IX-B and IX-C
But in 1943-1945 is a big problem stay in well protected areas and convoy battles are very difficult because Air cover make it near impossible , survive is not easy.
The wolf is now a vulture looking for food.
Stay in deck awash is a good advice, despite the fuel and speed reduction. because planes can jump over your back everytime and everyplace.
In the late years you need snorkel and radar detectors or go to far patrol grids, like africa or caribbean.