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Old 06-10-18, 06:37 PM   #2
Antonin Dvorak
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Join Date: Jul 2011
Location: Germany
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the very part were I am confused is , why is it possible to mark an x any were on the map and have that represent the position of your u boat ..
Situation: Freighter at 000, enemy course 120, 9 kn, distance 2,500m, straight course. You are submerged, course 000.

If you want to do a simple straight-into-the-side shot --- which, as you know, is indifferent to the real distance and insensitive to some degrees of error in enemy course --- what do you need to know and do?

Answer:
  • The AoB of the freighter (the bearing the freighter would see you --- if they could see you) is 60 at the moment. You will want it to be 90 when the torpedo impacts, as the freighter is showing you it's starboard side.
  • You also want your torpedoes to have as little gyro angle as possible, i.e. the tubes should point in the direction the torpedo takes.
  • The 90 impact angle you want is when the torpedo course and the freighter course are 90 to each other. With the freighter's course of 120 this (you being south of it) is 30. So come to a course of 30.
  • Now, the freighter will of course move while your torpedo is travelling, but --- and that is one of the advantages of a 90 attack --- the distance only matters if the torpedo runs out of energy before it reaches the point --- the angle torpedo course and target bearing from the torpedo stays the same at all distances.[1]
  • You will have to lead the target --- by that same angle --- and that is only dependent on the torpedo and the target speed. As the torpedo travels 30 'units' forward, the target travels 9 'units' sideways, from left to right. And that --- 30 : 9 --- is the lead angle (and also a right triangle). That means you should lead by 16.7, i.e. when the target is at 0 respectively 360 - 16.7= 343.3 (the freighter is going left to right in front of you) relative to your course of 030.

Now ... do you see anything that would be different if you painted your x 5, 50, 500, 5000 miles away from where you are? As long as the freighter is still due north, 2,500m away, course 120?

The only one I could see is if land or very shallow shallows are in your way while your chosen spot on the card is just showing deep water ... but if you operate in shallow water or next to the coast, you should really know where you are ...

By adding/subtracting the right number of degrees to all compass-measured bearings and wrapping any answer <0 and >=360 properly around, you can have the "North" direction your map wherever you like. But since that is going to confuse, we don't do that (much).


And that is why you can draw any X on a paper towel and work with that. You might prefer better paper and grids and the right tools, though.


[1] This technique, enhanced by using a compass instead of having to hold the course, also tells you if you are on a collision course: if the bearing stays (nearly) the same, you are on one. No matter what maneuvers you and the other one have done. Unless you are increasing the distance to each other.
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