JBDs (Jet Blast Deflectors) were installed on British carriers from the mid to late 50s onwards as Steam Catapults were fitted, and the original pattern was for a steel plate that could be raised and retracted hydraulically behind the catapult to protect deck crew from jet blast. Aircraft that were positioned on the catapult would throttle up to full power once the holdback and strop were secured, and although early RN jets such as the Sea Hawk and Sea Venom could be launched from carriers without JBDs (and with hydraulic cats), the incoming generation such as the Scimitar and Sea Vixen were much more powerful and their jet blast would cause great disruption on deck. The steel plate would raise to an angle of about 45 degrees, and would also be angled at about 30-45 degrees off the axis of the catapult so that the jet blast would be deflected upwards and off to the side of the ship.
Hermes, Centaur and Ark Royal had JBDs angled to port and starboard, on the latter two ships they were placed either side of the forward lift. Hermes had a deck edge lift so they were not 'squeezed'. Victorious had both her JBDs angled to port as the starbord one was very clos to the bridge, and they were both placed abaft the forward lift as well. Eagle, post her 59-64 MLU had both cats on the port side so both JBDs were angled to port logically. Hermes introduced new three part JBDs after her 64-66 upgrade which also gave her a new longer port catapult. The main JBD plate was now at 90 degrees to the cat axis as in US practice, but also had two smaller side plates at 45 degrees to 'scoop' the jet blast (the starboard cat only had a single side plate to port as the starbord side was close to the deck edge. Ark Royal's final JBDs post 67-70 were a further development of this idea, to cope with the extreme heat of the Phantoms which launched in full afterburner, her JBDs were water cooled (as well as the deck plates immediately in front of them) and the main plate was split into two that were angled inwards to contain the jet blast. This pattern had been originally designed for CVA-01 which was also intended to operate Phantoms.
When Eagle carried out Phantom trials in 68 only her waist cat was used (it was about 50 ft longer stroke than the forward cat, as in Ark Royal 70-78) and because her JBDs were not water cooled they were not used. The flight deck behind the cat was kept clear of personnel and parked aircraft during launches. The deck itself needed protection during these trials, so a thick steel plate was chained to flight deck immediately in front of the JBD to absorb the heat. The plate was observed to glow white hot, and after launch fire hoses were used to cool it down before the next aircraft could taxi onto the cat. Eagle had been due to recieve upgrades for Phantom ops including water cooled JBDs, bridle catchers and DAX II arrestor wires (a single DAX II wire was fitted in no 3 position for the trials, and remained with her to the end) during her 1968 refit, but this was cancelled to prevent the RN from keeping her in service beyond 1972. Her original paying off date after her five year rebuild would have been around 1984...
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