The rule is that if you double the wind speed, you quadruple (not double) the wind force. Therefore, if the boat sails well in 15K of wind, then to sail the same in 30K, you will need to cut the sail area to one quarter of normal full sail. For our smaller boats, this means at least 1 reef (2 if you have a second0 and a scrap of jib, or no jib. Sea state in 30K can prove interesting.