Continuity mistake: When they are attempting to get "between the screws" of the tanker, the underwater footage shows twin screws, but the surface footage only shows wake from 1 single mid-ship propeller.

Down Periscope (1996)
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Directed by: David S. Ward
Starring: Bruce Dern, Kelsey Grammer, Rip Torn, Rob Schneider, Harry Dean Stanton, Lauren Holly
Genres: Comedy
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Pascal: Buckman! There was a fingernail in my food, you fat-ass moron! Yesterday, it was a band-aid!
Buckman: I'm sorry, sir. The band-aid was holding the fingernail on.
Trivia: Tom Arnold turned down the role of Lt. Com. Dodge to play the lead in "McHale's Navy."
Question: Would pouring whiskey into the tank have really thinned down the mix, giving the submarine more speed?
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Answer: Adding alcohol to gasoline can lower the fuel's viscosity, thinning it to flow more easily.
raywest ★
But the question was, would it have actually increased the sub's speed?
Thinning the fuel mix would not increase energy or performance, so it would not make the submarine faster.