View Full Version : Question about Das Boot and real life.

08-05-2006, 11:30 AM
***Spoilers ahead*** Don't read if you haven't seen the movie!

Ok, I have seen the movie a while ago. When the boat sinks, they bring it up by pumping water out with bilge pumps and blowing ballast (I assume, since there was a lot of noise

pressurized air usually makes). So, in order to blow tanks, the air pressure had to be even greater than water pressure outside the boat. In order to contain that pressure, some sort of

container had to be inside the boat. Now, if the shipyard could build a container that could withstand that pressure, what about building boats that could do so without creaking,

leaking and the whole pipe and hull splitting business? Yes, I do know about the deep-diving VIIc/42

08-05-2006, 03:36 PM
Cost? Its not that expensive to build a compressed air cylinder that divers use, make it a bit larger - no problem, but making a large submarine is complicated as u have lots of things poking through its structure - periscope, driveshafts sort of thing.

08-05-2006, 05:17 PM
Good question, I used to be a welder and we had to know how these kinds of things worked. I know with acetylene gas you have an acetone which desolves the gas until use. However, this wouldnt explain the oxygen tanks in WWII. How puzzling. http://forums.ubi.com/images/smilies/16x16_smiley-mad.gif

In reply to #2, a normal cylinder for oxy-acetylene welding is approximately 1 foot in diameter and 4 or 5 feet high with maybe a quarter-inch of thickness. I assume these were pretty close to what they used for storage tanks in WWII maybe slightly bigger. However if you make a cylinder thats 20 feet high and 10 feet wide (a small submarine maybe) and kept the quarter-inch thickness of the same material you would crush or blow up under the same amount of pressure, depending on what you used it for. SO theoretically if they could've made a submarine with ratios all the same as a high-pressure cylinder but much larger it may have worked. However a high-pressure cylinder is made from drawn steel which is touchy as it is and once formed you cannot weld on it, so repairing a submarine made from a high-pressurized cylinder would've been impossible.

08-05-2006, 08:12 PM
Thanks, that makes sense. I still cant comprehend how powerful those pumps must have been http://forums.ubi.com/images/smilies/blink.gif

08-06-2006, 11:56 AM
I brought this subject up in company yesterday and we had a long chat about it... We figured out that the subs must take on just enough water at the surface to sink but keep neutral bouyancy. If they wanted to surface they could just engine their way to the top and then pump the water out of the tanks. Theres no way that they had pumps in WWII that could force the water out at 200 meters under the water. Thats over 20 atmospheres of pressure - thats ALOT of pressure.

08-06-2006, 12:48 PM
There were two different reasons to evacuate water from the submarine. The one we seem to be discussing here is the ballast tanks, which used compressed air to void the tanks. There was also the necessity of removing bilge water, water which inevitably trickled into the boat. This was removed by bilge pumps, which were mechanically operated, sort of like a fuel injector in a car engine, but operating in reverse, that is to eject water from the boat. Because these were machinery, they made noise and were shut down when the U-boat went to silent running. A U-boat could not operate indefinitely in silent running because the seepage of seawater into the boat over time made the sub heavier and caused it to sink. At some point the U-boat would become too heavy to keep at a given depth even with the force of water over the diving planes. At this point the bilge pumps had to be turned on again. But even these pumps had their limit. Below a certain depth they were not strong enough to overcome the external water pressure and could no longer operate. Diving deep was dangerous for this reason, as well as for the danger of the hull becoming crushed by external pressure. Sadly, SH3 does not take this into account, and we can spend hours merrily sailing along submerged in silent running mode without having to consider the seepage water weighing down the boat. Aces of the Deep took this into account; at some point if you were in silent running the chief engineer would inform you that the sub was sinking due to the accumulation of bilge water and would ask for your permission to turn on the bilge pumps. He would also tell you if you dove too deeply for the bilge pumps to work. If you stayed deep for too long, you could lose the boat because it was too heavy to ascend.

Here's something else that may boggle the mind. As the sub dived deeper, due to the contraction of the hull from water pressure, they had to pump water out of the boat, as the smaller volume of the boat made it denser than the surrounding water. When surfacing, the opposite effect was in force, necessitating letting water into the boat, so that as the hull expanded, the sub could become too buoyant and shoot to the surface in an uncontrolled manner and porpoise.

Scuba divers are aware of this principal, as they well know the effects of compression on the human body. There were occasions where the crew had to evacuate a submarine that was stranded on the seabed in water shallow enough that they could leave the sub via the escape hatch. During a pre-war training accident, a U-boat went to the bottom of the Baltic as a result of a collision with a surface vessel. The Kaleun gave instructions to the crew on how to exit the boat. Because the air inside the sub was pressurized, they were told to slowly breathe out as they ascended to the surface. This is because as you ascend from deeper depths, the air in the lungs expands due to less water pressure on the body. Some crew members in their panic held their breath and died because the air in their lungs expanded to the point where their lungs burst.

08-06-2006, 02:50 PM
...if you want to understand more ...read information on BOYLES law...i have had 2 cases of d.c.s =decompression sickness..aka the bends...divers sometimes call it bubble trouble..it's very painfull ..no joke.. http://forums.ubi.com/images/smilies/bigtears.gif

08-06-2006, 03:12 PM
As for the steel hull, it could have been done, but not only cost factors are involved. The sheer weight of a boat, built to handle enormouse pressures would be huge. The outer shell would still be just that - an outer shell. Lets face it, drowning is similar to suffocating, the hull not being ruptured & being at such a great depth just means you'd lay there waiting for the air to run out