no need to blow tanks?

Originally Posted by codmander

how come we dont have to blow tanks when surfacing ? at least no compressed air gets used unless you blow em under emergancy than you pop like a ballon to the surface,,, down 200 meters or so the tanks are filled with water and should have to be blown to rise thoughts?

Compressed air was normally NOT used to surface a boat! Propulsion and hydroplanes were used to bring the boat to nearly surfaced and exhaust fumes from the diesel engines removed all remaining water from the diving tanks.

From chapter 4, page 51 of U-boat Commanders and Crews 1935 - 1945 by Jac P Mallmann Showell (isbn 1861261926)

"Many science books suggest that air was blown into the diving tanks to bring a submerged submarine to the surface, which is not quite correct. Diving tanks need to be open to the sea, and water pressure decreases as the boat rises. Therefore, the volumn of air in the diving tanks expands as the boat rises, which will produce an uncontrollable upward motion.

To prevent the boat from coming to the surface in a mass of bubbles like a champagne cork released under water, a careful surfacing procedure had to be maintained, and this usually did NOT involve the pumping of air into the diving tanks. In fact the oposite usually happens: water is pumped out of the tanks as the boat rises to allow for the expanding hull.

Before surfacing it was necessary to determine whether it was safe to do so.......... The next step was to engage the hydroplanes to steer the boat to periscope depth, and then to the surface. The trim would have allowed the upper deck of the casing to come roughly level with the surface of the water, but the boat would have been too heavy to get much higher. At this stage the conning tower, with the ends of the ventilation shafts on the top, was clear of the sea, making it possible to start the diesel engines and to use the exhaust fumes for blowing water out of the diving tanks. This had the advantage of saving compressed air."