US Sub operation Reference
 

http://www.history.navy.mil/library/...b_doctrine.htm

Check this out.
Section 4
Operating Procedures to Avoid Detection

4401. The following procedures cover cardinal principles for effective periscope handling when it is necessary to avoid detection from that source:

i. Observations at three minute intervals or multiples thereof have the advantage of simplifying manual plots, but lose their effectiveness when they are allowed to become restrictive to the approach officer.

nice read. :up: