The Magic Of Pop-Pop Boats: Steam Power Simplified

Pop-pop boats, also known as put-put, toc-toc, or puf-puf boats, are jet-propelled toys that use simple physics principles to move. Invented and patented by an English engineer in 1897, these boats have been manufactured and sold in several countries. The boat operates using a small candle inside that heats the air inside the tank, causing it to expand and push water out of the exhaust tubes. This process repeats itself, with the water being drawn back into the tubes as the air inside the tank cools down. The popping sound associated with the boat is created by the tin diaphragm on top of the boiler.

The boiler doesn't fill with water

If your pop-pop boat's boiler isn't filling with water, there are several possible causes. It's important to identify and address these as soon as possible to avoid further damage to the boiler or the heating system.

Firstly, check whether any recent maintenance has been performed, including fitting a new boiler or components. If so, it's likely that something wasn't set up properly. Contact a professional engineer to describe the issue and arrange a visit.

If no recent maintenance has been carried out, there are several other potential causes. These include:

• A faulty non-return valve. If your home's plumbing system is fitted with non-return valves between the boiler and internal stopcock, and these are faulty, water may wash back into the system, preventing the boiler from filling up.
• A malfunctioning pressure release valve. The pressure relief valve is responsible for maintaining the proper pressure in the boiler. If it is not working properly, it can prevent the boiler from filling up with water. This could be due to a malfunctioning valve, a clogged valve, or a blockage in the valve's vent pipe.
• A malfunctioning water feed valve. This valve controls the flow of water into the boiler, and if it fails, it can prevent the boiler from filling properly. This could be caused by a malfunctioning valve or blockages in the valve or supply line.
• A malfunctioning pump. The pump is responsible for circulating water through the boiler and the heating system. If it is not working properly, or there is a blockage in the pump's water supply line, water may not circulate properly to the boiler.
• A damaged expansion tank. The expansion tank maintains the proper pressure in the heating system and prevents the boiler from overflowing. If the tank is leaking or damaged, the boiler may lose pressure and/or not fill up.
• Issues with the cold water feed. Blockages or leaks in the cold water feed pipe can cause a lack of water flow to the boiler.
• Low boiler pressure. Leaks in the system, an inadequate water supply, or a faulty pressure relief valve can all cause low boiler pressure, which may be why your boiler isn't filling up with water.

Air pressure and suction

The movement of water in and out of the boat is due to changes in air pressure inside the boiler. When the boiler is heated, steam is created, and the resulting steam pressure forces water down the exhaust tubes, propelling the boat forward. As the steam cools, it condenses, creating a lower-than-atmospheric pressure inside the boiler, which causes water to be sucked back up the tubes.

The popping sound that gives the boat its name is created by a thin copper diaphragm on top of the boiler, which makes a sudden movement in response to changes in pressure.

The forward motion of the boat can be explained by the fact that as water exits the boat, it transfers momentum to the boat, propelling it forward. However, when water is sucked back into the boat, the momentum is transferred in multiple directions, and so the net force on the boat is reduced. Additionally, the pointed front of the boat offers less resistance to forward motion than the flat back of the boat does to backward motion, further contributing to the forward propulsion of the boat.

The popping sound

This process repeats several times per second, creating a continuous popping sound as the boat moves forward. The popping sound is an indication that the boat is working correctly and is a unique and charming feature of this simple toy.

The boiler tilts up

As the steam escapes, it creates a tiny negative pressure system in the boiler, which then sucks water back up the exhaust pipes. The water is heated and vaporized, building up pressure inside the boiler. The boiler tilts up to utilize gravity and ensure that the water fully drains back into the boiler, where it will be heated and turned into steam again.

The boat moves forward

The forward motion of the pop-pop boat is a result of several factors, including the shape of the boat, the momentum of the water, and the exchange of momentum between the boat and the water.

Firstly, the shape of the boat plays a role in its forward movement. The bow of the boat is typically pointed or wedge-shaped, offering less resistance to forward motion, while the stern is flat, creating more resistance to backward motion. This design allows the boat to glide forward easily when water is pushed out through the exhaust tubes but resists backward motion when water is drawn back in.

Secondly, the momentum of the water columns in the exhaust tubes contributes to the forward motion. Even after the boiler stops pushing the water out, the water continues to move forward due to its momentum. This instability created by the water's momentum keeps the cycles repeating, with water being pushed out and then rushing back up the tubes.

Additionally, according to Newton's Third Law of Motion, there is an exchange of momentum between the boat and the water. When the water jets out of the exhaust tubes, the boat jumps forward. However, when the water is sucked back into the tubes, the momentum of the water entering from multiple directions largely cancels out, resulting in less backward force on the boat. This imbalance in the exchange of momentum propels the boat forward.

Furthermore, the oscillation cycle of the pop-pop boat's engine also contributes to its forward motion. During the cycle, steam pressure builds up inside the boiler, forcing water down the exhaust tubes and creating a propelling pulse. The steam then condenses, reducing the pressure and allowing water to be sucked back into the tubes. This cycle repeats several times per second, resulting in a net forward force on the boat.

Overall, the forward motion of the pop-pop boat is a combination of the boat's hydrodynamic design, the momentum of the water, and the exchange of momentum between the boat and the water during the oscillation cycle of its engine.

Frequently asked questions