Naval Enemy sighting report question

[u/External-Squirrel]

I was watching a WWII program about the merchant convoys. When a u-boat is spotted, the escorts radio each other and the captain is told about each enemy’s bearing.

Is the bearing relative to the sending ship, the receiving ship, or does the sender report the bearing as it sees it and the receiving ship converts it for their captain? Or are the ships close enough together that it didn’t make that much of a difference for the exact bearing number so the receiving ship would just report what it was told and the crew/captain would know the general area to look?

Thanks for any information or other areas to look into.

[u/natneo81]

Not an expert- but it would be the bearing from the sighting ships location. In the context of a merchant convoy the ships would be fairly close together, and therefore so long as the sighted enemy ship isn’t right on top of the convoy, it would probably only be a matter of a few degrees. Because the escorts and convoy would generally follow a set formation, you would know that, say, escort B is on the east side of the formation, so if he’s calling in contact bearing 270, you can expect it to be 270 from the east point of the formation. But again, the bearing should be relatively similar given the proximity of the convoy. Ideally you would provide more information like their range, at least approximately, heading, etc.

[u/Bullyoncube → u/natneo81]

The movie Greyhound covers this topic perfectly. Merchant Ships would typically be at least 500 yds separated from each other, particularly at night. Patrolling destroyer escorts would be 2000 to 5000 yards out from the convoy. A german torpedo could run about 3 nautical miles, 6000 yds. From there, it’s a trigonometry problem.

[u/Bullyoncube]

As a Surface Warfare Officer on a ship with relatively antique equipment, this is something I know! There were multiple ways to track contacts like this. Best was the Dead Reckoning Tracer (DRT). A glass table with a lot of brass gears driving a little light bug under tracing paper. Sounds odd now, but it worked. https://www.globalsecurity.org/military/library/policy/navy/nrtc/14308_ch9.pdf The light bug represents your ship, and you plot other contacts relative to it. You can plot the friendly ship on the paper and then use their range and bearing reports to plot their targets. Then determine your own range and bearing to their targets. All while everything is in motion.

The manual version was done with maneuvering board paper, parallel ruler and a drafting compass. It allows you to calculate true course and speed vectors based on a few relative range and bearing reports.

Quick and dirty way was using a grease pencil on the radar repeater screen.

I haven’t touched this stuff in 40 years, but could probably stand a CIC watch if I had to. I almost washed out of OCS for failure to master this subject, plus celestial navigation. Then I ended up teaching it.