What makes you think it will be difficult to detect on active sonar? It may be small for nukes but has a nice section of 7 meters in diameter nonetheless. There are much smaller diesel subs than that. I would immagine that the "little ones" produced in Europe in the 60s-70s (like the Type206 with 450 tonns or the italian Toti SSK with 580 tonns) were very difficult to detect on active sonar.
For example, from experience, three sikorsky sh3d could have a very hard time to get a fix on that little bastard Toti SSK down in the Med, if he had a layer. And this doesn't mean in littoral waters but in open sea. Many times in exercises with the 6th fleet those boats managed to sink the carrier, and quite a few times they did that without being counterdetected.
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