This unit was built back in the early seventies to power a 19" balsa wood model tug which I had been given - a Titan, built from a Veron kit.  (and later re-named Bluebell after the brand of of metal polish used so lavishly in the engine room)
  The requirement was that it should be fail-safe, particularly against propellor fouling in our local pond. It must drive the model at scale speed with power to spare for at least 30 minutes without attention and it must be clean, dry and run cool enough to avoid damaging a highly finished wooden deck and superstructure.
  Fuel would be methylated spirit (alcohol) and lamp control would allow "Full" or "Half" speeds ahead, under radio control.
  The space available was 9.5" x 3.75" with 3.5" height available for the boiler, dropping to 2.375" under the engine room hatch and it was decided to build the complete plant as a single unit which could be lifted from the boat for filling, cleaning etc.

   The very restricted height in the engine room led to the adoption of a Stuart Turner ST 7/16"x7/16"  double acting oscillator and an old engine was given a complete overhaul including an 'O' ring on the piston and in the gland.  Shaft bearings were drilled out and long phosphor bronze bushes fitted at each end with a fair sized oil reservoir between, whilst the old crankshaft was replaced by a new heavy disc crank extending back over the front bush and with oversized, hardened, shaft and pin pressed in. 
  Further wear at the gland neck was prevented by fitting a crosshead guide bar to rock the cylinder, driven by a bush soldered to the, new, big-end.  Ports were opened out to the maximum permissible size (just under 1/16") and a new 1/8" trunnion pin fitted; a small displacement lubricator finished the job and the engine was run on the bench.
Although plenty of power was now available, heavy slogging at 30 psi began to wear the soft brass trunnion bearing and allow the piston rod thrust to separate the port faces.  So a longer phosphor bronze bush was fitted, together with a hardened pin and a labyrinth groove turned in the port face.
   Unfortunately even this only alleviated the problem and under heavy load the port face still bubbled slightly. Although steam loss was now infinitesimal and the wetness would have been unnoticed in normal use, it was considered unacceptable in a balsa hull and the outboard trunnion bearing shown in the photo was fitted.  Pins and port face were lightly skimmed with the cylinder assembly running between centres and the centre in the outboard pin used to locate the steel ball which the leaf spring on the outer bearing bears on.
   Performance was now 100%.  There was no trace of leakage at 40 psi, whilst at only 10 psi it would turn a Stuart 2" x 5" propeller at 750 rpm.

  Attention now turned to the boiler and the twin drum design was adopted, again because of height restrictions and after some experimenting with steam drums and perforated pipes the curved connecting pipe was found to give the driest steam. 
   It worked well and the boiler could be filled almost to the top without priming, even in a 'rough sea'.  The one bad design feature is the 'remote' safety valve - this should have been fitted, horizontally, to the side of the curved pipe.
   The boiler was filled by an external hand pump, the high level check valve backed up, via a syphon, by the car tyre valve mounted on the base plate as a convenient, and dribble proof, coupling for the connector..
   To provide heat insulation the firebox is double skinned; the two tinplate skins are approx 1/8" apart and the air supply to the lamp is drawn down between them.  This was very effective, the top remained cool while at the hottest place, near the bottom, the temperature never went above 110 degs/F until just after shut-down when it rose to about 150 degs/F.
   Two stoke hold ventilators and the gap between the funnel and its liner provided all the air required and this allowed the superstructure to be sealed to the deck, providing a very sea-worthy ship.
   Draught is provided by the engine exhaust via an oil separator; the steam entering this via a tangential pipe and leaving through a perforated pipe in the centre.  Without this draught the lamp suffocates almost immediately to provide the necessary fail-safe. (and also make lighting up a rather elaborate ritual - see later)

   Early experiments with spirit lamps had shown that circular wicks were only effective if diameter was limited to about 1/4", above that size, the fuel evaporating from the centre didn't get sufficient air to burn properly.
   So long wicks were used, with air supply arrangements based on those of a paraffin lamp but without a secondary supply.
   These are divided approx' 1/3 from one end to provide 'full' and 'half' speeds and are fed from a standard 'chicken feeder' arrangement operating at two levels, controlled by a lever operated by the four station clockwork radio escapement. (which also provided Port or Starboard helm in the intermediate positions)   A balance pipe had to be connected between the feed chamber and the firebox to equalize the pressures and prevent the depression drawing neat fuel from the wicks, whilst cold air leaking into the box alongside the fuel pipes, prevented vapour locks.
   One rather strange feature was the effect of engine speed on steam raising.  Allowing the engine to race, greatly increased the draught to both evaporate more spirit and provide the air required to burn it.  I had certainly not expected to get involved in 'grate loading'.

   The rather complicated starting procedure was also vaguely reminiscent of loco practice - to the total bewilderment of the other pond-siders.  A plug cock, situated against the back head where it could be operated by a rod connected to a concealed lever on the superstructure, was connected to 'short-circuit' the engine and connect the steam pipe directly to the oil separator via a 1/32" hole.  This was the 'blower' valve.  Starting with the boiler empty and the whole unit removed from the hull to give access to the lamp lighting door, this cock was opened and a rubber tube with a bulb was attached to the separator drain valve.  The lamps were lit and the bulb was used to pump air up the blast pipe, drawing sufficient air past the wicks to keep them burning after the lighting door was closed. In a very short time the water tubes were hot and a single pumpful of water was fed into the boiler.  This burst into steam immediately it reached the tubes and after a few seconds warming up the plumbing, it reached the blast pipe.  The separator drain was now closed, the rubber tube removed, and, with the engine ticking over gently on the pressure drop across the 1/32" hole,  the boiler very carefully pumped to the level of the try cock.  The fuel tank could now be topped up, the separator drained and the whole unit wiped clean before the engine was stalled on a dead centre and it was fitted into the boat.  With the superstructure fitted and the boat in the water the engine was started again before closing the blower and, after a quick check of the radio control, away she went ----

   With Bluebell in the water several propellors were tried before arriving at that strange four bladed, flat tipped, 2.5"x4.5" 'heavy duty wheel' cribbed from a very old American 'Mechanic's Guide'.  This was the most satisfactory by far, the boat riding as steady as a rock with 15 ozs in the tow rope when pulling my two boys in a heavy canoe at just over 1 mph.  (scale 7 knots)

   Now we come to the really sad part  :-(

  Many early morning runs were made, with the plant's performance 100% reliable and consistent - and this was to prove my undoing - I got bored  and measured its performance.    65ccs of spirit was sufficient to raise steam and then run the engine at full power for 20-23 mins, a total work output, at the tow hook, of 1800 ft/lbs, giving an overall efficiency of 0.18%.  A whole order of magnitude behind IMLEC and a challenge.
   I decided to modify the engine for expansive working and spent several happy weeks designing and making a stainless steel plate valve fitting between the port face and the cylinder, and oscillated by an eccentric machined on the flywheel.
   This was an unmitigated disaster.  Not only did it leak steam but the engine would only run at high pressure and speed.
   Much time was spent on modifications and I did succeed in reducing leakage to a dribble - but the engine was still very inflexible and I was considering making new port faces and returning to non-expansive working when I noticed the damaged hull.  Several pond side knocks had allowed the balsa to get damp, this had broken the cement joints and things were coming apart at the seams!

   I gave up - wrote up the whole unhappy affaire in M.E.  (21-5-76) and forgot poor Bluebell -----
        Propellor test  (superstructure not fitted) - pond stirring at 1 MPH!