I enjoyed the film, but at several points found it a bit striking that the U-boats seemed to be doing things that made it much easier for the Americans to attack back rather than the kind of hit and run tactics I'd have expected. Did they really go after escorts so aggressively, and in ways that exposed themselves not just to depth charges but direct gunfire?
You are correct in your suspicion that much of the U-boat tactics shown were less than accurate (although I agree that it was quite enjoyable as an action film). However, to some extent the movie did, in fact, accurately reflect the tactics used by U-boats against convoys:
Firstly, no, U-boats did not aggressively attack convoy escorts as shown in the movie. They almost always avoided the escorts in favor of the real prize: the merchant ships. A destroyer effectively held all the cards against a U-boat: they were significantly faster, much more maneuverable, and much better armed. Destroyers could also see much better, as they were positioned much higher above the water and had many more lookouts than a U-boat. U-boats were fundamentally ambush predators. They only card they ever held against warships was that of surprise- and even then they often did not have that due to radar, sonar, and other means of detection.
Regarding attacking from the surface versus attacking submerged, some aspects of "Greyhound" were accurate while others were not. U-boats were technically not true submarines but rather submersible boats, meaning that they were capable of submerging but had to spend most of their time on the surface. If a U-boat was moving, it could travel submerged for perhaps 24 hours on a fully charged battery. If it was motionless on the seafloor, not using battery power, then it could last 48-72 hours until the CO2 reached dangerously high levels. Submerging was intended for either a stealthy approach or escaping, otherwise U-boats traveled on the surface where they were much faster (although new technologies in 1944-45 eventually enabled much longer submerged travel.)
Based on my recollections and a brief review of YouTube clips, the movie portrayed U-boats firing against the destroyers while submerged. This is accurate, since attacking a destroyer from the surface was suicidal if the destroyer knew the U-boat was present. Even attacking a destroyer while submerged was an extremely risky endeavor. As you may know, U-boats were not intended to primarily attack warships. Its primary prey was merchant ships, which were generally either unarmed or only lightly armed.
The ideal employment of the Type VIIC U-boat, which was the most numerous design and the type shown in the film, was group attacks against convoys. This is, in fact, specifically what the Type VIIC was designed for. U-boats were much faster and more agile on the surface, thus, like fighter planes attacking an enemy bomber formation, U-boats were intended to nimbly weave between the merchant ships of a convoy, firing torpedoes at the most promsing targets and diving when necessary to avoid the escorting destroyers. You are correct that these were fundamentally hit-and-run tactics: the U-boat was meant to sneak inside the convoy, usually at night, loose as many torpedoes at as many merchant ships as it could, and then weave back out.
There is a scene during the final battle where Tom Hanks' ship scores a hit on a U-boat that is right under the surface. The U-boat then surfaces and is promptly hit again and explodes. The first hit looked like a fatal one, so the boat was probably surfacing to abandon ship. This is true-to-life, as commanders of a fatally damaged submarine would nearly always attempt to surface in order to escape. It was rarely clear to Allied ships, however, whether a U-boat in such a situation was calling it quits or surfacing for a surfaced attack (the latter scenario was very rare, but nobody wanted to take chances.) U-boats in these situations were invariably turned into Swiss cheese until Allied gunners saw men start pouring out of the conning tower hatch. Even then, a combination of adrenaline and anger meant that they didn't always stop shooting right away.
tl;dr:
1.) No, U-boats did not aggressively attack escorts. Their primary targets were the merchant ships. Escorts were primarily to be avoided and evaded.
2.) U-boats did often attack merchantmen from the surface because they were much faster, more maneuverable, and had better visibility than while submerged. However, going toe-to-toe while surfaced against a destroyer was suicidal.
I'm currently writing a non-fiction book about U-boats in WW2, so please let me know if you have any other questions on the topic!
Sources:
Ten Years & Twenty Days by Karl Dönitz, U-boat Tactics in WW2 by Gordon Williamson, U-boat Commander's Handbook, The Illustrated History of Submarine Warfare by Edward Horton, Neither Sharks Nor Wolves by Timothy Mulligan
No, definetely not.
First of all, that movie is based on a novel from 1955 (The Good Shepherd). And then, German Kriegsmarine U-boat doctrine was laid out to cut off supports, not escorts.
The main goal of the German Navy in the battle over the Atlantic in 1942 was to prevent supports from the US, which came in on tankers and cargo ships, reaching the German west front. Those supports were in form of fighting men, material, oil... Basically everything to support the allied war effort. Destroyers didn't transport those supplies so they were never a real target.
German Navy submarines had the standing order to find convois, assemble at a location to build up an attack group (the so-called Wolfsrudel) to then intercept the convois.
They then always tried to slip through the escort defense lines and attack the cargo ships and oilers from within the convoi. Attacking an escort would have made such an effort impossible for the warning sign that would be. If and when a uboat attacked an escort, a destroyer it was usually a last exit measure to either distract, divert or get away unharmed. Also, some destroyers were hit by stray torpedoes that didn't hit the intended target and "found" an escort instead before running out of fuel.
Now for the surface warfare: That actually was a thing. But not as shown in the movie.
In some cases, after successfully infiltrating a convoi, and when weather and environmental conditions were good and tolerable, a uboat captain would surface and start a sprint towards his target. This was mainly done because of the speed difference of those old submarines. Submerged, they weren't really fast, making a silent five to seven knots on average. But surfaced, we're talking top speeds of up to 20 knots in perfect conditions.
So, a captain would sprint towards the target to lower distance fast and attack. The reasons were obvious: faster attack means more potential targets can be run at to be torpedoed. Also, the lower the distance to the target, the higher the chances for a successful hit. But all those targets were mentioned tankers and cargo ships. Running towards a war ship surfaced would have meant certain death.
There are a hand full of documented cases and incidents where US and British destroyers spotted "surfaced" German submarines in actual combat, though. But they were all spotted due to weather conditions, when a periscope depth submerged boat broke waves and the bridge, gun and parts of the deck became visible. The only reason why those submarines weren't destroyed on the spot were the weather conditions - because a 1930s/1940s destroyer is as unable to shoot and kill in high waves as that submarine.
The other spotted surfaced submarines were those of either very stupid or very unperceptive captains. While sprinting towards a target under conditions like a full moon, for example, even a German submarine with lights out may be spotted from multiple hundred to some thousand yards away. And if those crews didn't notice the thread immediately... Well, there's a reason for the high German uboat losses.
Hoping I could be of service and clear things up - at least a bit,
A German Navy LtCdr