use your old set of parallel rules to gently close against the shaft and then measure the distance between the rule sides.
What a clever idea!
I found myself measuring so many little things (shroud diameters, pin diameters, hose diameters, wire sizes, all sorts of things) that I bought a small dial-caliper and keep it with the tools on the boat. I use it all the time.
Mine are plastic, similar to this. I got plastic so I didn't have to worry about it rusting and becoming un-usable. I personally like the dial better than either vernier or digital.... ymmv.
https://www.amazon.com/General-Tool...dial+calipers&qid=1554139343&s=gateway&sr=8-3
The main thing is, for most things we're measuring, it doesn't have to be a precision instrument. I'm basically trying to determine if (e.g.) a shroud pin is 5/16" (0.312") or 3/8" (~0.375")... and this has proven to be super-useful doing things like that around the boat.
ObNote, in the past, I've used strips of paper to measure things like shafts and hose-ODs. Wrap a strip of paper around the thing, put marks where the paper overlaps, and then measure the distance between the marks. Divide that distance by 3.14 and you get the diameter of the thing (i.e., if the marks are about 3-1/8" apart, it is a 1" shaft...if the marks are about 2-3/8" apart, it is a 3/4" shaft) Crude, but...
$.02
Bruce