This is a complex issue because various cloth manufacturers calculate cloth weight differently. I reviewed this issue about 2 years ago and went so far as studing the data from several production runs of Airex cloth. As a result of that study and consultation with Airex we approved Airex 700 and 650 for use in J30 spinnakers. However, Airex 600 is not heavy enough to meet the current J30 specifications.
Larry Christy
J30 Class Measurer
