[UKönig]

The air mast, or schnorchel, as Dönitz called it, using a dialect word for 'nose', had already been fitted to some Dutch boats before the war, but these had been used solely for ventilation purposes. The air mast fitted to the German boats of type VII and IX was intended as well to supply fresh air to the diesels, making it possible for them to recharge their batteries while underwater, instead of only on the surface.
The snorkel consisted of two tubes, the first for air induction, the second and slightly thinner one was for carrying away the exhaust gasses from the engines. The tubes were raised and lowered together by means of hydraulic pressure.
Before the introduction of the snorkel, U-boats had to spend most of their time on the surface, and only diving to carry out or avoid an attack. With one, they were able to stay underwater for weeks at a time. The record was 66 days, the second place record was 59 days. But these were exceptional feats of endurance for it was soon apparent that snorting made heavy demands on the crew, and afforded by no means, complete protection from attack.
While snorting, a U-boat crew had to maintain a careful watch at the periscope to ward off surprise air attack, for although the top of the snort mast showed only as a pinpoint on the enemy radar screen, it was liable to detection from the air by the exhaust trail or wave wake it left behind.
In all other regards, the crew had to be continually alert. In no more than light waves, the air inlet on the induction mast would repeatedly flood, and if the diesels were not stopped in time, they would pull their air in from the only other source, inside the hull, until, with the reduced air pressure, the mens' eyes would bulge from their sockets, or eardrums could burst. But some loss of pressure was unavoidable, and after a time, the continual changes of pressure would start to affect the health of every man aboard.
In the early days of the snort, a further difficulty appeared. When a U-boat plunged deeply into a swell, the exhaust would be unable to escape against the weight of the water, and would blow back down inside the hull. If the diesels were not stopped in time, the engineers would collapse with carbon monoxide poisoning.
A further and perhaps more serious disadvantage was the effect that prolonged snorting had on morale. At periscope depth, the U-boats were reduced to walking or at best, cycling speed. They lacked an adequate field of view. In daytime, the area seen through the scope was tiny, compared to the vastness of sea that the enemy surface vessels could cover. At night, the U-boats were completely blind, and now the snorkel made them completely deaf as well.
When U-boats had recharged on the surface, at least there were lookouts on the bridge with binoculars, but now, recharging at periscope depth, the only means of keeping touch with the outside world were the hydrophones, now rendered useless by the roar of the diesel engines.