phavoc said:
It was a lot more submarine than the average U-boat.
You've never really heard of them because they weren't that successful. It was an
interesting vessel, but not particularly effective. It's very typical of inter-war vessels where they were still trying things out. She was also quite expensive.
Well that and she was French. Here in the Anglosphere, you don't hear very much about anything that isn't American or British at all. I'm constantly shocked at my utter ignorance about all kinds of things historical, technological, even in the way of current events if they don't involve the United States or the United Kingdom. The Surcouf also has a certain controversy because there's any number of theories (many of them pretty tinfoil hat) floating around about how she was sunk, what she was carrying, and where she was going. (Though there's surprisingly little written about the French Navy in general during WW2 - I suspect because of the UK's actions during the war towards the French Navy which are, putting it lightly, controversial.)
But back to the unsuccessful thing, they weren't really submarines, any more than U-boats. Like U-boats (and really pretty much every production submarine during the war), they were pretty much all designed to run on the surface of the water as a standard ship most of the time and only submerged for short periods to evade notice - you can pretty much identify all these boats because their surface speeds are okay, but their submerged speeds are terrible. You can tell the late-war 'usually submerged' submarines because of their low surface speeds and high submerged speeds, like the German Type XXI and the Japanese I-201s (both of which were built, but never saw real use).
Back to shortcomings, she was big, kinda clumsy in the water as a result, not particularly speedy, and despite being armed with ~200mm guns, it had the typical basically non-existent armor of a submersible, so pretty much any battle damage would mean she couldn't submerge, but that was mostly okay since the Surcouf was intended more as a commerce raider than something intended to fight warships. However, a U-boat was pretty much better at what they were intended to do - hunt cargo vessels; they were cheaper (so you could make a lot more of them) and pretty much as effective against unarmed or nearly unarmed ships.
Submarines that carried seaplanes were not that uncommon. Again, you'll hear a lot about the UK's attempts to have a submarine with seaplanes but they weren't particularly successful and the UK gave up on them and that's that, the Anglosphere would have you think. But the Imperial Japanese Navy had a number of models of submarines that carried them and they worked fine. However, the trade-offs necessary to have a seaplane hangar on a submarine made the IJN submarines that carried them larger, clumsier in the water, and more expensive -- again less effective as "submarines" - submarines, particularly of that era were best off being lean and mean. Then there's the boondoggle known as the I-400 class of the IJN, often touted as a "submarine aircraft carrier" (a phrase which is sufficient to make any man with an interest in military things turn into a starry-eyed little boy again), but was pretty much another Surcouf type thing - large, expensive ... and um ... "interesting" but not particularly useful.
