U-84 question

You usin' the Mirage PE set, one which is dedicated to the sub?   If not - you really should look into them.    IIRC they have the insulator yoke off the front of the conning tower.  They also have a PE net  cutter which is MUCH better looking than the plastic one in the kit.  The plastic is at the limits of injection technology.  The PE is finer & crisper and looks like it could actually cut a net.   Also very well done PE railings formed for the particular sub.  Then there are the PE decks ... magnificent.

Anyway from the insulator yoke forward, extremenly fine strtched sprue or invisible sewing thread.  From either sidfe of the conning tower aft,  over crutches at about the level of the screws to the deck at the level of the stern planes. 

There was only one U-84 in World War II; she was a type VIIB...here's direct link from uboat.net about this boat-

http://www.uboat.net/boats/u84.htm

All Type VII U-boats would have had these cables running from the tower to the aft end and forward end of the deck.  The bigger question is whether on the foreward part of the deck if they ended on the cable cutter, or went down into the deck itself. If your model include the upper bow cable cutter, I would run the line to that.

1/400th, eh?  stretched sprue, or very fine monofilament -like flyfishng tippet material may work.