Per Nordenberg:

Note that the Wiki stub does not mention if one or both boats were on the surface. I suspect that the R class was submerged, but the U-boat was on the surface.

Many sub vs sub encounters happened in both wars, but in all but one case one or both of the subs was on the surface (usually a submerged sub attacking a surfaced one, just like it would any other surface ship). Even in the one submerged sub-on-sub case, a British sub using its persicope attacked a German U-boat on schnorkel....in many ways this still counts as a 'surface' attack as the attacking sub relied on tracking the other sub via its surface image.

There are no cases in recorded history of one sub hunting down and atacking another sub when both are fully submerged, with no connection to the surface at all.

I doubt that even if the R-class had been successful that it would have influenced much - sonar was not accurate enough until the end of WWII to give an accurate enough fix for a firing solution, and most WWII torpedoes could not even be set to fire at a deeply submerged object: they ran in a straight line at a pre-set depth just below the surface of the sea. In other words, there was no means to track or attack a submerged sub from a submerged sub till after WWII (or at least till the very end of WWII).

The cylindrical cross section of the R class is similar to the very earliest, pre-WWI US boats, such as the A class. These very early boats also had submerged speeds higher than their surface speeds. Early experience showed that their round, streamlined shape was so poor in seakeeping that the subs could not venture far enough from shore to be useful: subs evolved into the narrow shape we know as the classical WWI - WWII submraine so that they could operate in the open ocean. Only when nuclear power finally divorced the sub from the surface did the well known round shape re-appear - and modern boats are still lousy at seakeeping in the open ocean.

Regards,

Kurt