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Originally Posted by Nisgeis
Are the fuel ballast tanks actually built as a U shape, with flat sides to the pressure hull?
Oh yeah I almost forgot. The conversion process for Fuel Ballast Tanks to Main Ballast Tanks was to remove some blanking plates - I'm assuming these were over the vents, as you wouldn't be able to replace ones over the flood ports at sea. The question is why have blanking plates - were the vent valves not secure enough?
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Okay, part two.
Fuel ballast tanks are a U-shaped, but the side that is against the pressure hull is not flat. It is contoured to match the curvature of the hull.
The necessity of having long range in the fleet boats really drove the design of the boats. In order to achieve the long ranges needed to conduct Pacific patrols, a vast amount of fuel needed to be carried. The voids between the inner and outer hulls provided excellent storage areas for fuel.
The problem that arose was one of compensation. Fuel oil has weight. As you burn fuel the boat gets lighter. If you burn enough fuel, you would eventually get to the point were you would be so light that you couldn't submerge. So, all the Normal Fuel Oil tanks are compensated with sea water. As fuel is used, water is let into the tanks to make up weight for the used fuel. Fuel floats on water so your fuel suction is at the top of the tank.
But now the exact opposite situation occurs. If all of the fuel tanks were set up like this, eventually you would get so heavy that you may not be able to surface! The solution was ingenious. A portion of your fuel tanks were made into a combination of fuel tank and ballast tank. Once the fuel was used up, you could convert the tank into a Main Ballast Tank which could be blown or flooded like any other MBT.
A Fuel Ballast Tank (FBT) has many of the same features as a MBT. It is shaped the same, it has flood ports at the bottom, and vent valves at the top. It has blowing connections so that it can be blown with high pressure air to surface the boat. Because it carries fuel for part of the voyage, it has some major differences too. Of course it will have suction lines at the top so that fuel can be drawn off as needed. However, unlike an MBT, an FBT has valves at the bottom that will close off the flood ports. This prevents a loss of fuel when the tank is full due to the rolling and pitching of the boat in heavy seas. A FBT also has a sea water compensation system to make up for fuel useage.
Once all the fuel in the tank is used up, converting it to a MBT is fairly simple and can be done at sea by the ship's crew. The flood valves at the bottom are opened and locked in the open position for the remainder of the voyage. Vent valves, which had their operating mechanisms disconnected to prevent accidental opening, are reconnected to the system. The vent valves themselves are located outside the pressure hull in the superstructure. Blank flanges are placed over these vents to prevent them from being unseated during a depth charge attack. If this happened on an MBT it was no big deal. If the vent became unseated on a FBT fuel would leak out through the vents (remember fuel floats on water and is always at the top of the tank). As part of the conversion process, a crewman would have to go into the superstructure (obviously while the boat was on the surface!) and remove these blank flanges from the FBT vents. Once this was done, high pressure air valves for the blowing system were unlocked and made ready for use. The last step in the conversion process was flooding and blowing the tank several times to flush out any residual fuel. Once done, your FBT was now a MBT and was operated in exactly the same way.