Various google results on the topic refer to an article by Gerr in Professional Boatbuilder magazine. Here is the link to the electronic back issue that is presently available to read free online:
https://pbbackissues.advanced-pub.co...=102&pageID=63
The reason for my choosing the fatter Schilling profile over the Thistle is an increase in effective rudder angle, from 40° typical for a Thistle to 70° for the bulbous nose Schilling, the penalty being increased drag. My application is for a narrowboat with a steering tiller; hanging off the stern with arm extended is common practice on the UK's narrow canals with the challenging sharp turns into junctions.
https://youtu.be/ziNRSQFHNJI
Thistle rudder profiles seem to be a more common choice for sailing craft for better maneouvering within marinas.
Hopefully someone will find these thouights to be of use. Happy sailing!