I do not recall RFB off the top of my head, but when you crash dive, take her all the way to 160 - at least for TMO (RSRDC) and FotRSU, else your boat might be seen underwater from a plane passing overhead. If you are in a sparsely populated area as far as planes go, you can generally come back up after about 15 minutes - if they didn't detect your radar or see your boat ~IF~... RR has a video on plane avoidance in the
Sub Skipper's Bag of Tricks thread. Just remember that CV generate airplanes in the game, as well as the airfields.