How to cure the 'torpedo using wrong speed' problem/bug

[Paul Riley]

I am sorry to ask this,as most of it was explained in my previous posts about problem torpedo settings,mainly with the ETorps.I was advised on this,and was told that sometimes mixed loadouts can cause problems with the torpedoes using the incorrect speed.What I would like now though,if you dont mind,is a final,step by step rundown on how to cure this problem,ensuring all torpedoes are correctly set for firing.
At the moment tube 1 is loaded with an ET,and 2,3,4 are loaded with standard STs.I can see in tube 1 that the torpedo is registered at 30kts,yet the red arrow inside the dial reads about 28kt (again explained before),and the other 3 tubes display their speeds correctly,ie,40kts for fast.What exactly causes this?

I also cant seem to change an ET for a ST mid patrol.Surely you should be able to do this.

I really dont want another repeat of what happened the other day,my miss at only 500m with an ET.

This would be a great help to me,and would be greatly appreciated