Hi Ian,
Thanks for the comments about my 1920's replica 'Jenny' in the other posting. Yes she will be r/c, and she is 640mm (25") long, and as such will only be suitable for use on fair days.
Came across this which may be of use for your 48" long 'J' class. Model boats magazine November 2007 have featured a lovely freelance 45" classic pond yacht, built on a GRP hull. 5 page item both photographic and build detail. Gaff rig with sail plan and a paragraph on making the sails.
Hope this may be of use.
If this item is of value and you have difficulty obtaining a copy in NZ, please send you address in a PM and I will send you mine. (They only get sent to recycle at the end of each month)
Happy model making.
Tony