Here is my proposal.
It’s in fact my the first ship on the forum (I’ve designed many more but never dare to post them) .
In this AU, the Sak class are scratch built subs build by West Papuan people to strike the Indonesian commercial interest in their island.
They’re almost giant torpedo made of steel tank section.
For security purpose the torpedo tubes are located in the conning tower so the integrity of the hull is not affected. So they are unable to fire on surface and only two torpedoes before being forced to reload, which is a quite long operation.
They got a strange triple drive system. Like their model (torpedo) they got a pneumatic power engine who is filled by the two trucks diesel engines combined with a compressor when surfaced. As an addition they got a short range electric drive when they need to move discretely.
Those very simple ship became great threat for the Indonesian shipping as the Jakarta force always thought the ship were sank by sabotage and never suspected Papuan would be able to handle, even more built, submarines.