Torpedo Deflection is a term of my own creation to explicitly refer to the form of deflection discussed in contemporary British sources when discussing Torpedo Control. They speak only of "deflection" in these sources, but by this they seem to be alluding to a Dumaresq Deflection or "speed-across". However, the important distinction must be noted that this is the speed-across that would result if Own Ship were not moving. That is, if a Dumaresq were set to the speed and heading of both ships, one would obtain the "torpedo deflection" by moving the own speed slider to zero.
The reason for this distinction is that in fire control applications, own ship imparts its velocity to the shells nearly completely, but in torpedo control, a torpedo's movement along with the firing ship is quickly negated by the drag of the water and has a very negligible effect in establishing the proper aim.