I have been reading about the pop pop boat on wikipedea. What I don't understand is after the hot water is ejected the vacuume gets created, so when the same amount of water will be sucked in the boat should move backwards. Wikipedea says,
...while the water pushed out carries away with it momentum, which must be balanced (by Newton's third law) by an opposite momentum on the part of the boat, the water sucked in quickly impinges on the boiler tank and transfers its momentum to the boat. The initial reaction force on the boat (which would pull it backwards) is therefore cancelled by the pushing of the water when it hits the inside of the boiler. The result is that the inflow of water causes no appreciable force on the boat.
Now when the vacuum generates inside the boat feels a net backwards force force by the pressure of water present towards its head. I think that since the area of boat is larger than the nozzle so the force exerted by water on its surface must be larger as compared to the force imparted by the water being sucked. So the initial reaction force should not be completed cancelled out.