After being charged down by a BB in my smoke and with no idea if he would get to me before my torps reloaded, I decided to do some addin up.
Knots and Km in same game is first problem, but I sorted that. At 30knots a ship covers approx 1 km per minute.
Torps go twice the speed, which makes predictions easier, so 60kn = 108Kmh = 1.8km/min = 30m/s.
A tinfish should cover 10nautical miles in close to 5 mins. But it doesn't, takes 90seconds.
Another way to look at it, 10KM = 5.3 nautical miles. At 60 knots, torps take 5.3 mins to cover 10km.
Is my math wrong, or is the mathematical perspective of this game skewed?

Ok! Here are the value for a full speed (no flag, no boost) Tachibana in a straight line. The method worked according to the following layout:
I started full speed near a corner of the map squares and sailed parallel to one of the North-South lines. In this way it was ensured my ship only travelled the distance equal to the edge length of the squares. The time was taken for the start and the end when the tip of the ship arrow was on the lines (you can also see an example of the timestamps in the top right corner). I used 5 squares as a distance because: a) precision, b) referring to the screenshot in my earlier post, the distance between the first and fifth line is exactly 12 km.
The Tachibana needed 147.94 s for those 12 km (check the picture above, too) at her 30 kn (30.1 kts according to the game UI). This leads to a speed of 81.12 m/s or 292.02 km/h. If you recall earlier values for the torpedoes, ships seem to have slightly higher m/s per kn-relation than torpedoes! I absolutely understand if anybody is baffled over the time objects in this game need to travel distanced. These numbers are indeed fishy!
TL;DR:
torpedo speeds relate to 2.6 m/s per 1 kn in game (or 9.40 km/h per 1 kn in game)
ships speeds relate to 2.7 m/s per 1 kn in game (or 9.73 km/h per 1 kn in game)
Edit: Derp!