I agree with previous post. The default speed for Ahead Standard for XVIII is like 15 knts but should be more like 9-10 knots. That is why you are burning so much fuel.
The XVIII is a "Fast Attack" submarine but travel speed should be about the same as other boats, 9-10 knts for best fuel economy. You can adjust this setting in the
Data\Submarine\NNS_Uboat18\NSS_Uboat18.cfg file.
BTW, I just posted a TurmXVIII fix that makes the Schnorchel work.
SH4v15_Stock TurmXVIII_Fix_byTheBeast
Enjoy!