Hello Maciek and Simon,
Maciek is 100% correct about the different thickness of sheet metal on the saddle tanks:
1. R1, R2, and Q Tank outter sheet metal is 11.5 mm thick
2. FBT 2 and FBT 4 outter sheet metal is 6 mm thick
3. The internal walls of Q Tank are 13 mm thick
However, let's examine the sunken Type VII U-Boat in question...
1. I would say the missing sheet metal cover on the port side of FBT 4 and Q Tank was the result of a depth explosion and not the effects of salt water corrosion over time. The reason for my conclusion is there are no internal support structures; they seem to be torn-off from the pressure hull and saddle tank.
2. If you look aft, the saddle tank FBT 2 port side, it looks to be intact.
3. If you look at the attached drawing, Q Tank was designed to withstand internal pressure because of the aft concave and forward convex sides.
4. When the depth charge detonated; the port FBT 4 was ripped open and the forward wall of the Q Tank was designed for the opposite stress forces. Therefore, the port Q Tank was ripped open and exposed the mushroom valve actuating mechanism. Q Tank has 1 internal rib type support structure mid way in the tank, but I highly doubt that it would hold under such devastating pressure.
5. I conclude that both FBT 4 and Q-Tank on the port side were completely blown apart and the complete devastation stopped at the aft Q Tank internal wall which was designed to withstand a force in that direction.
Now as to that rectangle hole in question - I don't know? Did I go wrong on my conclusions as to what we see on this sunken U-Boat? Please advise...
Regards,
Don
PS - after looking at mt trusty steel ruler with inches on one side and mm on the other side; we're not talking about sheet metal. The 6 mm plate steel for FBT 2 and FBT 4 is a little under a 1/4 inch, and the 11.5 mm plate steel for R1, R2, and Q Tank is a little over 7/8 inch steel. This plating would have to be rolled to fit the framing. You are not going to bend that into place...
