You get potential energy that is transformed into kinetic energy and cold air is heated.
The candle heats up air and makes the hot air rise. The rising air pulls in air from the front of the can. This is because the air that has risen has to be replaced with air. The heated air goes out the end of the sail and pushes the can.
The hot air wants to move up because it’s expanding/getting less dense, but when hitting the flap it changes direction and pushes a force on the boat.
Top speed depends on the force balance on boat between forward force from hot air and counterforce from drag in water and air.
Drag is dependent of shape of boat which for us a can, properties of water and air, and what portion of boat is in the water.