There are two mutually supporting Watkins websites,
http://watkinsowners.com/ which has a wealth of technical information about each of the Watkins
lines as well as history of the company and
http://finance.groups.yahoo.com/group/watkins/ which has an active subscribership of over 400 Watkins owners sharing maintenance, restoration and upgrade information. Go to the second one and that discussion of props and engine rpm is taking place right now among some very knowledgeable owners and diesel mechanics.