Our famous Archchancellor hasn´t already found the thread?
Err... emm.. I was busy.
Well, i´ll try to replace his wisdom as good as i can.
You did fine.
_Die_ _h_eilige Lanze.
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Why not use a technical term?
Remember, the terror-inducing weapons of not-quite-mass-destruction were called V-1 and V-2. (And the V was just standing for "Versuch" ("Experiment"). Even
Konrad Zuse called his first computing machines V1 to V4.
The urban myth goes that he saved his V4 machine thanks to its title - he said he had to move his V4 into a safe location. Soldiers assisted him (not knowing what he was really working on) believing they were securing a super-advanced prototype of the V missile series (and since the Germans only ever got the V2 to productive use, V4 sounded something insanely important and possibly crucial to win the war.)
Today, the computers are called (in his honor, I don't know if he himself renamed them or not) Z1-Z4.
So, I suppose you could also call the submarine V1, or VZ 1.
It is typical for German military (even today) to either use contractions and acronyms that can be spoken as a single word (like in the 3rd Reich "GröFaZ" => Größter Feldherr aller Zeiten => Greatest commander of all time) or to describe them with Noun and adjectives separated by commas.
Panzerfahrzeug, schwer (Tank, heavy) or even
Fahrzeug, gepanzert, schwer (Vehicle, armored, heavy)
or jokingly: Falle, Nagetier, klein, grau (Trap, rodent, small, gray = mousetrap)
Unterseefahrzeug, experimentell, zeitreisend = (submarine, experimental, time-travelling)
Versuchsfahrzeug, unterwassertauglich, zeitreisend (experimental vehicle, under-water-proof/capable, time-traveling)
Unterwassertaugliches, zeitreisendes (Versuchs)fahrzeug => UnZeitVerZeug, UnZeitVerFahr, UnZeitFahrzeug, UnZeitFahr, UnZeitZeug. I like the last one best, but I am not sure it would be "realistic". UnZeitZeug reminds me a little of "Ungeheur" (Monster).
So, yeah, if you want menacing, UnZeitZeug sounds fine.
1: "Was ist das?"
2: "Das UnZeitZeug, die neueste Entwicklung der überlegenen arischen Wissenschaft!"
1: "Ich weiss nicht, was es bedeutet, aber es klingt unheimlich."
If I hear "UnZeitZeug" in the context of a submarine, I think when it breaks the water surface or travels time, it must make a horribly, nearly screaming sound. It's probably mostly black, and you see a lot of strange (but vaguely menacing) installations mounted everywhere on the boat...
The time-travelling sound effect would be probably start with something similar to the Doctors TARDIS/phone box sound, but with a more... sinister touch. Maybe it should really sound like a screaming beast, mixed with something metallic screeching. Maybe a whale or elephant in pain (as depicted in movies) and metallic cratching...
And be sure to let someone say something like
Nazi Science sneers at causality!