NavList:

Frank is just winding us up by asking how it works.  But OK - I'll bite :-)

This is a useful explanation of the way the Mark V stadimeter works - for a general audience.

https://www.youtube.com/watch?v=zuN8rvSo8pg

If we think of it as just a tool for viewing two points at the base of a triangle, with the viewer being the apex, it becomes obvious why the scale of distance compresses as that apex angle gets smaller.  But that scale means nothing without an adjustment the known length of the base of the triangle.  This is provided by the drum.

If we were using a sextant for a horizontal or vertical angle to measure distance off, we would apply that adjustment mathematically or perhaps we might just apply the good old fashioned 5-6-5 rule (for charts in fathoms and feet) or the 54 rule for charts with metric heights.  With the Mark V stadimeter we wind that in with the drum adjustment.  We get the stability, viewing and double-angle benefit a gadget that uses mirrors like a sextant, without the need to do any extra calculations.

With regard to Gary La Pook's comment about holding station during WW2 convoys - yes - these were important and were on board most if not all escorts, but not always on board other ships in a convoy.  Of course they only worked in daylight and when there was no fog obscuring the view.  In the North Atlantic, and the Murmansk convoys, both fog and darkness - especially in the winter - diminished their practical use.

Prior to sailing, each ship was given its station and both skippers and navigators attended a briefing given by the convoy commodore.  In order for a stadimeter, or indeed a sextant, to be useful for a distance-off sight, they each needed to know the height of the object they were sighting above the water - or near enough.  For some standard ships, such as a Liberty Ship, the height of the top of the funnel was known.  But in most cases data would have to be provided before sailing.  Escorts held data in a file on board for the other military ships they were likely to meet.

Even then, and taking a liberty ship as an example, with a funnel height above the water of about 130 feet and a distance between ships on station of 1,000 yards, the angle at the triangle apex was small - say 8 degrees plus or minus.  With smaller ships - and there were many - and angles were smaller still.  Add a good seaway and  I wonder how mucg practical use was had.  In the right conditions, and given line of sight, you could use two sights from a polaris to do the same job - but like the sextant, you then had to crunch some numbers.