One key thing to remember is that all boats (except the S boats) are of the same type (long range ocean going attack boats), made for the same mission (scouting and screening force for the surface fleet). What they are, is design evolutions, with each class improving on the previous one. In SH4 that translates to:
Porpoise -> Salmon -> Sargo-> Tambor-> Gar-> Gato-> Balao -> Tench
The Salmon/Sargo's are nearly identical to each other. As are the Tambor/Gar's. So you could narrow this down further by saying:
Porpoise -> ( Salmon/Sargo ) -> (Tambor/Gar) -> Gato-> Balao -> Tench
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