[John Channing]

Originally Posted by joea

Because the German TDC was different? US boats did not fire spreads in the same way as Germans AFAIK. Not a good answer but there it is.

Sure they did! From…

Current Doctrine Submarines
USF 25(A)

Prepared By
COMMANDER SUBMARINE FORCE, PACIFIC FLEET
FEBRUARY, 1944

Chapter 5: Attack – General
…

Q. Spread -- A spread consists of a salvo of torpedoes fired to hit at different points along the length of the target or its length extended. There are three forms of spread:

1. Longitudinal spread. A pattern formed by firing a succession of torpedoes along a practically identical track. The submarine steers a constant course and uses the same periscope and gyro angles, but fires at different points of aim on the same target.
2. Divergent spread. A fan-like pattern formed by a succession of torpedoes fired at the same point of aim but with gyros set to such angles that torpedoes cross the target track at different points. This is not to be confused with the change in gyro angles necessary to make all torpedoes of a salvo hit the target at the same point.
3. Parallel spread. A pattern formed by firing torpedoes simultaneously from bow and stern tubes with gyro angles set so that the torpedoes run parallel.Spread angle is the additional gyro angle, over that required for hitting the same point of a moving target, set on the torpedo to make any type of spread.
   …
   The minimum spread used should cover at least 80% of the target length.
   Firing torpedoes on different tracks to hit the same point of a moving target is not considered firing a spread. A spread is fired only when the torpedoes are aimed to hit at different distances from the same point of aim.
   …
   Standard commands for torpedo control are essential for the same reasons as other types of standard phraseology and the following are prescribed:
4. "Make ready the (bow) (stern) tubes." This orders all designated tubes flooded and made ready in all respects for firing. Only certain numbered tubes may be designated if desired.
5. Set gyro angle(s) _____ degrees." Specific angles are ordered for individual tubes when necessary. This order is used when ships are not supplied with, or are not using, the automatic or "follow the pointer" angle-setting equipment. The order provides for relative settings being received in the torpedo rooms so that necessity for conversion by the torpedo personnel is obviated.
6. "Set spread angle _____ degrees." this orders spread angle to be set when using divergent spreads on submarines equipped with and using the automatic or "follow the pointer" gyro-setting equipment.

JCC