Nice picture, someone's got a sore ass over that one I'm sure!
So with the pictures you've provided we just need someone to make the model. As for adding the Walther Turbines it's not to hard.
1. Open up the boats .sim file
2. Go down to the unit_submarine node.
3. Add new child chunk unit_submarine. This new chunk should have the info for the Walther Turbine included in it.
4. Alter speeds and shp as you want. (note: The top speed of the Walther is directly connected to the top speed of the electric motors. They dont have to be the same number, for example on 688i Walther is set at 35kts and electric at 7kts.)
5. Now go over to the German equipment.upc file and copy the Walther Turbine out of there and paste it in the American equipment file. Give it a new equipment number and you're set.
6. Now you can go into the boat.upc and add the new Turbine in whatever compartment you want.
Now a little more info on the Walther. I did a test to see if it could be damaged and destryoed. On 688i I received no damage what so ever. So I tried the Type18 under a stock game, I did receive damage and they where destroyed but I could still use them at full power. So with a stock game not really destroying them anyways I stopped my attempts at putting them in the damage model.
About high speed depth control, this could be another issue of operator experience. But who has the time under battle conditions for your planesmen to finally learn how to drive the boat? The best you can do in the mean time is order up Ahead Flank, get your speed up and either go to All Stop or Ahead 1/3. This way when your planesmen finally pull out of the dive or rise, speed bleeds off and you have a more stable platform.