Chicago--A thick and persistent fog bank shrouded the southwestern shore of Lake Michigan today and prevented any of the 263 boats entered in the Sailing World Chicago NOOD Regatta presented by Mount Gay Rum from racing. The regatta, annually the biggest of the nine comprising the Sailing World NOOD circuit--will continue through Sunday.All four circles--the Great Lakes 70s have a circle to themselves--left the dock on time for a 10:25 a.m. first gun. But while the steady 10-knot northerly breeze was more than sufficient for racing, the visibility on the water was so poor that at times the race committee would've had a hard time making out the leeward end of the starting line. The respective committees flew the AP flag--used to signal a postponement--at the scheduled time of the first signal, hoping that the fog would eventually burn off as forecasted. However, the fog did exactly the opposite, intensifying to the point that it was impossible to see more than 100 feet into Monroe Harbor from the shore. Around 1:30 p.m., the race committees told the competitors to return to the dock, but that there was a possibility that they would try to run some races later if the weather cooperated. It didn't, and at 2:30 p.m. racing was postponed for the day.As has been the case in the past few Chicago NOODs, the Tartan Ten class is by far the largest. With 48 boats registered, this venerable class has more than twice as many entrants as any other class. Second is the J/105s with 21 boats. The S2 7.9s have 16 boats registered. Of the eight classes sailing in either PHRF bands or as a level rated group, the PHRF 34 to 66 group--which includes everything from a Mumm 30 to a Beneteau 456--is the largest with 15 boats.The first gun for tomorrow's racing--less fog is predicted--will once again sound at 10:25 a.m. The Hall Spars & Rigging Boat of the Day award, which is usually given out to the most outstanding individual boat on the first day of each NOOD Regatta, will be awarded tomorrow evening.