Hi Siara,
Whilst looking for some ref for you as regards the saddle tank flood/blow vents i noticed in this photo that the flood drain holes on the hull side differ to what you may have been led to believe. It may not be a problem for you, but i thought i'd give you the heads up on it whilst you can still do something about it.
There are more photo's here
http://www.u552.de/ hope this helps if you've not seen them already.
Sav.