The periscope had horizontal marks on it that indicated angles. The periscope observer noted how many marks, ie: what angle, the target waterline to mastehead top made in the scope. Then, he would either guess the size and height of the target, or, if fortunate, identify the target and look up the masthead height in a recognition reference of ships. The masthead height (guessed) and the angle it displayed in the persicope (measured) gave the distance to target. He could also make several obervations to help note changing distance, and make a judgement (guess) on the angle on the bow the ship made, giving the relative track of the target.

Combine that with the bearing to target and his own heading and speed, known torpedo characteristics, and a little luck, and you could figure out a firing solution.

Obviously there was a lot of guess work, especially in the masthead height and angle on the bow guesses. Nightime measurements were very hard to do well. Since sub commanders usually overestimated the tonnage of their victims, they probably also overestimated the masthead height a lot too.

Towards the end of the war the US deployed the \'ST\' radar, a simple ranging device fitted to the periscope head that gave exact range in all conditions, but it arrived so late the Japanese merchant fleet was already in tatters and the ST radar did not have much strategic impact.