The movement of the soap powered boat can be explained by a combination of three principles.
All liquids have
surface tension along the surface of a liquid, caused by inter-molecular forces within the liquid pulling liquid molecules together. Because of surface tension, liquid surfaces act like a kind of 'skin', able to support small insects and materials on their surface.
Isaac Newton's
third law of motion says
for every action there is an equal and opposite reaction.
The movement of the detergent provides a 'pushing force' backward into the water surface as the detergent molecules spread out, sending the boat forward... i.e. an equal and opposite motion.
The
Marangoni effect is also acting upon the boat. This affect describes how a liquid with a high surface tension pulls more strongly on the surrounding liquid than a liquid with a low surface tension.
If you change the surface tension of some parts of the liquid you introduce a difference in surface tension or 'gradient'. A presence of a surface tension gradient will cause the liquid to flow from areas of low surface tension to high surface tension. Your detergent molecules reduced the water surface tension behind the boat, causing the water (and the boat) to move forwards.
To repeat the experiment you may have to get clean water without detergent so that the surface tension is equal throughout the water.
Try different shapes of boats and amounts of detergent, does it make a difference?
