Shipbucket

yes,I mean Zwaardvis Class.

After all these,I think that Zwaardvis is a good choice.Timeline suits perfectly and RHN variant could be a cross between Zwaardvis & Hai Lung but without the towed sonar,28 torpedoes (later 22 torpedoes & 6 Adis II-U).Also they have a modified hull,capable of diving more than 300m and powered by triple TAG/MTU engines instead of Werkspoor.They are still in service,after a mid life refit.

The Tench-M (Missile).Is this modification plausible?

It doesn't look like it from SB scale.

But it might be IRL.

It is SB scale.Please find the original design and compare them.

Since real Hellenic Navy did had British submarines, the "V" Class, another choice could be the Oberon Class (which curiously no one suggested).

The scenario goes like this:

RHN (AU) received 6 Balao Class submarines from USA (Guppy I) in late 1950's to early 1960's. But a new-built submarine was needed. Among many designs looked in early 1960's, the British Oberon was chosen. Total 8 built, two first built in Britain and 6 in Hellenic Kingdom. These were not the first submarines built by license, a similar program was made in 1930's with French assistance.

Could the Oberon a good choice or not? With/or without missile conversion, names will be the same and last hulls decommissioned in 2004-2005.

The question is if an Oberon can attain 6 underwater-surface missiles in a lengthened hull, using compressed air system like that of Polaris ballistic missiles. The missile is the sub-Adis I. with about 900kg weight.

Next will follow French designs again, Agosta 70 and later Scorpene-H with AIP. And no AIP-Agosta.

Oberon's export success rather wasn't accidential, so I suppose it would be a good choice.

As for the SLBM-armed Oberon... I believe You mean something similar to Soviet R-11FM missile (submarine version of the early model of Scud tactical missile) on Pr.V611 (modified Pr.611 Zulu submarine) - only that R-11FM with fuel weighed nearly 6 tons (2 tons empty weight), not 900 kilograms. And had pathetically short range. (MGM-31 Pershing weighed almost 5 tons, UGM-27 Polaris 16 tons)
I guess it could work, but would be a complicated conversion, especially for country that actually isn't designing submarines on its own.

I think the only plausible refit of this sort would be an insertion of an additional compartment in the process of a whole-ship lengthening.

No, I did not mean ballistic missiles like Pershing or Polaris, these are not needed and very expensive. Look above in Tench to see what I mean, a vertical launched Adis I with pressurized air assistance for launching without surfaced like Polaris. Total weight of 6 Adis I is 4.5 tons, about the same with one Pershing. Do not forget that these missiles are 10m long, Oberon hull had 5.5m draught, Polaris or Pershing above hull extension would reach the height of the sail! Even in Soviet Delta Class the height of the missile compartment is less than half the sail. In my case, hull extension would be much less, my missile is only 0.9m longer.

The Tench-M above is a Guppy II lengthened to about III spec with the missile compartment.

If I needed an SLBM launched submarine in that era, I would discuss for a nuclear powered design.

Another option could be the SM-39, since HLK participated in Exocet program, perhaps SM-39 could entered service earlier and intergrated to these submarines during modernization program.

Pr.611 and other early Soviet SSB's had missile silos in the sail.
Anyway, a battery of VL launchers for SSM's would involve a plug in the hull anyway. Alternative would be a set of launchers in horizontal position, raised for launch (like Soviet SSG/SSGN's or US Regulus-armed subs).
Use of Exocets could make it much easier since they can be launched from torpedo tubes.