Category:Put-put boats

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Put-put boats, flash steamers or pulse water jet boats were a popular type of cheap motor toy boat in the early Twentieth Century.

Usually made of tinplate, these boats had a simple burner or space for a candle, and usually a coiled metal tube open at both ends.

Once the tube was hot, the boat would be placed on the water, water would flow into the tube(s), come into contact with the hot metal, some of it would flash to steam, and drive the rest of the water back out of the tube as a jet. Once the water had been expelled from the tube, the steam that had left of the hottest part of the tube would cool and contract, sucking water back in until it came back into contact with sufficiently hot metal to flash to back into steam, repeating the cycle. If not all the water was expelled, the momentum of the final length of water in the tube would act as a piston, further reducing the pressure of the steam and further encouraging it to condense in the cooler part of the tube.

Since put-but boats are steam engines and rely on the the gas in the tube being steam rather than air, getting the boat started might require temporarily pushing a length of rubber hose onto one of the tube outlets and sucking water into the system (removing air) to start the process. Some users apparently also had success with putting a finger over one of the outlets and allowing the initial heating to expand and expel as much of the initial air as possible before allowing in water.

Some put-put boats also had a chamber with a diaphragm: when the water flashed to steam, the diaphragm would pop into a position that allowed a greater volume, delaying the expulsion of the water down the tube and probably allowing a greater and more sustained jet. When the tube entered its negative-pressure or vacuum mode, the diaphragm would pop pack into its first position, probably delaying the reentry of the water and allowing the tube more time to get back up to temperature. The popping of this diaphragm made a loud sound that, with the Toc-Toc brand boats, was presented as an additional selling point.


Having two open tube-ends didn't just make it easier to remove the static air from the system, it apparently also allowed the engine to work asymmetrically: The theory was that once the gas in the tube ended up concentrated more to one side than the other, the "other" side would have a larger body of resisting water blocking the flash of steam, which would find it easier to expand along the arm with less water, force a jet of water out of the tube. During the contraction phase, water in the first arm of the tube would be nearer the hot zone, and would reach it first, and from that point on, one arm of the tube would work preferentially as an intake, and the other would work preferentially as a jet output.


Given the complexity of the put-put process, the arcane dependency on exact timings, the difficulty of working out the best way of starting the boat and knowing what the heck was actually going on at any time (due to the absence of any moving parts whose timings could be monitored and adjusted), and the impossibility of watching how the water was really behaving inside the metal tube, designing an efficient put-put engine was more of an arcane black art than a theory-driven science, and successful designs were probably the result of long hours repeatedly trying different angles of bent tube through a process of trial and error, rather than of solving equations on a blackboard.

The theory of the put-put engine was also complicated by the fact that the people who'd got it to work best didn't exactly go out of their way to explain the principles of why their engines worked better to potential competitors: it was a valuable trade secret.

With the advent of "Pyrex" borosilicate heat-shock resistant glass, one could probably now make a glass-tube version of a put-put boat, fit it with a miniature camera, and film the process and pay it back in slow motion to finally establish what happens inside the tube. However, we haven't yet seen anyone doing this.

The World War Two "Doodlebug" Flying Bomb

The put-put engine is related to the valveless pulsejet engine used on the V1 doodlebug flying bombs used by Germany during World War 2. In a V1 engine, flash ignition drives a jet of flame out of the back of the engine, after which there's is a cooling and contraction phase that sucks new air in through the intakes at the front at the back. This suction also applied to the large exhaust port at the back. However the lengthened exhaust port was occupied by a large body of hot gas already moving in the wrong direction, and since inlet path was shorter, fresh air reached the combustion chamber through the inlet while there was still hot partially-burned fuel in the outlet path, triggering a new combustion pulse. While part of the combustion flame might exit through the inlet valves, the narrow path meant that almost all of the jet found it easier to roar out of the wide-and-long rear nozzle. The "Doodlebug" design suggests that the rear exit might even have been designed with an additional shroud to further lengthen the outlet tube: while this wouldn't have made too much difference to the efficiency of the exhaust, it meant that the "suckback" phase found it even more difficult to pull back the exhaust gases, as there was then an even longer column of hot high-velocity gas in the tube that didn't want its path reversed, and which, after the ignition phase is over, acted as a piston, pulling more air in through the front.

Efficiency was achieved by exploiting asymmetries in the inlet and outlet tubes and the resulting timing asymmetries: the inlet was shorter and narrower, and optimised for allowing in comparatively small quantities of cold dense air, once the fuel ignited with the oxygen in the air, the hot gases were hugely increased in volume, and found it easier to gush out of the large-bore rear outlet. The replacement cold air found it easier to enter through the front due to the shorter path and the absence of a large body of exhaust gas moving against it ... and also the vehicle's passage through the background air, which resulted in cold pressurised air accumulating in the air inlets and trying to enter the engine once the exhaust flash was over.

All these little nuances and complications – using air as an engineering material to replace the action of conventional valves and pistons – meant that the V1 engine was the result of an awful lot of research and development ... however, after the design had been perfected, the lack of moving parts meant that Germany could then mass-produce the engines comparatively cheaply, which was important as each engine was only going to be used once.

External links

valveless pulsejets:

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