Torpedo Time-to-Target Calculator
by Bill Johns

Two big, fat targets dead ahead, and I'm in the perfect firing position. Unfortunately, there is a destroyer closing fast. He hasn't detected me yet, but he will at any second. Better get the torpedoes off quick.

I line up on the distant target first, naturally, and fire off three fish. Then three more at the closer target. Scope down. 45 seconds latter comes the satisfying rumble - "Target destroyed, sir!" - scratch one tanker. But what about the other tanker? Up scope - it's turning away, scared off by the sinking of it's sister ship. The torpedoes miss.

How often has that happened to you? If only there was a fast and easy way to make all the torpedoes strike simultaneously, so there is no warning at all...

You could do the math in your head - a Mark 14 travels at 46 knots, one knot is 0.562 yards per second, the more distant target is 1214 yards further away... uh... Well, maybe if you're an idiot savant. But when things are happening fast and the destroyers are closing in, there isn't much time to fiddle around, so I created this little thingamabob (a highly technical term if ever there was one) to do the math for me. You can make one too: it's easy, dirt cheap, and uses authentic WW II technology (it is essentially a slide rule).

HOW TO USE IT

There are just three simple steps.

1. On the Range (bottom) scale, line up the range of the closer target with the range of the more distant target,

2. Read the delay off the scale for the proper torpedo type.

3. Wait that long after shooting at the distant target before shooting at the closer target.

For example: Let's say the closer target is 1100 yards away, and the more distant target is 2000 yards away. In the above image, you'll see that the number 1100 in the Range (bottom) window is lined up with the 2000 underneath the window. Looking at the window labeled M-14, we see that the pointer is pointing at about 35 seconds. If we were firing a Mark 10 torpedo, the delay would be read of the M-10 window, 45 seconds, and the electric Mark 18 torpedo would be read off the M-18 window, about 48 seconds.

There is one thing to remember - the distances used should be to the point of impact, and that is not always easy to figure out. But if you just use the distances given by the TDC, you'll usually do all right, since the targets are usually traveling in formation and their positions relative to each other (and thus the difference in distance) aren't changing much. However, if one target is moving toward you, and the other moving away, the calculator may be way off.

It can also be used for three or more targets, just by calculating the delay between adjacent targets. For a single target, it can be used to calculate the time-to-target directly, just by using zero as the distance to the closer target, but this may not be accurate unless you estimate the point-of-impact correctly.

Using this contraption, I routinely hit two ships within two or three seconds of each other.

MASTHEAD HEIGHT RANGE CALCULATOR

Fine, you can now time your torpedo attack for simultaneous hits. But to do so you need the range to your targets - and how do you get that?

You could simply use the TDC on auto, but for a much more realistic flavor, I've used the Procedures for Manual Ranging described by Frank Kulick to calculate range based on masthead height. It seems to work pretty well, but again it involves more math than I can do in my head quickly, so I added a range calculator to my thingamabob (there is that word again).

HOW TO USE IT

There are just three simple steps.

1. Identify the target and look up the masthead height on the table below.

2. Measure the height of the ship through the periscope.

3. Plug those numbers into the calculator and read off the range.

For example: If the target is a standard tanker, the masthead height is 120 feet. Looking through the scope, I measure the masthead height to be 1.25 divisions. In the upper window on the calculator, I align 1.25 in the window with 120 under the window, and read the range indicated by the arrow in the bottom window. If I am at 4x magnification, I read it off the arrow to the right (labeled 4x), and if at 1x I read it off the left (1x) arrow. In this case, at 1x the range would be about 1900 yards, and if at 4x, about 7600 yards.

Between 100 and 1000 yards, the divisions are in 100 yards. Between 1000, and 10000, the divisions are 1000 yards. You'll have to estimate where in between, but an estimate is good enough, since the periscope reading is none too precise anyway.

Make sure to read the notes from Frank Kulick's tutorial.

HOW TO MAKE IT

That is easy, too.

1. Download the plan.

2. Print it, preferably on stiff paper.

3. Cut it out.

4. Cut out the windows (use a VERY sharp knife and a ruler).

5. Fold the frame (top part) along the solid horizontal lines, and tape.

6. Fold the slide (bottom) along the solid horizontal lines.

7. Slide the slide into the frame.

HAPPY HUNTING!

And that's it, my little contribution to the world of Silent Hunter (or any other sub game, really). Hope you like it.

–Bill

P.S. Comments can be directed to [email protected]. I am especially looking for a better name for this than a "thingamabob."