Pontoon Boat Propulsion

My Dad is building a pontoon boat and is not sure what to use for a propulsion system. He's using 55gal steel drums welded together for the pontoons and I believe it will be about 26-30 feet long and 9 feet wide. He knows of people that have built similar boats using a paddlewheel to drive it. I'm thinking that this is probably not the most efficient way of doing it. Will he be better off using an outboard motor? He plans on using it for joyriding, so it doesn't have to be all that fast. He's thinking about 10-20 km/hr. Has anyone ever built anything like this, and whats the best/cheapest/or most efficient way to do this?

 

a small outboard would probably be the easiest to use. I wouldn't go more than 10 hp.