Scaling up the putt-putt

How about a steam boat with no moving parts? 

It's called a putt-putt, or pop-pop boat.

The principle is very simple:  a pipe, open at the stern of the boat has a closed end inside the boat that is kept very hot.  Water enters the pipe, and when it gets to the hot end it flashes to steam pushing the water out the pipe and propelling the boat forward.  The pipe is left with only steam in it, which cools and condenses, drawing water into the pipe again, and the process repeats.  Because water is drawn in from all directions at the end of the pipe, but ejected in only one direction, there is a net forward thrust from this cycle.

This cycle is not unique to the putt-putt.  It is essentially the same as that which takes place vertically in a geyser, and it is analogous to the operation of a valveless pulse jet where the pulse energy comes from burning fuel instead of flashing water to steam.

Putt-putt boats have been around for over a century, originating with a design patented in 1891 by a British inventor, Thomas Piot.  They became popular as toys in the early 1900's.  I bought one of these when I was a child by sending in a dollar along with a box top to a promoter for breakfast cereal.  Here is a video of one just like the one I had...

    http://www.youtube.com/watch?v=-2iithgVT0k

You can buy one for $3.98 at...

    http://www.tintoyarcade.com/search.php?search_query=pop

Many factors conspire to make the putt-putt principle only work well as a toy.  However I believe that each of the challenges can be met with creative engineering and I intend to build a putt-putt boat big enough to ride in.  The essential characteristic I aim to preserve is that water flashes to steam directly forcing water out of a nozzle to propel the boat, whether it uses valves or not.  My first priority is to make a water pulse jet that can carry me across a pond.

I've been having a lot of fun with various designs, dealing with such issues as pipe size (if the pipe is large enough, the water will lie on the bottom and not behave like a solid slug), energy lost to cooling of steam if the cycle is slower (it will be), and how quickly one can flash enough water to produce a man-propelling pulse.

I wrote a pretty long blog entry detailing how various pipe sizes might produce various amounts of thrust under various pressures, and therefore how much water needed to be vaporized at each cycle and how much heat would be required.  Without going through all the details, it appears that 1-2kW of heat (easy for a propane burner) should be able to produce about 5-10 lbs of thrust, and that should be enough for a leisurely canoe ride.  If it seems to slow, we can burn more propane or drink more wine.

Ordinarily I would delight in presenting those calculations, but there is one significant problem in scaling up the putt-putt cycle that has nothing to do with pipes, valves, calories or thrust equivalents.  The problem is how to flash something like one cc of water to steam in less than a tenth of a second.  In the case of the putt-putts, only a tiny amount of water gets vaporized and, to judge from the sound of it, the flashing takes only 10-20 milliseconds.

If you have ever dropped water on a hot skillet, you know that it bounces.  While it's a lot of fun to watch, it prevents the droplets from turning to steam quickly;  they simply ride around on a cushion of steam made from the first contact.  In the little putt-putts, that first contact produces enough steam for the cycle, but we need to produce 100-1000 times more steam in not much more time.

So I am now meditating on various boiler designs that might work.  Here is a picture of my first serious prototype.  The note on it is because I knew it would invite TSA inspection in my suitcase...

Interestingly, this "Leidenfrost effect" (thanks to emddudley for the ref) is the same problem that is involved in making the hot-head steam engine practical.  So I would be interested to hear any suggestions or actual experience in how to make this work.  Hopefully my first design will let me try out a lot of things such as surface texture, cavity shape, temperature ranges, etc.  So future posts on both the hothead steam engine and the man-sized putt-putt await a solution to the Leidenfrost effect.