Thanks to all the folk who've helped me out on my enquiry regarding the intervals on the skeletal frame of the hull and saddle tank of the VIICs. Unfortunately one answer seems to raise more questions.

The majority of model builds I have seen, indicate that the oil canning effect on the saddle tank was two frames apart. Unless my eyesight is really bad photos of the saddle tanks of the U-555 seem to indicate that they were one width apart in line with the skeletal structure

Can anyone shed light on why we have these differences and tell me which is correct? Please help stop me from going mad and allow me to start building my u-boat!!!!!