score:8
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.
Upvote:1
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.
Upvote:2
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.
Upvote:3
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.