[TorpX]

Originally Posted by razark

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Can someone explain why? Is there something in the geometry of the approach that makes the normal approach course better than another method? Why 90 degrees, instead of 80, or 70, or any other value?

I worked out the geometry related to the "normal approach course", some time ago when I started getting serious with SHCE. I can't remember the trigonometric particulars, but suffice it to say, it is indeed a matter of geometry. You probably already know this, but the word "normal" relates to the course being normal (perpendicular) to the target, not the course being usually or always used.

The course allows an interception to be made at a minimum of speed, provided an interception is possible. It is not a tactically optimum course or a default plan. There are too many unknowns to allow for this. The NAC is more a product of math, than tactics, if you get my meaning.

Also, in what situation is the NAC used?

If a sub is operating on the surface, it would normally have a speed advantage and has many options for making an intercept and attack. The same is true, if it is submerged, but on, or very close, to the target's track. When the target is detected in a less than optimum situation (a distance off the target's track), where the sub must operate submerged, and with a very limited speed, the NAC is often very valuable. ( I've made use of it many times.)

I can think of 5 general cases:

Case 1.
You start on a NAC, but the range is steadily increasing (the target is on a divergent course). This means you were caught napping and the target basically, "got by". You would have to chase after it (at considerable speed) or let it go. The NAC will not help you here.

Case 2.
You start on a NAC, but the range is holding steady, (the target is on a parallel course). Interception is impossible at this speed, (but can be achieved at a greater speed).

Case 3.
You start on a NAC. Range is steadily decreasing. The target is loosing bearing. This means interception is assured (unless the target changes speed or course). If you continue at this speed and course you will pass in front of target.

Case 4.
You start on a NAC. Range is steadily decreasing. The target is holding bearing. Interception is assured. You are on a collision course.

Case 5.
You start on a NAC. Range is steadily decreasing. The target is gaining bearing. Interception is not possible at this speed, but an attack may be possible. Target will pass in front of you.

Case 2 is mathematically improbable. If you are searching continuously and are alert, so is case 1.

Would the NAC be constantly recalculated to keep the target on the 90 degree bearing, or would it be calculated once?

It could be done either way. If you are gaining bearing [Case 3], and you hold the same course you will reach the target's track and pass in front of him. You could do this in order to make use of stern tubes or simply reach the area first and slow to minimum speed and wait. However, you could also adjust bearing once or more times. In this way, you would intercept sooner. If you were gaining bearing, you could also reduce speed, and still make an interception.



BTW, There is also, a Normal Evasion Course. (I think that is what it is called.) In this case, taking a course 90 deg. going away from the "target" allows one to get as far away as possible at a given speed. This can be very useful at times.