NavList:

Murray B, you wrote:
"Frank is just winding us up by asking how it works." 

Absolutely not. I thought it would be easy to understand, but I did not see it that way when I looked a little closer. So I decided to see if anyone else could explain it. And no, "here's a youtube video" is not a successful attempt. If you can't explain how it works in a couple of paragraphs, then arguably, you don't understand it yourself.

The underlying geometric principle has to be something quite simple, of course, but then why not just use a sextant? Why this very odd device? If I have a sextant, and I want to know the distance to some vessel with a known mast-height (or any size of any feature that is relatively perpendicular to my line of sight at its "base", then I can measure the angle it subtends with my sextant. And if that angle is smaller than about 5°, I don't even need to bother with any triangle and trig functions. For those cases with angles less than 5°, I can use
   angle (m.o.a.) /3438 = size / distance,
or
   distance = size · 3438 / angle(m.o.a.).
Note that "angle(m.o.a.)" here means the angle in minutes of arc, and also note that distance off will be in the same units as the size of the mast or other feature. Feet yield feet; meters yield meters. But expressing this basic geometric relationship (or its very slightly more difficult triggy version for larger angles) does not mean that I understand --or anyone else here understands-- this stadimeter. Yes, it resembles a sextant, and yes, the underlying geometry has to be similar, but how does it work?

There is another distinct geometric principle that we can apply to to measure distance with a sextant. I think Alex E. alluded to it, and it is essentially identical to the range-finder principle in Gary L.'s tank. That works by instrument parallax between the two lines of sight in the sextant. For sextants of normal size, we're dealing with very small angles and low accuracy, but it works. The big advantage with this secondary range-finding principle is that it does not depend on any known size of the target, and it requires no re-adjustment for different targets in the same field. All ships in a convoy, for example, would show the same angular offset with a parallax-based range finder at the same distance --instantly. The math is essentially the same as above, but in this case the "size" is the length of the baseline --the separation between the two lines of sight in the range-finder-- which for a common sextant is rarely greater than a few inches or 10cm so, again, low accuracy in terms of the resulting distance off. There were range-finders, with optics much like sextants, like this aboard ships decades ago with baselines on the order of six feet (two meters) and they apparently competed with this handheld stadimeter. I'm sure there were cases where each excelled.

A case where the handheld stadimeter may have been preferred was during fueling operations. Maintaining proper distance during fueling from a tanker sailing alongside at speed was critical, and if these Schick stadimeters were used only for that singular scenario, then that was worth their development and distribution to the fleets. Fueling is sensitive.

Frank Reed