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Light Stuff

20 mm "Doorknocker

Lightest anti air weapon on board. The noise of these firing is said to have been the most feared sound on the ship. If these are firing, the bad guys are within a mile of the ship. Called the "Doorknocker" because they politely let the incoming aircraft know the ship sees them, but are too light a weapon to do serious damage. It was also called a "revenge weapon." It could not stop an aircraft from delivering a bomb or torpedo, but might damage the aircraft that delivered it sufficiently that it couldn't reach it's base. Revenge weapons were considered worth while early in the war. Japanese kamikaze tactics increased the desire for heavier anti-air.

AA Director
Anti Air gun director, Mk 52

Used to coordinate the quad 40s. The quad 40 mounts have two gunner's chairs on each mount. One gunner could manually crank the vertical position of the gun, the other the horizontal. If the automated tracking systems were down, the guns were fired this way. However, usually the quad 40s were fired from a gun director, such as the one shown above. A targeting pip was projected on an early version of a heads up display. If the operator pivoted the director to keep the pip on the aircraft, several nearby quad 40 mounts would in theory be on target.

This director, however, is not in good condition.

The same director, with more paint.

I returned several years later to find the maintenance crew had visited this director. It had also been turned well to the right. One can better see the radar dish. This is a Mk 52 gun director, the heavier and more complex of the two models used on the to direct the quad 40s. Direction is determined using an optical site mounted on top. The radar determines the range.

Mk 51, optical only director.

This is the lighter version, the Mk 51. No radar dish. The Mk 51s are mostly mounted on the deck level, protected by splinter shields. The Mk 52s are generally mounted higher on the ship, and note how low the splinter shield is on the above Mk 52 picture. I'm not sure just how much microwave energy was being generated by the Mk 52's radar, but bouncing it off a splinter shield seems undesirable.

The optical site of a Mk 51

While this display has been yellowed by time and weather, it reminds me of the reflective gun site mounted in the Sky Warrior's T-34 Mentor trainer aircraft. (See Flight by Committee.) The glass seen just inside the site is mounted at a 45 degree angle to the bore. Images are projected onto the glass from below. These images are visible only if the operator's eye is directly aligned with the center line of the site. The target pip would be projected to generate the proper lead to hit the target.

Quad 40 tub left, director tub right.
Near each quad 40 is a Mk 51 or Mk 52

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