1: Yes, Japanese merchants in both tmo and stock have hydrophones. I'm not sure if it's something that starts at a certain year or from the beginning of the war. Personally, I just assume they're listening at all times.
2: No, the merchants have hydrophone sets of lesser quality than the destroyers. You will be able to hear the surface vessels from *much* further away than they can hear you when you're running quiet.
3: Based solely on my own experience using tmo (and this will vary from mod to mod), I would call *safe* distance at about 6 miles during daylight and 5,000 yards at night, assuming clear skies for both. You can get a lot closer than that though, it depends on aspect ratio crew rating of the enemy vessel, weather, your speed etc. If everything goes right, it's possible to get within at least 1,500 yards of a merchantman on the surface at night with clear skies. Just keep in mind that you are *much* smaller than they are. The next time you notice a sampan, notice how much closer you had to be to spot then than an 8,000 ton freighter. Same principle.
4: About 10 miles for the early war set, further for the late war set.
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