the correct german term would be:
"Äußerste Kraft"
complete list of german speed terms:
forward (from slowest to fastest; voraus = forward):
"Kleine Fahrt voraus"
"Langsame Fahrt voraus"
"Halbe Fahrt voraus"
"Große Fahrt voraus"
"Äußerste Kraft voraus"
backward (from slowest to fastest; zurück = backward):
"Kleine Fahrt zurück "
"Langsame Fahrt zurück"
"Halbe Fahrt zurück"
"Äußerste Kraft zurück"
see this image for reference, U995 in Laboe:
http://www.pbase.com/kevbelcher/image/74884359
[edit]
"Wahnsinnige Fahrt", as in Das Boot, definitely is no correct term, but Slang. As far as I remember, they say "Beide dreimal Wahnsinnige" and also, as addition (but here I am not sure if it is in the same scene in the movie) "Alles was drin ist".
A translation would be "both (engines) three times extreme/crazy/mad" and "as much as possible". So the Kaleun here tells the LI to make the boat run as fast as possible, but in no usual or correct nautical terms.
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