Here I am a nice HOT (96) day to be sailing on the Potomac river, moving along a 5kts. Watching a large cabin curiser (on my port) coming up the river at around 20kts, big wake. Watching him for 3 miles. At about 1/2 mile I expect him to change course a little, 1/4 still nothing. Now I'm starting to wonder about this person. At 200 yards I come off of autopilot and make a 110 deg turn to starboard to parallel his course. Somewhere between 100 to 50 yards they wake up and start a turn to the right to pass behind me. When I see him turn, I turn back to my original course. They pass with 30 yards at full bore throwing up a large wake. Tell my wife to hold on. We were rocked so bad that someone could have been hurt. I was unable to catch his boat name and there were two people on the bridge just looking around. No use hollering on the radio I find most of these people either don't have it on or the engines are so loud they could hear anyway.There was not another boat within 3 miles of us....I was telling this story to a friend this morning and wonders why boaters don't have to take a license course before getting behind the wheel. I told him it wouldn't matter look at the things people do behind the wheel of a car and they have to be licensed.Jim - just venting a little steam.S/V Java