I watch a show about WWII submarines. After Pearl Harbor they send 4-5 submarines of a new class with large dimension so can have bigger autonomy range. This was related to the first German waves to the USA East Coast and the great success of one Nazi captain because american didn't have enough resources to defend their coast.

But there are some incidents make me wonder what happen.

1) During day the submarine stay on the sea bed and go hunting during night. But on one opportunity the submarine went up too early and still was some daylight and one American plane detect it and throw some explosives.

Why the submarine go all the way up? Why don't go to periscope level first to take a peek? That would be enough to realize there is still daylight and even if were already dark watch for any close threat.

2) After sunk a ship instead of killing the survivor the submarine just go away. I have seen other episodes where the submarine crew use machine guns to kill all floating survivor. Is that a tactic implemented later? On this case the survivor went and alert about the submarine presence.

3) With very low number of torpedo remaining, the submarine decide to use the boat deck gun against one small cargo vessel but when the gun didn't work right away they went for the torpedo.

Why they didn't continue shooting with the boat deck gun? My guess is if the submarine has that weapon is for something, but if can't handle a defenseless boat why they put it there.

4) At the end without any torpedoes and low fuel they were returning home but were spotted by a whaler transformed into oil tanker, the whaler was going in a collision curse and because the submarine has engine problem was going 1 knots slower than the whaler. Finally they fix the engine and could escape by going 0.5 knots faster than the whaler.

But why they don't use the deck gun to scare the whaler?

  • 2
    re #1. German subs didn't get schnorkels till 44 or so. That was a pipe that would bring in air to the diesels and allow recharging of the batteries. Prior to that, subs had to surface to recharge their batteries. No idea if that is the case here, but running at periscope depth wasn't always an option if your batteries were low. Commented Jan 27, 2018 at 2:09

4 Answers 4


From the events you've described, you are talking about U-123's 7th Patrol.

For Question #1, it is likely they did go to periscope depth first. Aircraft would be hard to see from a periscope. Wakes from vessels are somewhat luminescent, and can be spotted from the air in low light conditions.

For #2, Machine gunning survivors was not a common tactic. Killing survivors in the water was not something either side looked kindly on. If it was discovered, then the Allies would reciprocate, and with the U-Boat loss rate what it was, it was entirely likely that they would be in a lifeboat of their own eventually.

For #3, the deck gun is generally used for smaller targets. For this particular target they underestimated its size, and thus chose to use the deck gun on a ship that warranted a torpedo.

For #4, the U-123 had its deck gun mounted ahead of the conning tower. As they were heading away from the Whaler, the gun would not be able to traverse to hit the Whaler. If they turned to bring the Whaler into the firing arc, they would lose speed, and the Whaler would probably catch the U-Boat and engage in a physics problem the U-Boat was unlikely to win.

  • Yes, you are on point was the U-123. Regarding #1 I know a plane can also see a submarine when close to the surface, So maybe they misrepresent the fact in the tv show because they show the crew detecting the plane from the deck. But still looks weird they surface while still daylight Commented Jun 21, 2017 at 16:44
  • Never mind close to the surface, a plane can spot a submarine down to several tens of metres if the water is reasonably clear. That is perhaps a quarter or fitth of its diving range. Thsubmarine itself is also about forty or fifty feet in height, all of which is closer to the surface than the submarine's own estimate of depth, its bottommost outer shell. Commented Jun 21, 2017 at 22:33
  • I'd add to #3 that while a submarine is on the surface using its deck gun, it is vulnerable to being spotted; there might be ships and aircraft on their way to answer a distress call. The captain won't want to hang around pumping gun rounds into a ship that won't sink, and a torpedo becomes worthwhile.
    – Schwern
    Commented Jun 23, 2017 at 19:09
  • ...while the cargo ship is transmitting the contact and its exact location via radio. Even with an unarmed cargo vessel, you need to silence it quickly if you didn't want to be bounced by an aircraft, or worse yet, a nearby destroyer. (That -- and basic humanity -- is also why you don't hang around firing small arms at shipwrecked sailors. You want to clear datum ASAP.)
    – DevSolar
    Commented Jun 26, 2017 at 14:25
  • 2
    @mart: You are referring to the Laconia Order ("Triton Null"), so named after the sinking of RMS Laconia and the unfortunate chain of events following it, namely the bombing of U-Boats engaged in rescue operations by the USAAF. As usual in war, both sides have to tell a story.
    – DevSolar
    Commented Jun 26, 2017 at 15:19

