Powered by Boiling Petrol.

The main motivation for this system was to evade the USA regulations that required even the smallest steam launch to carry a qualified steam engineer; this stipulation was not lifted until 1912. Other countries had no such regulations , and the use of boiling petrol outside the USA was minimal.

Above: Cross-section of the Boiling-Petrol Launch. Note that the petrol tank is at the front, well away from the boiler. This was probably not purely a matter of weight distribution as the tank would have been empty at times; it was probably a safety-measure. The "condensing pipes" for petrol vapour leaving the engine are not easy to see, but they pass downwards from the front of the engine, run aft to just in front of the propellor, then double back to the fuel tank in the bows.

This thing was probably expensive to build: there is no such thing as a free launch.

Above: The Boiling Petrol System. Petrol is pumped from the tank in the bows by air pressure, generated by a hand pump, and passes through a coil boiler. Part of the petroleum vapour issuing from the boiler is fed to the burner that heats the boiler, and the rest drives a three-cylinder engine. The long U-tube at the bottom is the condenser. Note the whistle top right, I thought at first it blew with petrol vapour, (which might have somewhat hazardous if any one on board was smoking) but Ken Helmick points out that on the diagram above, the whistle is connected to the compressed air line that pressurises the fuel tank.

An interesting feature of this boiler-engine combination is that there is no throttle valve. With small steam launch engines with flash boilers it was quite practical to operate just using the reverse gear and the burner control, as the amount of working fluid in the boiler was very small, and the response to burner modulation therefore reasonably quick. The same applied to the boiling-petrol system.

Above: Another View of The Boiling Petrol System. Petrol is pushed into the boiler by pump P, through pipe R. (This pump needed to be larger than in steam practice because of the low specific volume of the petrol vapour) When vapourised, it enters B, where most of it descends through pipe D to the engine valve chest. The valves C were said to be slide valves, but the drawings here do not make it clear how they were oscillated; probably by some sort of cam action. Note the peculiar ball-joints at the upper ends of the connecting rods. A proportion of the vapour is diverted from B through tube T, and is squirted through injector P, entraining air from the funnel shaped thing A and driving the petrol/air mixture down pipe H to the ring-burner K, which heats the boiler tubes. The exhaust petrol vapour, on leaving the engine, was condensed in "tubes outside the launch" that were under water, and returned to the tank in the bows. Condensation did not create a vacuum, and the back-pressure on the engine pushed the condensed petrol forward and up into the bow tank. The wheel M controlled the valves to allow forward and reverse running. The boiler pressure varied between 14 and 70 psi, regulated by the burner control valve attached to injector P. There was no throttle between boiler and engine.

Petrol consumption was said to be between 17 and 20 pints per hour, generating 6 horsepower at 233 rpm. The launch would carry two people at about 5.5 knots.
The petrol acted as lubricant as well as working fluid, and it was said that "the density of the petroleum employed should be between 0.68 and 0.70. Below the former figure the petroleum does not lubricate satisfactorily, and above 0.70 it does not evaporate sufficiently rapidly." This seems like a very small range of density to specify in an age when the refining of petrol was very much in its infancy.

The dangers of this system hardly need enumerating. Most steam engines have little puffs of steam coming out of various crevices, such as the piston-rod packing. I see no reason why a petrol-boiling sytem should be any more leakproof, so it would seem to be only a matter of time before escaping petrol vapour was ignited by the nearby flame on the boiler, with dire consequences. J A Crabtree, however, was more optimistic, pointing out that there was a relatively small amount of petrol actually in the boiler, and the main risk was in starting without adequate pre-heating, when burning liquid fuel might run into the bottom of the burner casing and spill into the bottom of the boat. Crabtree states that anyone familiar with flooded vaporisers in paraffin-fired steam boilers would have been aware of the risks; this seems unduly complacent to me, as petrol and paraffin are very different in their flammability. Crabtree also says "It was hardly possible to apply these power-units for anything but marine work because of the need for infallible condensing arrangements." ie an ample supply of cold water.

Turning to the possibility of excessive boiler pressure, a steam boiler explosion is quite bad enough; a petrol boiler explosion would add conflagration to detonation and would be considerably more dangerous and destructive.

Left: External View of The Boiler, with the engine under it.. A is a sight-hole to allow inspection of the flames. E is a hand air-pump used to pressurise the petrol tank for starting up. The whistle can be seen just to the right of the base of the chimney.

The Scientific American said:

"These little boats have won for themselves an astonishing record. They seem to be as absolutely secure from accident as any kind of power driven craft can be. Every possible precaution is adopted..."

This seems an extraordinary conclusion to draw. There are many questions about the whole business, and it may not prove easy to answer them.

HISTORICAL NOTES

This is taken from a 1965 article called "Naphtha Launches" by F L Herreshoff in the journal Rudder, from the USA:
"Certainly as we look at the naphtha launch today it seems a most infernal machine. Strange to say, however, these launches had few explosions or fatal accidents 'though most of them blazoned up occasionally. Many were copper sheathed in the engine compartment so these blaze-ups were rather laughed at or thought to be part of the game... The real, and possibly the only reason for the naphtha launch was to get around the law that required a licenced engineer on a steam launch... The genius who worked up the naphtha power plant to evade the law was a German named F W Ofeldt of New York and I class him as a genius because he arranged or designed launches that had few serious accidents and because the power plants were novel and built at little expense... The naphtha launch was smaller, lighter, cheaper and, of course, much quicker starting than the steam launch. They sold like hot cakes right up to around 1900 when the internal combustion engine began to be reliable and lighter than the naphtha ones."
Hopefully that extract is short enough to avoid copyright issues.

From the Eagle Harbour website:
"The commentary on the smaller boat hulk was very interesting. My father who began to spend time at the Harbor in about 1905 always told the story that this boat was a "naptha launch" that exploded and burned at a dock on the west end of the Harbor with several fatalities. It was later towed to the east end and sunk by the coast guard. As I recall he said that the boat was owned by a prominent copper country family."
Which sounds like the sort of accident that would be all too likely. Clearly this "blaze-up" was not a laughing matter.

An extract from a news report in 1900:
"July also featured the Larchmont Yacht Club "Race Week." Thursday, July 25th, was a scheduled break for the big sailing yachts and raceabouts. Rowing contests and three events for powered craft were scheduled in their place. No gas engine launches raced but the naphtha launches were divided into two classes, those over 21 feet and those 21 feet and under; Intrepid took the over 21' title and Trochilus took the 21 and under title. The third race was for Alco-Vapor launches, launches powered by alcohol vapor.
The development of the camp and cottage life which is now the great feature of the river has been made possible largely through the introduction of the older types of pleasure craft, first, the primitive steam launches, then the "naphtha launch," and the "Alco-vapor launch," and more recent years the gasoline launch with explosion motor."

THERMODYNAMICS: CARNOT EFFICIENCY

You might think that a liquid with a low boiling point- preferably just above ambient temperature- would be ideal for use in a heat engine. Less thermal energy would be required to boil it and produce a vapour under pressure that could drive some kind of motor. However, you would be wrong. It has been known since Frenchman Sadi Carnot published his famous book in 1824 that the efficiency of any thermodynamic cycle, no matter what the working fluid used, increases as the temperature difference between the temperature at which heat is put in (as in the boiler of a steam engine) and the temperature at which it is taken out (as in a condenser) is increased.

Where Th is the input temperature and Tc the heat output temperature.

The Carnot efficiency is a maximum limit. In practice efficiency is always lower.