I think I found the answer. Stor means large in Swedish. Tumlaren means Dolphin. Apparently Knud Reimers (a Dane) designed the Tumlaren class in 1933 to marry the Koster boat, long and narrow, with the speed potential of a Scandinavian Skerry cruiser.
Adlard Coles gave the name Cohoe to all his boats - after the fastest of the salmon.
Back to researching Tumlare.
Yep, you got it.
There are subtle design differences between a Stor-Tum and a regular Tum - mainly to do with the Tum being originally intended to be a 20m2 class racing yacht whilst the Stor was more... "comfortable".
Koster's are quite something (particularly the Norwegian wooden ones) and there are a couple of (very quiet) people I know on this forum who are lucky enough to own them.
To sail a Tum is an experience in itself.. ask anyone who has owned one. A little like driving a sports coupe along a winding ocean road, it's a whole new enjoyment of wind and water. I'd love to have one - and there are a fair few around to choose from - and they are certainly a lot quicker than my old bus, but one Classic at a time is enough for me.
As I understand it both of Coles' Tumlare were designed by Knud Reimers. Zara was less than 22' on the waterline. Cohoe won the TransAtlantic race of 1950. Both Zara and Cohoe were raced extensively in open ocean, and Coles loved the light feel of the tiller. I remember a picture in the early edition of Heavy Weather Sailing showing his hand on the tiller in a gale. He was also quoted as saying they were as wet as a half tide rock.
Well, a half-tide rock isn't always wet all of the time..
There's been a bit of unpleasantness around our clubs recently over the fact that the Tums don't have self-draining cockpits and are thus deemed by some to be "unsafe" in the rough stuff. Interestingly though, over the course of their +80 year history in this country not one has ever been sunk in a storm. IIRC, three Tums competed in a long-distance race on our Port Phillip back in the late thirties in which one other yacht sank and half the remaining fleet turned back, yet all 3 Tums finished the race just fine... and, as you point out, Adlard Coles took his safely across the Atlantic.
Mr Reimers certainly knew how to design a racing yacht.