SUBSIM Radio Room Forums - [rel] SlideRule MOD(β)

Here is one way to utilize a Slide Ruler. Puster Bill wrote this some time ago, and its brilliant. I belive you can find the original post with images by searching for Slide Rule Targeting using the search tool for the forum.

Slide Rule Targeting

For Silent Hunter III / IV

Did you ever want to increase your game immersion, and your accuracy, without using modern technology? I certainly did when I got the GWX upgrade for SHIII. I had been using this wheel for targeting:

It worked well enough, but it took me ‘out of the game’ because it looks like it was printed on an old dot matrix printer.

After trying to make a couple of slide rules (and succeeding, somewhat), I decide to just go ahead and buy one. I checked some local antique shops until I found this:

A Sterling “Precision” slide rule. I put the word precision in quotes because Sterling has just about the worst reputation for accuracy. However, that doesn’t really matter for the job we have to do, and I was able to get it for $5. A good condition, more accurate, and more authentic slide rule would have cost a lot more. No matter whether you buy one or make one, it has to have the C, D, and S scales on it. It would also help if it has the A and B scales (just about every rule does). Any rule that has the word ‘trig’ in the name will work fine.

Anyway, down to the business of sinking ships…

In order to get the information that you need, you will have to make two observations. Let us call them A and B. We need to know the distance to the target and the bearing for both observations, and the time between those observations.

Let us suppose that the distance for our first observation is 7000 (don’t worry about whether it is meters or yards yet), on a bearing of 65 degrees. Start the stopwatch, lower your scope, and go get a cup of coffee, take a leak, apologize to your wife, or whatever. Let’s just say you missed the opportunity to use the 3 Minute and 15 Second rule to get target speed, so your next observation, B, occurs at 4 minutes. When you take the B observation you get 4000 yards at a bearing of 45 degrees.

I write this stuff down on paper, and this is what it would look like:

O RNGE BRG AOB TIME (seconds)
A 7000 065 ??? 0
B 4000 045 ??? 240

You now have everything you need to determine AOB and speed of the target. We whip out our handy-dandy slipstick, and note that there are 20 degrees between 65 degrees and 45 degrees. I move the slide until there is 20 degrees difference on the S between the 4 and the 7 on the S scale. I have to use the A scale on this particular rule, as it has a ‘double length’ S scale that goes from about 45’ to 90 degrees. Most rules have a regular S scale that goes from a little under 6 degrees to 90 degrees, and an ST scale to handle the smaller numbers. If your slide rule has a 'regular' S scale, you have to use the D scale instead of the A scale.

You will note here that it works out that when 23 degrees on the S scale is under the 4 (for 4000) on the A scale;

That 43 degrees on the S scale falls directly underneath the 7 (for 7000) on the A scale:

We now have our two AOB observations. Strangely enough, the AOB for observation A is actually under the distance for observation B, and vice versa. We then fill the information in on our paper:

O RNGE BRG AOB TIME (seconds)
A 7000 065 023 0
B 4000 045 043 240

If you are pretty good at guessing the AOB, align your guess of the AOB from observation B on the S scale with the distance from observation A on the A (or D) scale. That should save you some time fiddling around, as you are close to where it should be.

Now that we have the range and AOB, we have to figure the target speed to input it into the Vorhaltrechner/TDC, as the case may be. As a side note, I usually pause the game right after the second observation so that I can make the calculations. On a real submarine, the captain wouldn’t be making the calculations, so I am in effect doing two jobs! You don’t have to, though. If you leave the periscope at the heading of the last observation, you will be able to enter all the information, then swing the ‘scope back on target, with the information all updated by the TDC.

To calculate the target speed in units per second, we need to find the distance the target has traveled between observation A and observation B. Without moving the slide, we move the cursor of the slide rule down the S scale until it is right above the difference in degrees between the bearings of observation A and observation B. For this example, that is 20 degrees. We read the speed off of the A scale (or D scale, if you have a ‘normal’ S scale). In our case, it reads 3.5, which we know will be 3500 units (either meters or yards):

We now have to find out how fast that is in units per second. On this particular rule, I have to flip the slide over the get the B and C scales:

To find the target speed, we have to align the ‘1’ on the B or C scale with 3.5 on the A or D scale. I generally use the A/B scales, as they double, so that I don’t have to worry about the result ‘falling off the rule’, and we don’t need the greater precision of the C/D scales for this work. It should look like this:

Notice that the 3.5 on the B scale is under the middle 1 on the A scale. We then slide the cursor over to the 2.4 on the B scale (standing for the 240 seconds between observations. This gives us a result of about 6.85 units per second. I usually round decimals, so lets call it 6.9 units per second.

Now, to find the target speed in knots we have to know whether our distance unit is a yard or a meter. If the distance measurement is in yards, you have to align 5.6 on the B scale with 1 on the A scale:

You then slide the cursor so that is over 6.9 on the B scale, and you read the speed in knots on the A scale, in this case about 12.15 knots.

If the distance unit you used was meters, you have to align 5.1 on the B scale with the 1 on the A scale:

Again, you slide the cursor so that it is over the 6.9 on the B scale, and read the target speed on the A scale, in this case about 13.5 knots.

You can now plug the distance, bearing, AOB, and speed into the TDC in manual mode. Don’t forget to switch it back to Auto!

Happy Hunting,

Puster Bill