Throughout the Second World War, the US Navy sought a solution for the problem of night time aerial attacks as well as the improvement of anti-aircraft weapon accuracy overall. Early attemps, such as the Mk 49 director, were not entirely successful, and the Mk 51, being strictly a visual system, did not provide adequate capabilities at night or in conditions of low visibility. Work on improved systems continued, and by January 1945, the Mk 63 Gun Fire Control System had been completed and its installation authorized for use with twin and quad 40mm gun mounts.

The Mk 63 GFC consisted of two exterior elements; the Mk. 51 Mod 6 director and Radar equipment Mk 28 Mod 2. The Mk 51 Mod 6 was the same chassis as earlier Mk 51s, but with the Gun Sight Mark 15 Mod 12. The Mk 28 Mod 2 radar was not mounted to the director, such as in the Mk 57 director, but to a 40mm gun mount itself. One Mk 63 could be used to control multiple mounts, with the understanding that the farther away from the actual gun mount the director was, the more probable parralax error would occur (Parralax error is an angular error that happens when two devices are sighting on the same object from different points. In the case of a gun director, it would cause the shells to consistantly be "off" from the target).

As mentioned earlier, the Mk 63 Gun Fire Control System was primarily designed or 40mm gun mounts, but it was also authorized for shipyards to wire them to also control 5" mounts or even 3" mounts if time permitted. Limitations were placed on this, however, and they were not meant to be used as the primary director, only a back up, as the trajectories of the various shells were different and parallax error was probable

Information for Modelers:

The modeler should be aware that the Mk 51 should always be pointing the same direction as the mounts they were directing, as the two were supposed to be aimed on a parallel line and were in fact calibrated to each other. It was possible for the radar on the gun mount to point "offset" from the gun, as it was gimbled so that it could offset to compensate for taget motion.

Galleries
Drawings
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The Mark 51 Mod 6 director was similar in many respects to earlier Mark 51s. The earliest models lacked the computerised gun sight and used ring gun sights. This unit also includes the Range Receiver, Mark 2, Mod 0, to allow it to receve informationf from the gun-mounted radar.

Crew shot useful for placing figures on models or dioramas. Documentation called out for a seven-foot working area for the director and crew, but permitted the use of six-foot working areas if space was an issue. This works out to a XX" diameter tub, for those that want to scratchbuild their own.

This image demonstrates that the radar could turn independently of the gun but that the director itself was parallel. Presumably when not active the radar would be "caged" to prevent damage and wear to the servos.

This image ties together all of what this Gun Fire Contol System was doing; compensating for target speed, wind, bullet drop, in a manner that was efficient and accurate for gun crews.

Rough wiring diagram for CV-16 Lexington in May 1945 showing the Mk 63 systems installation. Note that not all quad 40mms had Mk 63s. Note too, that there were interconnects for the Mk 63 to control other mounts, but this would not have been a regular procedure due to parralax error, and only would have been performed if the regular director for the mount was out of service.
Sources:
1) "Operating Instructions, Gun Fire Control System Mark 63" USN, March, 1945
2) C-S71-3(665fl) "Installation of Gun Fire Control System - Mark 63" dated 29 January, 1945