Well, off the top of my head...
Default spacing is usually 500m between ships, in each and every direction (i.e. 500m from the nearest ship in the front/back/side).
But it is highly recomended that you download an improved convoy mod, or RUb for that matter, because that would cause most or all of the spacing to be 900m... Which, if it is or isn't realistic, i don't know, but it does make it easier for the game A.I. to navigate their ships... You know, when you go around creating traffic jams with your torpedoes!
