 crew. Yarrow (Y-ARD) were then brought into the programme to refine the design which became the Niels Juel
Drawing Note: the sketch design used for this drawing closely corresponds with the initial B.3 sketch design. It featured very generic launchers for the Harpoon (which in 1973 had not entered service) which looked a lot like those used by the Standard SSM variant. B.3 had a "75mm automatic gun" but its likely the 76mm OTO was intended from the start. The Sea Sparrow and AWS-5 radar are evident on the sketch design.
By 1971 Sweden was also looking for a flotilla leader corvette of around 700 tons displacement which would also be capable of relieving the ageing destroyer fleet from escort duties. The design was around 75m long with two 57mm Bofors with a single-barrel Bofors 120mm A/S rocket mortar. Diesels and gas turbines would be fitted. Dubbed K1 and K2 the two vessels were not built at that time.
Work continued into the later 1970s, under the Ro 1 designation. The design evolved with newer model gun mounts, still including the single ASW mortar. An addition was two anti-ship missiles, at this time most probably the Rb11 SKA which was being developed for coastal defence use, a weapon in the Harpoon class. Work on this weapon was later abandoned and replaced by the RBS-15 which entered service in the early 1980s.
Drawing Note: this drawing is based on a plan of the Ro1 which featured a SAM armament replacing the aft 57mm gun, the mount matching Sea Cat which was about the only SAM then available for corvettes (although around 1977 Karlskrona were the first to offer a SAM-armed FAC when they proposed fitting Blowpipe to Spica-M for export customers). The drawing also seems to have four SSM cannisters, with two right aft replacing the depth-charge racks so I have done so here, quite likely it was a growth option. Note what seems to be a VDS mounting aft.