How plausible is a Penus-class semi-submersible torpedo cruiser?
Short answerIt is a deeply unattractive solution: by the time it looks sensible, the wider naval situation has probably deteriorated beyond recovery.
NotesThe objection is strategic rather than merely technical. The concept is treated as a desperate hybrid, analogous to a battlecarrier, whose appeal signals that the navy considering it has already exhausted more sensible options. Its value therefore depends on the circumstances that would make a navy adopt it, without an unsupported calculation of machinery or weapons. Exotic hybrid concepts can signal institutional or strategic failure when their appeal depends on a navy having exhausted more coherent options.