Hi,
Martin took a video in 2010 at Wickstead and posted it on You-Tube under "Steam Jet boat".
I'm afraid there was an incident on its very first run when the electric water pump blew its fuse and the boat had to be rescued by a certain person in waders, who unfortunately interpreted the boiler burner flame as indicating that the boat had caught fire. Subsequently, he pushed the boat under water to douse the flames and in doing so saturated all the boiler lagging.
I managed to dry out all the electronics and put the boat back on the water (see video), but the saturated boiler lagging made the boat very heavy and absorbed a lot of the burner heat.
I have subsequently made many modifications to the system to try out various ideas and have come to the following conclusions:-
1). As a pure jet, the combined steam/combustion gas exhaust (at 240mph) contains mostly kinetic energy and little momentum. It is the momentum derived force that accelerates the boat.
2). The kinetic energy only becomes a force when it impinges on something not rigidly connected to the boat, e.g. another model boat on the pond, or a turbine on the boat.
3). The engine "stalls" if its exhaust is restricted by too much water, i.e. exhausting under water.
4). If a pure steam jet is used against the surrounding water most of the energy is lost by condensation, i.e. little kinetic or momentum is exchanged between the steam and the pond water.
Any pure jet propulsion system becomes more efficient the faster the vessel travels, because the relative speed between the vessel and exhaust velocity decreases and less energy is expelled in the kinetic form and more as momentum. This is why jet aircraft require full throttle on take-off and throttle back when upto speed where they are more efficient. For passenger aircraft the momentum is increased by using fan engines (contained props), which accelerate a large mass of air by-passing the engine core. Fighters still use a pure jet engine for high speed.
The only way to extract the energy, for the boat, is to gradually extract the kinetic energy without stalling the engine. One could put the gas exhaust through a highly geared turbine driving a conventional prop or, alternatively, gradually introduce water droplets that could pickup the kinetic energy. This way, the engine shouldn't stall and the kinetic energy will be recovered.
In the video, you may notice the unconventional "rudder". This is of my own devising and vectors the force by using the "Coanda" effect. The "Coanda" effect was discovered during the development of the first jet aircraft in 1910. The rudder was successful and doesn't restrict the exhaust, especially when going straight ahead.
The steam jet engine has extreme steam conditions of 710degC and 20bar, and the back end of the boiler runs at temperatures in excess of 910degC (the melting point of brass). Silver soldered joints do not survive!
I still want to develope the concept, but I must finish my monotube boiler boat first (see "Edwardian Steam Boat "thread). The monotube boiler has been successfully developed and is to transferred from my experimental boat "Vital Byte" to the new launch.
Thanks for your interest.
Ian