NavList:

The only thing I can remember about my first day as a trainee navigator-radar in 1968 is that I was issued with a brand new pair of extremely expensive looking proportional dividers by “Threadwell” in a deep blue velvet and satin lined maroon leatherette case (see link http://www.ebay.co.uk/itm/VINTAGE-PROPORTIONAL-DIVIDERS-by-THREADWELL-LONDON-/281756867219?pt=LH_DefaultDomain_3&hash=item419a049a93) , and being told I didn’t have to sign for them, and I wouldn’t be required to hand them back at the end of my time.  They’re still about the houset somewhere, but it’d take me all day to find them.  The case wasn’t as sturdy as the dividers, and it fell to pieces some years ago.  We used them to take radar significant items from large scale maps, 1:63,360 and later 1:50,000, and position them on 1:250,000 topos, the scale of H2S MkIX, so that we could draw target identification overlays in Indian ink on clear acetate sheet.  We also used them to identify significant echoes on radar PPI photos at ¼” or 1/8” scale from larger scale maps. 

Thi thread has prompted me to calculate how the scales worked. Taking the overall length of the dividers as L, for small point distance (SPD) to equal long point distance (LPD), the pivot should be L/2 towards the small point (SP).  For the SPD to equal ½ the LPD, the pivot should be 2/3L towards the SP.  So on and so forth until for the SPD to equal 1/10 of the LPD, the pivot should be 10/11L towards the SPD.  One of you clever mathematical chaps will be able to tell me what sort of a series this is.  However, there’s not room alongside the pivot for the scale selection mark, so the whole etched scale has to be offset by the distance from the actual pivot to the selector mark in the direction of the mark from the pivot.  My “Threadwell” dividers also have a squared scale for areas and a cubed scale for solids.

Turning to the Weems & Plath dividers, all proportional dividers follow the formula LPD=SPD x scaling factor.  E.g. if the mark is on 2, LPD=2xSPD.  Speed time calculations follow the very similar formula Distance=Speed x Time.  Therefore, one only has to fit appropriate navigational values to the scaling factor.  E.g. using the picture on the front of the Weems brochure, 60 mph is one mile per minute, so the scaling factor is one and miles to go equals time to go.  600mph is 10 miles per minute, so time to go (SPD) is distance to go (LPD)/10.   This same basic explanation works for all the other examples too; try it.  The S-N mark is for changing statute miles to nautical miles on the same scale.  The mark occurs at 6080/5280 = 1.19 on the scaling factor scale. I’ll leave you to guess what the N-K is and where it has to be positioned (Hint 1km = 3281ft).

However, although these dividers look very handy, I’d never use them in an aircraft or a yacht at sea.  Firstly, it’s difficult to flip them whilst measuring a long leg in 50 or 60 mile steps.  More importantly, consider turbulence or rough weather.  If thrown forward towards the nav table whilst using them, it’s too easy for the uppermost points to catch your face or pierce your life jacket.  DaveP.