Quote:
Originally Posted by Paul Riley
I am surprised,I never knew about fixed wire method,what is this exactly?.
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You set your periscope forwards or to the rear, and set your course until the view is right infront of the targets bow. Start your clock when the bow touches. Stop it again as the stern (backside) leaves the line. The ship's overall length, divided by this time is the speed of the target in meters/second (or whatever units used). The value in knots is almost twice this m/s value. Basically you have just made an imaginary stationary wall through the water with your periscope line. The periscope must be along your bow/stern centerline to nullify the distorting effect of your own speed you would get if the scope is looking sideways. But if you know your speed well enough it is 'rather easy' (with Hitman's Attackdisk, ISWASWHAT-EVER
, slideruler) to calculate the speed correction based on viewing direction.
Real uboats had a moving line in the scope optics that was slaved to the gyrocompass. So any small course deviations of the uboat would have been cancelled out. We need to make sure we don't turn, by keeping the rudder amidships, or submerge to soften the wave ridding. They called it 'feste Linie im Raum', translated to 'fixed line through space'