2010/04/23 - 1st Tour of the 2010 Season

From DE Docent 5K Walk

I gave my 1st tour of the season. The tour was on the East shore of the river. The construction crew was putting the finishing touches on the new mooring, so the ship was delayed in moving from their winter berth (on the Rensselaer side of the Hudson River).

The group was a composed of Engineering Students, so I had an opportunity to discuss the mathematics of Naval Fire Control. It appeared that they had trouble with transportation earlier. When the bus appeared they were anxious to get aboard before the first group. I rushed the final three stops and got them on the bus. Jack Madden's group lost the race (I don't think they realized there was one), but Eric gave them they got a tour of the Motor Room and Engine Room while they waited for the shuttle bus to return.

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Regarding math and Naval Fire Control here is a good example of the mathematics involved: ".... A differential equation is necessary to solve the problem of predicting the location of two maneuvering ships because of the changes of course they can make; only in an ideal situation will both of them be moving in a straight line.

Consider the following example. A firing ship is steaming course 000 (North) at 12 knots. Its target is steaming course 315 (Northwest) at 12 knots. Initially, the relative bearing to the target ship is 090 (East) and the range 10,000 yards, just shy of 5 nautical miles (4.937 nautical miles, to be more exact).

If neither ship turns, then their relative positions five minutes later may be calculated with trigonometry. The firing ship will have moved 1 nautical mile along course 000 and the target will have moved 1 nautical mile along course 315. We know that it has therefore moved 0.707 (sin(45) = 0.707) nautical miles along course 270 (West) and 0.707 (cos(45) = 0.707) nautical miles along course 000. Therefore, the distance between the firing ship’s track and the target ship has decreased to 4.230 nautical miles and it has fallen behind by 0.293 nautical miles. These distances form the two sides of a right triangle, with the hypotenuse being the linear distance between the two ships.

Trigonometry and the Pythagorean theorem again allow us to determine the length of this hypotenuse and the angle of bearing of the target ship This distance is 4.240 nautical miles ((0.293)2 + (4.230)2 = (4.240)2), and the angle is 86 degrees (arctan (4.230/0.293) = 86). As this is the angle between the firing ship’s track and the target, which is now slightly astern, the relative bearing becomes 094. The distance converts to 8,588 yards.

This simplest of examples illustrates the basic principles involved in plotting the movement of two ships. A significantly greater magnitude of complexity would be introduced if the target ship were turning to come onto a parallel course to the firing ship An accurate calculation would then require a differential equation. Effectively an infinite series of trigonometric calculations like the example above would have to be calculated as the target ship traced through the arc of its turn. ...."

A few more relevant links:

• Naval Fire Control Systems
• USS Slater 3" Fire Control (Mk52)
• USS Slater Flying Bridge Layout
• US Navy Battle Doctrine between WWI - WWII
  
• Training Video - Fire Control, Part 1
• Training Video - Fire Control, Part 2
• Mathematical Discussion of Range Keeping
  

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.