I’ll be touching on the First World War. For some background information I recommend these previous answers of mine. This first one is about the submarine technology available to both the Central Powers and Allies, and how materially there wasn’t a huge gap between them in terms of raw technology. This second one provides background into how submarines were used by the Central Powers and Allies. Furthermore, I do have this answer which is about why German leadership decided to wage a campaign of Unrestricted Submarine Warfare.

So with that out of the way, I’ll get to the meat of your question.

But how did WW1 U-boats performed?

What has to be kept in mind here is that “performance” is very relative. In war, the situation is fluid and tactics and strategies are constantly being renegotiated in the face of new challenges – including tactics and strategies of the opposing forces, new technologies designed to counter your current tactics and strategy, and so on. To demonstrate this I have two charts.

This first chart details, over time, the amount of tonnage sunk by German U-Boats. This one is for the number of ships, it’s effectively the same chart as the tonnage one as tonnage is the carrying capacity of a vessel, so measuring the tonnage gives a better picture of how much shipping was reduced in terms of cargoes able to be shipped.

Here you can see that the peak month of sinkings was in April 1917. You can see a slight peak in 1915 when the first campaign was instituted as well. After April 1917 there is a downward trend in terms of sinkings. So by this metric alone, the U-Boats seem to have performed their job pretty well. They were able to sink hundreds of thousands of tons of Allied shipping, making the Allied war-effort much more difficult.

That’s only part of the picture however. Germany wasn’t seeking to only frustrate the Allied war-effort with submarines, they were seeking to end the Allied war effort. And the U-Boats were not able to achieve that, and I’d argue that’s not due to the skills of the crews of the submarines themselves – but rather because the Allies were able to effectively counter the U-Boats. I will note that the German strategy was a long-shot, it only would have been possible in a vacuum where the Allies sat around and did nothing.

To see what I mean here is a chart of the number of U-Boats sunk during the war. As you can see there is an upward trend that starts really once the second Unrestricted Campaign was started in February 1917. Of course, this isn’t simply just because the German submarines were being defeated by the Allies – there were also more U-Boats out at that point in time. But, compared to the earlier periods, the difference is staggering. By that point in the war, you start to see the deployment of better depth charges, more sophisticated anti-U-Boat Patrols, and most importantly you see the introduction of convoying.

Convoying wasn't an offensive measure, it was defensive. It made it more difficult for U-Boats to locate ships, and when they did find a convoy the convoys were protected by a number of military vessels. U-Boats were not hunting in wolfpacks like in the Second World War meaning they were lone targets, easy prey for a handful of destroyers. A combination of Allied tactics and technology were able to overcome the worst of the U-Boat threat and thus make them perform less well in their goals. The Germans were also not fielding their whole U-Boat fleet at once and were instead only truly able to field a handful at a time, thus diminishing the overall effectiveness of the fleet.

Overall U-Boats performed their jobs as submarines fine. Where they faltered was in effectively being able to overcome Allied countermeasures in 1917 and 1918 to further the Germany strategy of cutting the United Kingdom off from the rest of the world. While there were still plenty of advances to be made in submarine technology, they were at a point where they were not simply novelties but had to be taken seriously.