The T-valve snorkel is the same basic design as modern snorkels, so it was indeed a very promising design since it is still with us. The head is only slightly larger than the air inlet pipe since the T-valve closes on the top of the pipe. It is about half the area of a ball valve snorkel and a lot lower. The T-valve was operated via a conductivity sensor which was placed below the top of the head. The degree of valve closing could be controlled based on the amount of time the sensor was immersed. Together with an appropriately designed bilge system, the valve could be left open to allow swallowing small amounts of water. If the water taken in exceeded the capacity of the bilge pumps, the valve could be closed or the U-boat could go up a bit to stick the snorkel further out of the water. This method of operation allowed the snorkel to be sticking up just a little over the average wave height.

The ball valve was a purely mechanical device which would close the moment a swell hit it. This obviously causes a lot of pressure differential problems in the U-boat. To avoid the constant on and off, the head had to ride quite high over the waves, so it had to stick out quite a bit more than a T-valve snorkel plus the head was much larger and thicker (higher) due to the diameter of the ball.

As it was, the ball valve was missed 95% of the time by Allied radar. The T-valve would probably have been missed more than 99% of the time. It must have been very hard to find since half a dozen type XXIII operated with it on 10 combat patrols in 1945 and sank 6 ships with no losses. The British wrote all 6 losses off to mines. There must also be plenty of data available on the problems of detecting T-valve snorkels by radar since everybody was using them by the 1950s.

The issue of heavy seas is easy to answer, you do not snorkel, but stay below. In medium seas, the snorkel is stuck up above the waves. I described above how the optimum height is found. This is quite safe since the radar does not penetrate the waves so the effect is similar to having the snorkel low in a calm sea. Also note these U-boats could recharge in about 4 hours, so that is all the time they needed to snorkel. The rest of the time they were below snorkel depths using their sonar to find targets.

BTW, the magnetic pistol was not a failure. The problems were fixed by late 1942 and it was routinely used by the U-boats. It was the American magnetic pistol which was not fixed until after the war.

The 37 mm AA gun was a dumb idea to start with. Any U-boat to airplane duel with guns is a dumb idea and that should have been obvious from the start. The whole thing was a move of desperation.

The T-5 (Zaunkoenig) was not a complete failure. The way it was used almost insured that the premature detonation problems would not be noticed. Why the U-boat was supposed to dive to 50 meters just after firing is an interesting question. The excuse was to prevent the torpedo from looping around and hitting the U-boat, but that could have been easily fixed by making the arming distance adjustable. Considering all the other strange things which happened in the torpedo inspectorate, I am more inclined to think in terms of sabotage. After all, treason was already established with the Oslo Report. In any case, it took a long time before the premature detonation problem was diagnosed. Once it was, Zaunkoenig II was developed, but the war ended. Much more could have been done on the torpedo side if the priorities had been higher and everyone was giving 100% for the Reich.

BTW, the big difference between torpedo R+D and U-boat R+D was the timing of the ideas. All the torpedo concepts in advanced development or prototype at the end of the war were already known in the 1930s and being worked on, but the work proceeded too slowly, mainly due to priorities. On the U-boat side, the idea of the electroboat did not pop up until 1942, although a careful analysis of the situation should have brought it up in the late 1930s. The realization of the concept proceeded rather quickly considering the conditions. The snorkel was already suggested in 1933, but not seriously taken up until 1943. A serious mistake in judgement.

Regards,
SuperKraut