Hi Geoff,
That makes sense.
I was thinking more on the u-boat's Schnorchel to be honest, I read they were a constant problem, the diesels when running on the surface would take air direct through the sub.
The intake pipe did not lead directly to the diesel engines. This was to prevent unnecessary stoppages as it was common for the intake valve to open and shut as the ocean waves swept over the snorkel head, periodically submerging it. Instead, the intake pipe led to the inside of the boat, providing breathing air for the crew as well as the engines. The engines would draw air from the interior of the boat as usual, where the entire volume of the boat itself acted as an air buffer. In the event that the valve was suddenly closed, the diesels could run on for a short time, but this cannot be sustained for long. If the air supply was lost, the diesels had to be immediately disengaged to prevent the entire internal air from being extracted. Obviously, this would have catastrophic consequences for the crew. Thus, during the initial installations, the air supply was constantly monitored in order to stop the diesels whenever the valve closed, but this was later automated.
Apologies, it's not quite the topic Cowl Vents...