Ahoi Ubootfahrer
i think i am going crazy. Not so long ago i spotted a small convoy of 3 ships. I was on same course and i got their speed. So i thought:"Hey would be good if i can pinpoint them, moving to an ideal position for waiting".
So i started drawing on paper and soon i realized that it is just too complicated for me

. Still i am not sure if this is right. Due to the fact that i am at work and i do not want to waste my precious time at home with doing paint and stuff, i drawed my thoughts here, hoping someone could help me. I guess its just a simple mathematical issue
So my question would be: how long i must advance or, how far i must go to reach the point X (or the length of a) so i can start going into position which would take 20nm (here sm, because in Germany we say Seemeilen).
Any ideas?