# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Traditional navigation by slide rule**

**From:**Paul Hirose

**Date:**2016 Oct 18, 01:52 -0700

On 2016-10-12 11:59, Bob Goethe wrote: > > I am attaching the current draft of the user manual. I welcome feedback, relating both to substance as well as presentation. In the time / speed / distance problems, I think you are working way too hard. It's easier if you forget about multiplication, division, and formulas. Instead, think in terms of distance on D and time on C. You always know one distance and the corresponding time. When you bring them together, all distance / time pairs on C and D are equally valid. For example, solve for distance traveled in 5h 27m at 6.1 knots. Since 6.1 knots is 6.1 miles in 60 minutes, set 60 minutes on C to 6.1 miles on D. Now observe 12.2 miles on D at 120 minutes on C, and 3.05 miles on D at 30 minutes on C. In fact, all possible combinations of time and distance at 6.1 knots are visible simultaneously on the rule. (Well, almost. You have to exchange C indices to read them all.) Thus we need only find 327 minutes (= 5h 27m) on C and read 33.2 miles on D. Let's try a very different problem. Suppose a race car takes 40 seconds per lap on a 2.5 mile speedway. What is its average speed in mph? Set 40 seconds on C to 2.5 miles on D. Read 225 miles on D opposite 3600 seconds on C. The answer is 225 mph. Sometimes the answer is off the scale. For example, how long does it take to drive 12 miles at 75 mph? Set 60 minutes on C opposite 75 miles on D. Find 12 miles on D. Oops, the slide is too far to the right for a reading on C. So set the cursor to the C index, then slide the other C index to the cursor. Now you can read 9.6 minutes opposite 12 miles. What I outlined is the standard algorithm for time, speed, or distance on an E-6B aviation slide rule. Since it's circular, an "off-scale" never happens. To make life even easier, the E-6B has a large black arrow at the 60 minute mark on the disc, since in most cases the minute is the most convenient unit of time. There's also a small arrow at 3600 seconds. Furthermore, an auxiliary scale just inside the main (outer) scale on the rotating disc is graduated in hours and minutes. For example, 5:00 lines up with 300 minutes on the main scale. This helps in cases like the first example, where I had to mentally convert 5h 27m to 327 minutes. It's equally true that the auxiliary scale is minutes and seconds when the main scale is seconds. As with the C and D scales on a straight slide rule, the units are whatever you want them to be. For unit conversions on the E-6B, you set, for example, the KM gauge mark on the disc opposite the NAUT or STAT mark on the body. This makes it easy to remember which unit goes with which scale. A similar aviation computer is the CR family. Their time / speed / distance scales are the same as the E-6B, but the CR is much smaller due to a totally different method for solving wind problems. Incidentally, either type of computer can solve current (set and drift) problems. The 500 knot mark can just as easily stand for 5.00 knots.