Bouncing bombs and U-boat pen entrances

Why was sending bouncing bombs inside U-boat pens difficult?

Small entrances and bends in the approaches made it hard to align a bomb for a direct entry.

A bouncing bomb would need accurate alignment to enter a U-boat pen. The openings were comparatively small, and their approaches usually included a bend. Those features made a direct run difficult. No operation is identified for the claim that attempts were made.