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Originally Posted by johndallman
Presumably you're talking about the passive sonar? The hydrophones should work, but the signal processing will be optimised for mechanical, rather than biological, noises. That will reduce detection ranges, which will depend on how much noise the swimmers make. Submarines apparently routinely detect whales and the like, but the sonar team don't report them to the command unless there's some pressing reason to do so. Obviously, different signal-processing software could be written for the job, but I have no idea if the Brazilian Navy is set up to do that for themselves.
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Well, passive sonar is what would detect threats, but I also don't know how good the active sonar is at locking on man-sized swimmers at close range.
Pinging like mad might be fairly successful as an area denial weapon, but might not be feasible during diving operations. GRUMEC divers fall under the Submarine Command in the Brazilian Navy and apparently operate from submarines on occasion. In setting, it was during GRUMEC diving operations at the wreck of a civilian vessel that the Navy lost a submarine in 2009, it is believed to hostile action from the underwater entities.
I imagine that preventing such an event from repeating itself has driven quite a lot of quiet adjustment to Submarine Command vessels.
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Originally Posted by johndallman
They could eject grenades through the signal launcher, decoy launchers, and trash ejector.
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In ten years, I expect that they might have developed purpose-designed anti-swimmer charges and/or micro-torpedos designed to target swimmers.
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Originally Posted by johndallman
They could also simply go "all ahead flank". A Type 209 can make 21+ knots underwater, and since they don't seem to use pump-jets, catching living creatures in the propeller suction should result in them being chopped pretty well. Of course, there are easy countermeasures for that.
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While capable of full power maneuvers, the submarines are at much less danger from the mysterious swimmers. It's primarily while engaged in investigation, research or diving operations (generally aimed at gathering data on the Invisivle Residents) that the submarines need to defend the waters around them, instead of steaming away faster than swimmers can follow.
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Originally Posted by johndallman
While completely immersed in a conducting fluid? Seawater is a reasonable conductor, and the charge will simply leak away through the water . . . and probably not preferentially through creatures adapted to life underwater.
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Yes, that would probably be too much trouble for too little benefit.
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Originally Posted by johndallman
Given the lack of ability to aim, underwater claymores are more like it.
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Underwater claymores sound lime a good idea.
As for aiming direct fire weapons, however, there
are remotely controlled turrets for surface vehicles that seem like they could be adapted. They'd likely only be used during diving operations or similar situations where the submarine is acting as a mothership for smaller research craft underwater, so they could be extended from openings that were not needed during such operations.
Even with cavitating bullets, maximum range underwater is unlikely to reach even 100 meters, more probably half that, so there are probably sensors that could give enough resolution at those ranges for targeting purposes. Of course, aiming by composite sonar picture gathered by all arrays on board would probably be preferred, but a redundant secondary targeting sensor housed in the turret itself seems reasonable, given that there is a strong correlation between frequent equipment failures and hostile encounters.