[Bilge_Rat]

TMO and RFB are both pretty much the best in terms of tweaking the AI in SH4. It is really a matter of taste.

The TMO AI is more challenging, but not unrealistically so. I use TMO 2.5 and have managed to get within 1500 yards of IJN DDs on the surface at night without being spotted when stopped, although I have been spotted by IJN warships at 8,000 yds at night when moving at flank speed.

Underwater, if the AI detects you or knows where you are, it can be deadly. I have been nailed by DCs at 400 feet as well, although that was only once. Usually, I manage to stay undetected and escape.

I just went through an attack by two IJN ships in july 43 around Truk. I was at 350 feet the whole time, silent running. They never detected me even though they went right over me a few times and dropped all their DCs far away.

The Merchant AI is a mixed bag. I use RSRDC so I don't encounter them that often. I have tracked them on the surface at night getting as close as 3,000 yds without being spotted. On my current patrol, I recently sank a 2,000 tonner during the day submerged. As I recall, the range was down to 1,500-2,000 yds, again without being spotted. However, I have been spotted submerged at night at around 2,000 yds.

One good tip is to limit periscope exposure to the strict minimum, just quick peeks to confirm your firing solution. I also dont use the "periscope depth" setting but choose a depth around 5 feet deeper so the scope is just breaking the surface. That is a trick I picked up from SH5 and SHO where you can choose how far to extend the scope and it impacts how far your scope can be spotted. I don't know if it is modeled in SH4, but it is safer to assume it is.