The addition is a fixture that could be assembled from a pegboard and a few pegs, to facilitate bending of rectangular brass tubing into a variety of configurations that could be tested to find out if any of them could be used as unit engines for pop-pop boats produced by putting waterline models of old steamboats and ships on rafts.
David Halfpenny posted this suggestion on the cardboardshipsandboats group a while back:
I have an untried suggestion that is relatively crude to start with, but which one could make more sophisticated with time and experience.
Basically, make the Working Parts on a separate waterline-shaped sheet of plywood. Have any propellor shafts and jet nozzles fixed and sealed through this alone.
If a live-steam or I/C model, shroud the hot bits with shiny reflective metal foil, and put the exhaust in a sensible place.
Make up the paper ship with a big hole in its keel, big enough to let it slip over the mechanism with its uptake over any working exhaust. Draw round the waterline onto the ply.
Devise a quick-release way of fitting the ship to the ply. I suggest fixing two dowels to the ply and cutting two corresponding holes in the keel. Then the ship lifts off, but won't fall off.
Cut out the ply, and shape it like the bottom of a ship. Paint red or black to suit.
Float it in water, complete with burner, batteries etc, and ballast the ply until its top surface is only just above the water surface all round. (If it won't float, start again with thicker ply. If it's seriously unbalanced, rearrange the working parts and exhaust.)
Waterproof the ship. I suggest shellac for a ship that will be painted, or cellulose lacquer for a pre-printed one - as long as it doesn't strip off any ink or paint you need to keep. If ink is a problem, experiment with different inks by photocopying (heat-set toner) or scan-to-inkjet (water-based ink).
Find ways of adding discreet ventilation if a live-steamer or I/C model.
Float the keel, and start the motor, then drop the paper model onto the dowels (or whatever).
Just like a real ship, she'll float higher in the water as the "coal" gets used up."