no ship there

[Mittelwaechter]

Hi buddha96, welcome aboard.

In Stock SH3 there is a little tail attached to the contact squares popping up on your map.
This indicates the wake of the contact - so the contact moves into the opposite direction.

Slow means up to 7 knots (slower than 5 is very rare), medium 8 to 11 knots (often 8 and 9) and fast 12 and more knots (mostly troop transports).

Plot the contact course through the little tail and the square itself into the opposite direction. Assuming you are in range to intercept the contact - let's say it's slow - just double the speed (~ 13 knots) of your U-Boot and make your course double the length the contact will travel to your prognosed interception point. You can do it perfectly constructing the geometrics with your map tools - or simply estimate it roughly and dive at the interception point to listen for the contact.