I don't know why I hadn't asked this detail, because it does loom large at times, but what depth were you at for the approach, the attack then at PD, and what depth was your egress? Do you remember the range to your target when you shot, and the approximate angle?
I have done several tests on the CH-28 with only FotRSU on June 7, 1942 1300 hours. I have the CH-28 facing north, engines at zero, not anchored, and I approach at PD from behind, from in front, and from the two sides. The behind approach, as expected, I was all of the way alongside the ship, roughly 300 yards off his beam, and he detects me as soon as my motors are alongside roughly his bridge area. He takes off in a straight line initially, drops four charges from the racks, then shoots four from the Y-Guns, which is what he almost had me with
- I forgot about those things. Anyway, as soon as he started up, I went Ahead Flank and crash dive (from PD). As soon as I hear "Passing thermal layer" (184 foot), I go to Ahead One Third, hard right rudder, and call for 250 feet, going to Silent Running shortly thereafter, holding a course of roughly 30T then. The CH-28 circled several times, and dropped charges and shot with the Y-Gun just about the same place every time, but I of course, was well clear of that spot after his 2nd pass, and he didn't get close after.
Approaching from the front was not close to that. He kept chuffing his exhaust, and would move a few feet, then wait, like listening again. He heard me good enough apparently when I was still a bit out, and bee-lined me while I was roughly 2000 yards away. He basically followed me as I did a similar move as above, and did do quite a bit of damage on my boat. Instead of dropping four and shooting four, he would drop about 8 and shoot maybe a dozen, and seemed to vary the depth, making it difficult to decide what to do when he passed over each time. I did finally lose him, but only with the help of the external camera, and knowing which way he was turning each time, and I was finally able to put some distance on him by the time I exited. btw, I had gone below test depth with the Sargo after about his 3rd pass, and it still seemed like he was dropping them below me... We'll have to look at that for FotRSU. That should be later in the war.
The approaches from the side were similar to the front, only I did manage to get into about 1200 yards before he came after me. I had no problem with him prosecuting attacks. However, he was not "attached" to a group. I'll try some of those in a few days, but the ship itself seems fine. This is NOT an indictment of the other mods, just an initial observation. Testing with a group is needed. What I CAN tell is that the ship is practically deaf from behind, and from the sides up to about 1500 yards, during this time period. Just don't get in front of it, because it can lay down a LOT of depth charges, concentrated in a small area.
A few notes, but in 1942, the ship carries Type93-1P hydrophone ("P" for Passive), which are their "basic issue" device, and were not that good. The IJN didn't see the need for them. The sonar is Type93-1A, with the "A" meaning "Active", and is also not very good. I did not get pinged by the CH-28 but a couple of times, so I suspect they generally did not "know" where I was, since there was never a "visual" cue to go by to initiate targeting. Give them a periscope and / or a torpedo track, and that would probably change. No radar until 1944. More later.