[CCIP]

Yes, hydrodynamics were well understood and in fact the very early subs, such as the early Holland types, were quite hydrodynamic. There were also motors powerful enough even in WWI to achieve some quick speeds.

The issue, as noted, was one of doctrine and efficiency rather than technology. Surface performance and submerged performance are necessarily tradeoffs. An early hydrodynamic sub, which would've been small, would essentially be a sort of 'trap' restricted to a very short operating radius and inefficient at traveling any distance on the surface. To get around that issue, like the XXI, it would need to be very large, expensive and complex, while still sacrificing surface performance. Why do that when you have the technology to build subs that are smaller, faster on the surface, and have excellent sailing range, for just a small fraction of that cost? At the end of the day, a submarine's job is to sink ships without being detected, and escape without being sunk. It also needs to be able to return to port quickly, be prepared for the next patrol quickly, and get to its patrol area and sink ships again. A complex sub which compromised surface performance may be great at sinking ships and escaping, but in absence of fatal danger on the surface, it was a waste of time, money and energy getting it to and from patrol, and getting it in and out of base quickly. A sub fleet's function is to sink as many ships as possible, not as impressively but as efficient as possible - especially to the WWI/II mindset, tonnage war was not a psychological campaign but a very cynical game of numbers. The one who can outsink the enemy's production capacity wins. So it was a natural thing that navies operating submarines were looking for submarines that were the best compromise of all factors except overall fleet efficiency. What emerged, especially in the case of Doenitz's U-boat force (which was from the beginning was to be a commerce warfare unit first and foremost), is the ideal weapon for that time and those circumstances. It was a submarine that could efficiently go to sea, fight, survive and come back - and maybe it didn't fight as impressively as it could've, but in the circumstances and in the view of the doctrine, it was good enough to work to a devastating strategic effect. Unfortunately (for the U-boats) those circumstances changed quicker than the subs.

It's just like today, the idea of a supersonic transport aircraft is neither novel nor impossible. The Concorde has come and gone. But the fact is that there's no incentive to introduce another transport like that when what we have on hand are much cheaper, more reliable, and arguably still very fast subsonic airliners. The situation with subs was much the same, until technological change and vastly effective countermeasures forced the germans' hand and made them do their darnest to give the XXI all the performance they could, while investing a lot of effort into making its production faster and more efficient. But it was still too late.