What was the low-pressure blower?

The low-pressure blower was also located in the pump room. It took air from inside the boat, compressed it to about 10 PSI and dumped it into the ballast tanks. The manifold was in the control room just aft of the high-pressure air manifold. This allowed them to save some of the high-pressure air used to surface. The process took some of the stale air from inside the boat and dumped it into the ballast tanks, which started refreshing the air in the boat.

The low-pressure blower was usually run until they saw air bubbles coming up from most or all ballast tanks. This didn’t cost any high-pressure air and ensured that the boat was fully surfaced. When the ballast tanks were empty, the boat rode more efficiently on the surface.

As indicated above, it was possible to surface the boat using only the planes and the low-pressure blower.