'One of the most critical parameters for delivering Upkeep was to release it at precisely the right distance from the target. It needed to “bounce” three times, each contact with the water’s surface allowing the backspin of the weapon to retard its forward velocity.

If released too late (inside 425yd/388.6m) when it hit the dam wall the impact forces could destroy the device or cause it to “bounce off” the vertical surface instead of “rolling down” the surface – one key to effective destructive force was physical adherence to the dam wall. If released too early (outside 475yd/434.3m) the device tended to begin veering to the left which then made accurately hitting the centre of the dam – another requirement for effective use of its destructive force – problematic.

Therefore accurate determination of range from the dam wall was a critical parameter – one that was met with a rather simple solution. This was provided by Wg Cdr Charles L. Dann, the Supervisor of Aeronautics at A&AEE Boscombe Down. Since distance between the Möhne’s sluice towers was known, as was optimum release distance, it became a rather simple trigonometry problem. The solution was a small wooden hand-held Y-shaped angular sight. The ‘Dann bombsight’ was held by a thick dowel mounted beneath its junction, a peephole was mounted in the base of the stem and a small white dowel projected from the end of each arm, angled apart 29.5 degrees. As the Lancaster approached the target the sluice towers would begin to spread and when they touched the dowels, the weapon was released.'