Target Course Calculator

aka The Whiz Wheel

Recently I've started playing Silent Hunter with the TDC set on manual. I went from Sea Ace to Sea Arse in the click of the mouse. Torpedoes flew everywhere - except, of course, at the targets. At first I even had a hard enough time just getting in range!

This little device, made of nothing more than paper and which uses genuine WW II technology (actually, the circular slide rule goes back to the 1629, but that's another story), is designed to calculate the course of a target, using two observations of bearing and range, and with it I've managed to do pretty well.

Using the Whiz Wheel

• Observation 1: Bearing 213 degrees, range 9500 yards.
• Observation 2: Bearing 224 degrees, range 7200 yards, estimated AOB 45 degrees.

Angle on the Bow

• First, we note the difference between the two bearings; 11 degrees.
• As the estimated AOB is 45 degrees, we start by aligning that angle with the first range of 9500 yards (not the second range, as you might think), and check to see that there are 11 degees on the inner wheel between 9500 yards and 7200 yards on the middle wheel. As it turns out, there are 13 degrees in this range.
• After adjusting (fiddling with) the inner wheel, we find that by aligning 40 degrees with 9500, we get the correct number of degrees between 9500 and 7200. 40 degrees is therefore the corrected AOB.

Speed

• Without moving the wheel, read the distance the target has traveled off the middle wheel across from the number of degrees the target has moved. In this example, we read the distance 2820 yards on the middle wheel across from 11 degrees on the inner wheel.

• Next, we align this distance, 2820 yards on the middle wheel with the number of seconds between observations on the outer wheel; say, 5 minutes, or 300 seconds.

• Across from the index (1) on the outer wheel, we read the speed off the middle wheel. In this example, 9.4 yards per second.

• To convert this to knots, align the index (1) on the middle wheel with 5.70 on the outer scale. Read the speed in knots, 5.35, off the outer wheel across from the speed in yards per second, 9.4, on the middle wheel.

Intercept Course

• To plot an intercept course, align the target AOB on the inner wheel with your speed on the middle wheel. A Gato class sub running all head full underwater makes about 6.25 knots, so in this example, aligning 6.25 knots with 40 degrees, we find 33.5 degrees is opposite the target speed of 5.35 knots. You'll have to use your noodle to realize that the target will be off your port bow, so the actual bearing will be 326.5 degrees (360-33.5=326.5).

• You'll know you've got a good intercept when the bearing to the target remains pretty constant. If the bearing remains exactly constant, you're on a collision course. Slow down before you get there or you'll be run down!

Refining Speed calculation

• Adjust your course until the target remains at a constant bearing.
• Align the AOB on the inner wheel with your speed on the middle wheel.
• Read the target speed off the middle wheel opposite the target bearing.

Note: this method is very accurate - so much so that I often skip the above method for estimating speed.

Making the Wheel

It's easy. Just print out the plan, cut it out, and mount the wheels through the center cross hairs on a pin or thumbtack. All the L pieces go on one side, and the C pieces go on the other. (The C side is a Nasmith Director, or course finder, of my own design - described here.)

I mounted mine back-to-back on a piece of cardboard, but you don't have to. For an axle, I used two push pins, one of which I had "depinned" with a pair of pliers.

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