I left Port Washington, WI with (I thought) a full tank of gas. Range on my Irwin 32 was 60 - 70 miles. I was headed to Racine, WI 45 miles away. Conditions were calm and foggy, so with light wind we motored. 5 miles out of Racine a mayday call was sent from a power boat that was sinking less than 2 miles from my present position. I pushed the A4 to its limits and headed towards the distressed boat. After about a mile I ran out of gas, to my surprise, and sat dead in the water. The fog was fairly thick, but we could see above it, and, in time, a CG helicopter arrived on the scene. The mayday had been attended to, but we were engine-less in calm air and 5 miles from port - late in the day.