So, the XVIII featured in the game was in fact not just never operational, but not even finished. But the XVII, equipped with a Walter turbine was finished and used by the KM for trials

, even got numerals as U-792 to U-795. Any modder would be interested in trying to make its 3D model for the game? We already have a working Walter turbine with corresponding interface, so it could be a nice semi-realistic addition
http://www.uboat.net/types/xviia.htm