Acgoldis wrote:
Yep, very

Also highly impractical to the extreme. Not “ASB” but not really plausible due to numerous issues.
The Germans have U-boats. Did anyone ever consider the possibility of trying to attack convoys by approaching a vessel underwater and boarding it from underneath, blasting a hole in the side of the ship and entering that way?
Standard convoy ship (average speed is over 10 knots, escorts by their nature are pushing over 15 knots) is going to be so much faster than the U-Boat (maximum underwater speed is less than 10 knots usually less than 8) can be, plus the freighter is moving, (hence you have water flow around the hull which precludes “attaching” let alone getting close to the freighter) and maneuvering (zig-zag at random would preclude attachment) and that’s all before we get to the mass difference, (typical freighter is going to be massing several thousand tons whereas the U-boat is will be less than a thousand tons) and those are just the most obvious issues.
It sounds ridiculous, but who knows?
As a tactic for war it is pretty ridiculous I’m afraid. There’s a reason “boarding” as a standard tactic had fallen out of favor several hundred years earlier

Now if we’re talking “pirates” that’s a possibly different kettle of aquatic lifeforms…
The reward would be great if they pulled it off and managed to capture an enemy ship without anyone noticing. They can then surprise the rest of the convoy by having a friendly ship shoot at their own tankers and stuff.
Er the ‘reward’ is losing a significant portion of your overall crew because it is going to take a good number of people to man and run the ship you just took and it’s going to be rapidly destroyed the moment it opens fire by the rest of the escort. There is literally no ‘upside’ here.
And of course if they manage to blast a hole in the side of the target vessel and are not able to board, the U-boat can just bank off and let the target sink.
There’s these nifty devices that can accomplish that at long range called a torpedo…
Oh and I’ll address the inevitable question of why the U-Boat would have to ‘attach’ to the victim; it’s the only way to transfer the crew to the other ship when both are moving. You also can’t just ‘blast’ a hole since that will immediately alert and prime the victim’s ‘crew’ since you can’t immediately start transferring crew.
Acgoldis wrote:
True, but how many defenders are armed at the time of the attack? I don't know how much of the crew had sidearms in this era.
It’s worse than that, who needs side arms when the very FIRST action the crew will perform is closing water tight doors which can in fact be ‘locked’ from either side? Even assuming you can find a spot where you can ‘blast’ your way in without sinking the submarine itself, (a very non-trivial task) you are likely as not to come into a compartment that is difficult if not impossible to get out of once those doors are closed. “Sidearms” The Marines are ALWAYS armed with serious weapons not just "sidearms" and are both highly familiar with and have practiced combat operations inside the ship you just boarded.
Another option would be to board the target ship during the night from the sub and use people who can speak the language of the defenders without an accent. Hide in plain sight if possible. Then you just go to the bridge or something (unless that's off limits), take over, and start issuing orders and at the very least you'll have a very confused crew wondering whether they should be following the new orders (which works to the attackers' advantage). Granted, this would also be difficult as presumably there are people looking for subs and so forth at night.
First and foremost you just “blasted” a hole in the ship, night or not everyone is going to be VERY alert and VERY non-trusting of a ‘new’ face. Again it’s an almost plausible concept for a pirate action but not during a war.
Be advised I'm not exactly a guru on this. It's just a crazy idea. I also have no idea if they had water locks in those days which allowed people to enter through the walls of the ship without flooding a compartment.
Technically there were ways to penetrate a hull without sinking the ship but they were huge and bulky and NOT very stealthy. Now you have to have a plausible method of getting through the hull that doesn’t require ‘blasting’ and doesn’t alert the crew which is the real question. Most hulls weren’t the modern ‘double’ hull but they still had spaces that would have to be breached and crossed. Here I’ll point out your ‘sub’ is going to also cause a huge increase in drag so there are other issue to address.
The scene where the submarine gets boarded in U-571 comes to mind as a case where a small team comes aboard and occupies a larger vessel. However, I don't know how accurate this depiction was.
It wasn’t at all. Such an assault would have a very small chance of success and require a HUGE amount of preparation and effort which does NOT lend itself to ‘opportunity’ action. (First thing I noted in the movie was the 'longboat' had FAR to many people in it and the Germans would clearly have noted that fact)
Takonas wrote:
There are stories of Italian frogmen boarding via underwater apparatuses. However, their success rate was less than optimal with many reported failures. However, they did experiment with the idea of blowing their way into enemy ships but it was considered an almost suicidal route into an enemy ship due to the armament of personnel of enemy warships.
So, there was a technology by the Italians to board enemy vessels underwater but was deemed to be 'suicidal' or ineffective in actual use.
I don’t think those actions actually ‘boarded’ while underwater but “sneaked” near the ship by travelling under and then boarded by means of grapples or something. And there was a good reason for expecting ‘blowing’ your way in to be suicide since the required charge to breach a hull will pulverize divers in the water nearby. They also never attempted or planned an operation while a target ship was underway.
Again we’re talking nonmilitary operations it’s a maybe but it will take work.
Randy