Tour 9/22/2007, 40mm Bofors

Another beautiful fall day, not too hot and not too cold, and perfectly clear. Tour-wise it was pretty slow day. We had plenty of guides and only a few visitors. I only gave one tour to a small group. From a visitor's perspective a small group is great. You get to go at your own pace and get details on any topic you want. After six years I can answer a lot of questions, but I still have a lot to learn.

During the tour, while sitting on a 40mm gun mount (trainer position), a young visitor pointed to an instrument dial and asked what it was for? I was stumped (while on active duty I worked on 5" mounts and had limited exposure to 40mm mounts). I asked a few other Slater volunteers, and they appeared to be stumped as well. Finally, Eric Rivet (Education Coordinator) told me it was used to measure the deviation that occasionally occurred between the Mark 51 Fire Control Director and the 40mm Gun mount.

Each 40mm could be aimed automatically by a Mark 51 Fire Control Director. The pointer (the sailor who sat on the left side of the mount and moved the gun up and down) and the trainer (the sailor who sat on the right side of the mount and moved the gun right and left) would raise a control lever to the remote position and the 40mm mount would track automatically based on input from the fire control director. It appears that sometimes the 40 mm mount would lag from the intended aiming position that was transmitted to the mount by the Mark 51 Fire Control System. In extreme cases the mount could drop the signal altogether. The gauge in question would measure the lag and would be monitored by the pointer and trainer for obvious safety reasons.


Links:

Wikipedia 40mm Bofors
USS Slater 40mm Page
DE 220, Destroyer Escort Fire Control
Navy Weapons - Mark 51 FCS
Navy Weapons 40mm Bofors
Swedish web site regarding Bofors
Photo Site - source of 40mm Gage Picture

On the next week, 9/27 I managed to do a little more research. The proper name for the instrument is a 40mm lag meter. This text is from the WWII-era 40mm technical manual (page 161):

"The lag meter is primarily a milliammeter which operates in automatic control to indicate if the gun is in or out of synchronism with the director (i.e., the error). The pointer and trainer both have lag meter units (as shown on figure 130). Only early mods have the rectifier circuit.

If the pointer's meter reads to the left of zero, it indicates that the gun is elevated higher than the gun order. Conversely, if the meter reads to the right of zero it indicated gun elevation is less than gun order. When the gun position corresponds exactly with gun order, the pointer of the lag meter is stationary at the zero mark. The meter pointer may be adjusted for zero position by inserting a screw driver through an access hole in the case and turning the zero adjuster screw. ......."

This weekend I also had an opportunity to talk with John - Mechanic For Hire, the other USS Slater Blogger. It seems like his next project may be a cut away depth charge detailing the firing and safety mechanisms. This would be a great prop and it would add a new dimension to the tours. I'm looking forward to this addition.