Thats another good thing about SO. You can literally shoot with zero visibility, still hightening your chances of actually getting a kill. I got my fattest merchant on this patrol, by the help of periscope actually. The one modification that helped accuracy, was that I raced at flank underwater, and still the PK was off, so once I became close to about 3000 meters, I raised scope, corrected range and sealed the SO. It was by visibility this time though.. But certainly, when I get the electric eels, I'll even engage topside without DEs being able to find me. A radar also improves ranging, since the mast, gives a near center lock on the target, as its the hoist thru which radar emits...
to mid-end 1942...