Gas leaf blowers will work if you point them off the stern. In the late 80's there as a guy that used one on a SC 20. I am guessing it produced 3-4 kts under dead calm conditions.

Some of the newer leaf blowers produce a lot more thrust. I bought a big backpack mounted one to help with the Hurricane cleanup. Standing on a skateboard, it produces ~10 mph. I am guessing it produces about 25 lbs of thrust. If you buy 10 of them, you fly around and be on the news like that guy with the ballons and the lawnchair