[mookiemookie]

Early in the war, the accepted U.S. doctrine was to attack submerged, well below periscope depth. Hydrophone bearings only were used for TDC data. As the war went on, more aggressive captains met with greater success by using the surfaced attack method. The "Mark I Eyeball" was found to be a much more effective means of gathering torpedo firing solutions than trying to use only a hydrophone. A submarine operating on the surface was not vulnerable to hydrophone detection, and its low profile was very hard to spot in darkness. Sub captains found that they could get in close, get an accurate firing solution and escape on the surface without fear of detection.

So there you have it.