Yes that might be it, the 20 yards turn being the radius when a torpedo is moving at a low speed. I don't know though if the torpedo runs at a lower speed while turning and then accelerates, or if the engine runs with full speed immediately..?

I've just found a number of 100m radius stated for one of those zigzaging torpedoes while doing one of their turns, and they are of course running at a fairly high speed.

Leif...