Saturday, September 8, 2012

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.

13 comments:

  1. Hi Dan,

    Early in your post, you mentioned geysers that can in fact fire a huge amount of water in the air. They do this, because the water at the heat-source is under pressure and therefore able to overheat. As soon as the pressure lowers because some water is pushed out, a huge amount of water turns into steam instantly. Maybe pressure can help with a larger putt-putt boat, too, although the cycle would become quite slow.

    Another thing with geysers is, that they heat the water not solely from one direction like a plate does, but the whole hole is hot. In that way the amount of water that directly absorbs the heat is larger. Maybe not using a plate but heating up parts of the whole pipe might help your efforts.

    Just some ideas I had while reading your post.

    ReplyDelete
    Replies
    1. If you had two separate engines, that both led to a single pipe, with a pressure valve switch in between, you could let pressure build in one as it was released in the other. In theory it could double the speed of your boat and give you enough time to build the pressure needed, rather than worrying about cutting the time down for a flash steam. I'm not an engineer, so I don't know how feasible the idea is under test, but I think it might work.

      Delete
    2. This comment has been removed by the author.

      Delete
    3. If you had two separate engines, that both led to a single pipe, with a pressure valve switch in between, you could let pressure build in one as it was released in the other. In theory it could double the speed of your boat and give you enough time to build the pressure needed, rather than worrying about cutting the time down for a flash steam. I'm not an engineer, so I don't know how feasible the idea is under test, but I think it might work.

      Delete
    4. If you had two separate engines, that both led to a single pipe, with a pressure valve switch in between, you could let pressure build in one as it was released in the other. In theory it could double the speed of your boat and give you enough time to build the pressure needed, rather than worrying about cutting the time down for a flash steam. I'm not an engineer, so I don't know how feasible the idea is under test, but I think it might work.

      Delete
  2. Thanks for your comments, Benjamin. By analogy to my earlier post on boilerless steam engines, one could build a putt putt driven by superheated water. Right now I'm trying to find a solenoid valve that can deal with 700-degree water at 3000 psi. I think I'll get a couple of diesel fuel injectors and try them out at various pressures and temps.

    ReplyDelete
  3. Dan check this out:

    HotTug: A Floating Hot Tub and an Electric Motorboat All-in-One! (12/7/2012) - an interesting way for scaling up the putt-putt version? http://inhabitat.com/hottug-a-floating-hot-tub-and-an-electric-motor-boat-all-in-one/

    ReplyDelete
  4. Hi Dan, et al....
    I am Buzz ... owner of "Buzz's BoatYard". You have some interesting ideas about scaling up the putt putt engine. I sell the little toy putt putt boats by the thousands on my site. Over the past 20 years or so, I've made a lot of my own engines. The most efficient engines I've made are from coiled copper tubing. The normal putt putt boats use a diaphragm of .005" thick copper flat stock. There are many misconceptions about the diaphragm. Some think it adds a lot more power....but it really just makes noise. So, to your thoughts of a scaled up model to power a canoe... can and has been done. I strongly suggest using an aluminum canoe so you don't burn a hole in the bottom. There are many sizes of soft copper tubing out there and the idea is to keep a very high temperature where the tubing coils. If you use a small canoe, you could probably get away with using 1" copper tubing. The problem then becomes the thickness of the tubing walls. That is why you have to keep the coil almost red hot to make it flash in a faster cycle. The reason a putt putt boat defies the idea of an action creating an equal and opposite reaction. The reason? When the water is flash boiled and blows water out of the exhausts, it creates a vacuum inside the coils. That sucks cold water back into the engine, it hits the red hot coils and flash boils again. If you think about it... the above noted theory is defeated. The fast flash of water out of the engine, starts the small boat moving moving. Obviously, the boat has mass. When the vacuum sucks water back into the engine.... it is not strong enough to counter the forward movement of the boat because the boat already has momentum and much more mass moving forward than the reaction (suction) can overcome. We have made a scaled up coil tubing motor which moved a very short one man canoe. It moved forward very slowly... but it did move and it did so silently. Best,
    Buzz
    Buzz's BoatYard
    www.buzzboats.com

    ReplyDelete


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