in
Data/submarine/NSS_Uboat18/
you will find
NSS_Uboat18.sim
open this up with S3D.
go to Unit_Submarine
and click on the little "i" icon under it on the left-hand side
You will see a drop-down menu on the main screen.
there you will see things like
Unit ship.
Front/rear diveplane
E_propulsion.
Hydrogenperoxid_Propulsion
STOP! RIGHT THERE!
open up the dropdown and increase your range and engine RPM.
hit enter after you do and save before exiting
now test it ingame.
and have fun. you have just modded sh4 (welcome to the bigleagues
)