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Originally Posted by Nisgeis
Hi Dave,
Do you know what shape the Main Ballast Tanks No. 1 and No.7 that are part of the forward and after torpedo rooms were internally? The drawings in the fleet boat manual has them shown as a U shaped tank, with the outer sides being rounded and the top and middle sections being flat plates, the upper two parts of the U forming the deck of the torpedo rooms. This doesn't seem like a very good shape to withstand pressure, from an engineering point of view as they have large flat surfaces and sharp angles in the corners. There is no signs of any extra thickness in the pressure hull at the top/deck of the torpedo room to compensate for this shape.
Are the fuel ballast tanks actually built as a U shape, with flat sides to the pressure hull?
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Wow! Will asked a pretty good question that I will have to answer in a couple of parts. Here is part one:
On the Gato and Balao class boats, the pressure hull was not a perfect cylinder. In order to accomodate the necessary tankage in the forward and after ends of the boat, the designers were forced to radically alter the shape of the pressure hull to accomodate the tankage. Thus, the pressure hull had several kinks in it at the ends. Obviously this was not an optimal situation from an engineering point of view, but it still worked pretty damn well.
MBT #1 was u-shaped and this was done to create a "pit" under a removeable deck in the forward torpedo room. This allowed access to the lowest torpedo stowage racks and provided some stowage for torpedo handling gear. When you were in the pit, you were standing on the pressure hull. MBT #7 was flat topped.
All main ballast tanks are always either completely full or completely empty. Thus, their structure on three side does not have to resist sea pressure. Only the fourth side does and this will be part of the pressure hull. In the case of MBT's 1 & 7, the pressure hull portion of the tank formed the deck you walked on in the torpedo rooms. In other words, the top of the tank was also the bottom of the pressure hull. All other MBT's were in the void space between the inner pressure hull and the outer hydrodynamic hull amidships. A cross section of the hull would make these tanks appear like giant U's, but in reality they were divided into two separate tanks at the keel.
Open flood ports at the bottom let water into the tanks when vents were opened at the top, allowing air that was holding the water back to escape. MBT's #1 & 7 were located under the pressure hull and did not extend up the sides of the hull like the other MBT's. The problem here is that there was no place to put the vents! The solution was to attach large vertical pipes (called risers) to the outboard sides of the tank
inside both torpedo rooms. The risers ran upwards through the rooms and out of the top of the pressure hull into the superstucture. Vent valves were placed at the top end of these riser pipes. Once the vents were opened, the air in the tanks would vent through the risers and the tanks would flood.
This was not an optimal design because when the boat was submerged, you had full sea pressure inside these riser pipes, which were
inside the pressure hull in the torpedo rooms! Any damage at all to these risers from depth charge attack and you had a big flooding problem really fast (water in the "people tank" = very bad).
As with any problem, there was a solution. The subsequent Tench class boats had their tankage re-arranged. MBT #1 was moved forward so that it could vent directly overhead, eliminating the risers from the forward torpedo room. MBT #7 was found to be redundant and was converted to a variable fuel oil tank.
For a visual representation of what I am refering to click here:
http://www.maritime.org/fleetsub/app...es/figa-04.htm
And here:
http://www.maritime.org/fleetsub/chap4.htm#4A
Scroll down to figure 4-1 and you will get an idea of what I am talking about.