A good parallel to what you are talking about are US submarine tactics during the same period. A good book about this is the book Wahoo: The Patrols of America's Most Famous World War II Submarine, by O'Kane. It is a great glimpse into how complex and intense submarine operations were during that period. The parallels to your questions are:

1) Submarines spent most time surfaced and running on diesel engine power. This required constant and careful watch, but it kept the batteries topped up and they had better speed on the surface and that helped long cruises to patrol areas. If a target was spotted, they would usually run fast on the surface to get in a good position and then submerge get final position and attack. Some German and US captains preferred night attack while surfaced, for better situational awareness and speed.

2) Highly rare. Usually a sunk ship would have had an escort not far away, and the submarine was very interested in getting away before the escort got involved. It was not safe to linger close to the scene of an attack. As mentioned in other posts, this was also a war crime. But it did occur in some instances.

3) Deck guns were used for smaller targets that were alone and undefended. Wahoo's patrols actually had quite a bit of deck gun action, but only against lone vessels without equivalent guns and were not big enough to warrant an expensive torpedo. I think German subs during this period began counting their deck guns as a AAA battery due to the air threat at the time.

4) As mentioned above, the main deck gun was mounted forward and was not meant to fend off chasers. A turn to bring the gun to bear would allow it to be used, but the gamble would be lost if it couldn't stop the whaler from ramming them, which was fairly likely. A medium-caliber deck gun firing from low and forward was probably not going to stop a decent sized ship.

  • As to #2, it actually occurred on the Wahoo. There is a famous incident where Mush Morton machine gunned what he thought were Japanese sailors that instead turned out to be Indian POWs. It is most likely the reason he (Morton) never received the Congressional Medal of Honor despite the Wahoo's war time record.
    – ed.hank
    Commented Jan 10, 2019 at 15:21

The US made an unexpected entry into WW2, with Germany declaring war on it shortly after the even more unexpected Pearl Harbor attack. As such, the US was completely unprepared for modern anti-shipping warfare. Consequently, the German U Boats didn't need to change tactics, they just took advantage of the very unprepared state of the US maritime shipping.

No convoys, very few patrol boats. Ernest Hemingway joined in with some fellow adventurers in a fishing boat, armed with a few depth charges and tommy guns, to go hunting U-Boats. Fortunately for them, they never found one.

Making the situation worse was the head of the US Navy at that time: Admiral Ernest King. Bull headed, and an Anglophobe (didn't like the British), he ignored the extensive experience the British had, and US shipping suffered accordingly. Eventually, the US did get its act together, but for a few months, the U Boats had a second 'happy time'.

As for shooting survivors in the water... not only inhumane, against the rules of war, and contrary to good discipline (the Kriegsmarine was a very disciplined service) it was usually not necessary. If survivors of a torpedoed ship in the Atlantic weren't picked up immediately by the escorts, they almost always died.

U Boats were all equipped with a 'sky periscope', essentially a second periscope with a fisheye lens. It gave a complete view of the sky and horizon. They used this to scan for aircraft before surfacing.


I can answer part of your question.

2) After sunk a ship instead of killing the survivor the submarine just go away. I have seen other episodes where the submarine crew use machine guns to kill all floating survivor. Is that a tactic implemented later? On this case the survivor went and alert about the submarine presence.

Shooting survivors of a ship that you sunk was against the laws of war. It was a war crime. Only someone evil would do it. Therefore, not every submarine crew always machined gunned survivors of ships they sank.

  • Pretty damn pointless too. Why waste ammo you might need later to save yourself on a helpless target that will most likely die on their own anyway?
    – T.E.D.
    Commented Jun 21, 2017 at 16:34
  • Did they even bother carrying small arms on a U-boat?
    – user13123
    Commented Jun 22, 2017 at 3:43
  • only reason I can indeed see for killing the survivors is to prevent them getting to shore and raising the alarm, which would only make sense if the sinking is done in shallow waters close (within easy swimming distance) of a populated shore line. It will give the submarine more time to get away...
    – jwenting
    Commented Jun 22, 2017 at 6:52

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