I'm having trouble grasping how this could have been the intended manner of using their explosive. 20' is clearly not enough distance away from such a powerful charge, and the people of that era were not fools. I've read an account of of a man on board the CSS Virginia in which he noted how people would be killed or injured from the rounds they fired into the turret of the Monitor, so they knew about shock transfer.
Having such a large explosive charge deliberately held at 20' away is a suicide mission. Is there no evidence of any sort of release method?
The sub was supposed to trail a long lanyard to trigger the device to be deployed after the torpedo was planed. That plan backfired, literally.