6 X 48 = 288 per mothership's contingent. Have a 6 mothership wolfpack, and that becomes 1728 torpedoes running all over the place. Even the best aimed torpedoes will miss if the target(s) have a chance to take evasive action, but if the seas are filled with torpedoes (some of which are aimed right on) then there is a good chance that a ship that evades it's intended torpedoe might just blunder into another torpedoe's path.
As for the multiple submersible method, that is what I call the 'Convoy Crushers' for use against massed merchantmen out in the mid atlantic.
The most commonly used German WW1 submarine torpedo was the G/6.
Diameter 0.5 meters, length 6 meters. Comparing it to other torpedoes I estimate a weight of 850-900 kilogram per torpedo.
So your submersible carries a weight of at least 41 metric tons in torpedoes alone. Add torpedo tubes, a hull, engines, batteries, instruments, a crew...
(Late in WW1 the Germans considered the G/7 for submarine use. 0.5 meter diameter, 7 meters long, weight 1.3 metric tons. Which then evolved into the G7a used in WW2. Diameter 0.533 meters, lenght 7 meters, weight roughly 1.5 metric tons. So any improved torpedo likely will weight more than the WW1 G/6. Adding even more weight to your submersible.)
A 1930s early German coastal submarine type IIA had a displacement of around 300 tons just carrying 5 torpedoes. And had a submerged range of just 35 sea miles. A late WW1 German coastal submarine type UB III had a displacement of around 500 tons carrying 10 torpedoes.
So what do you want to remove from them to
a) carry 48 torpedoes and
b) let 6 of them being transported by one "mothership"?
Such a mothership must be huge to transport 6 of them. Just remember, 41-75 metric tons weight
just for the 48 torpedoes of one of them. That´s 246-450 tons for all 6. An ocean-going German diesel engine submarine in WW1 and WW2 had a total displacement of 1000-2000 tons in comparison.
And needing quite some machinery to even reach acceptable speed (not to mention range), I´d say. Don´t forget that reaching the location of a detected convoy was the main problem for submarines in WW2.
And you can probably forget trying an emergency dive with one of them.

If a British destroyer surprises a mothership surfaced ...
And WW1 and WW2 submarines - unless you invent the snorkel early - are really "surface ships". Speed and range submerged aren´t that good.
And in rough sea? The snorkel submerges under the waves, closes down and the diesel engines will suck in the air from inside the submarines.
What I have in mind is a ToT (Time on Target) assualt upon
all the major fleet bases of both the UK and French navies. By whatever means needed.
Scapa Flow, Portsmouth, Brest, Toulon?
And whatever else is used during wartime in Europe? Rosyth, Harwich?
Yes, I agree that the small craft will have only a limited duration, but a properly built mother ship should be able to sustain them just outside the heavily patrolled perimeter for quite a while. And if there is proper rotation of mother ships, they should be able to remain on station indefinitely.
Well, as long as there is no suitable radar available they might be able to surface during the night. Reload the batteries, fresh air. Of course there´s still the danger of accidental detection. Passing ship, patrolling airplane, fishing trawler.
Supplying them will have to be done during the night too. Which will add to the danger.
However you now add another requirement in addition to 48 torpedoes.
A crew large enough for shifts. And enough space for them. For working, sleeping, cooking and eating, storage room for food and water, toilet.
You can´t strap them into a seat and tell them not to move for days.
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I´m having real problems imagining such a mothership with 6 submersibles each carrying 48 torpedoes. Sizes, weights, available engines during that period don´t fit in my opinion?
For the mayor naval bases I see maybe two (or three) options:
- the Italian manned torpedo approach (first used in 1918). In WW2 some Italian submarines supposedly got hull additions to carry 3-4 of those manned torpedoes close to their target?
- a Pearl Harbour surprise air attack in the first hours of the war? Long range land based aircraft and / or seaplanes with torpedoes (from seaplane tenders). I assume Germany doesn´t have carriers, right?
(- Depending on technology available, midget submarines might be a choice too.
http://en.wikipedia.org/wiki/Seehund )
With submarines waiting outside the bases.
Maybe building two different ocean going submarine classes?
1. Long range submarine with 6 torpedo tubes forward. 24 torpedoes on board. Roughly 2000 tons displacement. Essentially the WW2 type IX class with 6 bow torpedo tubes. Each Wolfpack from the start supported by 1-2 type XIV supply boats. Additional surface supply ships "parked" before the war in different locations. Used against convoys.
2. Mid / Short range submarine. Trading range (and maybe size?) for two additional torpedo tubes. Carrying maybe only between 8-16 torpedoes? Used against naval bases in the British isles. Maybe in the Mediterranean assuming an ally there? Kind of like the 1970s German U-Boat 205 or 206 class?