Strange as it may seem, after reading this, I checked and discovered a contradiction in the Class specs. The J/30 LOA is listed as 29.83' (29' 10") in J/30 Journals #2 (April 1981) and #3 (8/20/1982) and an original J/30 brochure (6/84). There were no rules in Journals #4 & 5, but from #6 (Nov 1984) on it became 29.92' (29' 11").
Personally I think the 29' 10" is correct and would attribute this to a transcription error when the class took over the publication. I have no evidence
J/Boats changed the specs.