The exercise (as opposed to an operation) involved four aircraft from 617 Squadron lightly modified to carry a boom patrol boat in the bomb bay. This was a light boat whose bow was filled with explosive, the intention being that it was dropped, complete with a crew, who would then motor the boat towards its target, bailing out before it hit and exploded.
The original plan called for the four aircraft to fly down to Exeter where they would take part in the drop just off the coat. Two would head back to Woodhall Spa where the equipment was removed, the other two would land at Exeter Aerodrome for two other crews to repeat the exercise the next day. On the day (January 17th) the four flew down to the south coast in formation and after a check, made their drops from 2000ft, which overall went well. The two subsequent drops were never made and the plan dropped. Incredibly, on each drop, a Royal Marine was aboard each boat during the flight and for the drop, in contact with the crew via the intercom.
In the case of the BPB, however, air-delivery created an added hazard for the pilot. Furthermore, it was accepted that even if he survived the drop and the attack, he would almost certainly be taken prisoner – and there were many cases, including that of the “Cockleshell” raiders, in which captured British commandos had been executed out of hand by the Germans. In air-dropping experiments, more than one BPB prototype with a dummy pilot was smashed to pieces on impact with the sea. Three extra-large parachutes (of 96ft/29.3m flat diameter) were found necessary for safe delivery of the boat: while for the preservation of the pilot a padded cockpit with safety harness had to be added amidships. On landing, the pilot must trigger the quickrelease gear that jettisoned the parachutes, unstrap from the midships position and scramble into the control cockpit aft.
The first manned drop was made on 10 June 1944, when Lt D. Cox. RMBPD, climbed through a hatch leading from the fuselage of an Avro Lancaster bomber to its bomb-bay, where his BPB was suspended in a cradle; strapped himself into the forward cockpit, with a personal parachute which he might have the chance to use if the main ’chutes failed; and was launched by the Lancaster’s bomb-aimer from an altitude of c.5,000ft (1550m) Cox’s successful descent lasted for some five minutes: if he had been dropped by a low-flying bomber over an AA-defended harbour – even at night, as was planned – his chances of survival must have been small.
Late in 1944, six Lancasters of 617 Squadron (the famous “Dambusters”; but at that time sometimes known within the RAF as the “Suicide Squadron” because of the scale of losses incurred in their low-level operations) were fitted out to carry BPBs. Under cover of a conventional bombing raid, it was planned to para-drop the explosive boats into the harbour at Bergen, Norway, to strike at the U-boat pens and depot ships there. First postponed, when two of the modified Lancasters were lost on conventional operations on 12 January 1945, the mission was abandoned when reconnaissance revealed that the German defences had become so strong as to render a BPB sortie both ineffective and truly suicidal.