Sorry about the delay, just got this answer
As far as I know the word ‘Scratcher’ derived from the fact that he was scratching after the Coxswain Job’ Being one myself many years ago I did the job of running the boat on several occasions until I was recommended for Cox’n. When I became a 2nd Cox’n I was only a UW3, so never held a 2nd class rate becoming an Acting Chief from then on as a Cox’n,. My trade certificate says I was a Cox’n for 11 years and 7 months but I was only made a proper Chief for the last six of my Service.
‘Outside Wrecker’ is of course a different kettle of fish, in the first place it was always an ERA1 whose job it was to look after and maintain all the machinery not in the ER and as ‘tiffies’ had a reputation of ‘If it don’t work, get a bigger hammer!!?’. Hence the word Wrecker.