When I was younger and lived on my grandpa's mountain I spent a lot of time by myself. At one point I decided to craft a raft out of logs and spend a day floating down the river just as Tom Sawyer or Huck Finn would have done. I figured that when I reached the fork in the river where the lumber mill stood (that replaced the village of my mother's ancestors) that I could ditch the logs on the riverbank and make the eight mile walk back to the house.
As I did not possess my own chainsaw, I took an axe out into the woods and chopped down three or four trees of about 50-60 feet tall (if you decide to try this, you need to be careful not to let the tree "hop" off its stump and crush you). I chopped the trees into equal lengths and dragged them half a mile to the river.
Because I couldn't wait to test it out, I lashed together about six of the logs and tossed them into the river. I attempted to ride the logs. Very soon, I realized that even if I had fifty logs strung together side by side that they still would not float above the surface of the water.
To make the typical log raft that you see throughout your life in television cartoons would result in a boat that could technically float but would also leave the riders waterlogged for the duration of the voyage. The log raft idea is crap. It no does work. You are much better off using a cheap rubber raft from the big box dealer near you. Otherwise you might want to make a raft like the one in Race For Your Life, Charlie Brown comprised of a wooden deck on top of four rubber intertubes.
Consider this Friendly Advice from someone who has wasted a lot of time and trees and who gets a bitter feeling every time he sees a cartoon raft made of logs bobbing buoyantly, high and dry, down some skyblue waterway.