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Steam Powered Underwater Jet Engine
Bob Vila's Hammer writes "An Australian engineer, Alan Burns invented a very efficient underwater steam powered jet engine. "Steam that is produced from a petrol or gasoline fueled boiler emerges at high speed from a rearward-facing ring-shaped nozzle into a cone-shaped chamber. Shock waves created as the steam condenses are focused by the chamber to blast water out of the back. Besides powering watercraft pretty efficiently, it can also be used as an extremely robust pump. Pretty Cool."
The engine
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Bill Hamilton invented the jetboat back in the 1950s. It uses an impeller (not a propellor) to provide thrust.
Boats driven by jets are useful since they have better water clearance and can be used in shallow waters. Edmund Hillary (of Everest fame) took a fantastic boat trip up the Ganges river, as far up the headwaters as they could go, which turned out to be pretty far...
Such technology would be fantastically useful in the Florida Everglades for example, where conventional outboard motors wreak havoc with marine life, particularly the dugong.
If anyone ever gets to New Zealand on vacation, make sure to go on the Shotover Jet boats. They do a full 360 at high speed; can't do that with a conventional craft.
STF
ahh; sorry to be a dork, but this is the article on Scientific American about this stuff. Very good read.
My life in the land of the rising sun.
The air bubbles are not mixed into the water flow, but instead more or less surround it. As the bubbles (with their water payload) enter the heating chamber, they are heated and expanded by steam. The expanding bubbles displace more water, and cause the water to speed up as it moves into the exhaust chamber. As the steam gives up its heat to bubbles (and water), the steam recondenses, hence the need to transfer the heat to air beforehand.
The reason the motor has an upper scaling limit is probably because as the size of the central tube increases, the ratio of bubbles to water would decrease.
It seems likely that as in turbojet engines, the motor's efficiency would increase along with an increase in the motor's forward speed.
My understanding of the system may be lacking (I've only been able to see the diagram, just like everyone else), but I just don't see any "shock waves" occurring, being used, or needed for the engine to work.
This has been your cowardly anonymous tech reviewer, AC.