Originally Posted by I'm goin' down
Assuming, "this" refers to Robbins' explanations, the short answer is that it works because the TDC, being a machine, calculates the lead angle (9 degrees in his examples) even though the PK is not activated (i.e. the red light on the TDC is not turned on.)
Hypo 1:
A 450 foot wood duck, quacking wildly, is traveling from left to right at 8 kts, range 2,000 yds., bearing 270 degrees and closing. (It is lost and not flying very fast.) You put your shotgun away because it does not have enough fire power to bring down waterfowl of such immense size, and forget that it is Thanksgiving. You decide to take it out with a torpedo. You set the scope to 350 degrees, set Aob to to 80 degrees starboard (i.e. the duck is showing you its starboard feathers), and set range to 2,000 yards. Even though the PK is not activated, the TDC, which never sleeps, calculates a lead angle of 9 degrees. If you fire when the duck crosses the wire (i.e. at 350 degrees), the duck and torpedoes should rondeveous at 359 degrees.
Hypo 2:
Same duck as hypo 1, but you spilled your coffee and did not fire when it crossed the wire at 350 degrees. Swivel your periscope to a bearing of 10 degrees. Reset Aob to 100 degrees starboard, as the duck is flying away from you and still showing its starboard feathers. Click range. The TDC will recalcuate the lead angle to approximately 19 degrees (note, this should read, "approximately 9 degrees" and the term "19 degrees" is the result of a typographical error. See Robbins' comment in post no. 12, infra, where he caught the mistake.) Fire when the duck crosses the wire (i.e. 10 degrees) and the torpedo should hit the duck on its starboard side at a bearing of approximately 19 degrees [wood duck crosses wire and torpedo fired at a bearing of 10 degrees + 9 degree lead angle = torpedo and wood duck rondeveous at a bearing of approximately 19 degrees], as the TDC, even though the PK is not activated, calculated a lead angle of approximately 9 degrees.
The key is that the TDC will calculate the lead angle since it operates even if the PK is not activated.
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