speed of torpedo?

[Pawian]

Hi

I play silent hunter 3, patch 1.4.

I elaborated complicated system of torpedoing. Distance from u-boot to enemy ship, and angle between him and our uboot.

Then I use trigonometry to count distance, where enemy ship will be when our perisopce is 0.

Now i need tip how i can choose torpedo speed, so i can fire it at the appropriate time.

I dont ask about TDC and - slow medium or fast, but mannualy choosing torpedo speed for example 20 kilometers per hour or 30.

Help pls.