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...
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.
Just as a graphical scene can be built up from primitive graphical elements and other structural elements such as transformations, superpositions, etc., so it is also possible to build up a wave sculpture by combining various primitive and compositional elements.
An Example Wave Sculpture
This can all be understood fairly intuitively by considering a sculpture such as that shown below...
Figure 1. An Example Wave Sculpture 15m x 30m
The basic shape is a promontory of approximately the same extent as the typical distance between wave crests at the beach. The choice of scale is such that different features of the sculpture are engaged in succession as the wave approaches the shore. We believe that an ideal choice in this regard is one where the promontory is somewhat less than the distance between incoming crests. This would afford a period of rest between each cycle of activity on the sculpture.
Next note that the promontory is wider at the base so that the sides are somewhat oblique to the incoming waves. This is to minimize the effect of one cavity "shadowing" another behind it. Each cavity, or cell, in the structure should intercept a relatively fresh and energetic section of the incoming wave.
Now note the location of the cavities: their placement dictates a certain rhythm of activation. For this design, cells a and b would act together as the wave first engages the promontory, These cells might be blow-hole cells to give the effect of smoke coming from a dragon's nose. After this cells, c, d, e, f, g, and h would act in sequence on alternate sides of the sculpture. An effective choreography would be for these cells to be jets, designed to arch over to the cell on the opposite side that is about to shoot. The overall sequence would a back-and-forth jumping motion with the arching trajectories creating a sort of arbor way as the wave comes in.
Finally, cells i and j will act together, and cells k and l at the end. i and j might produce large forward- or outward-directed jets, and k and l might be a pair of ram tubes feeding a jet aimed straight into the air. Alternatively, i and j might be scoops that fill the top level of a cascade that drains for a while until the next wave comes.
Composite Shapes The example of figure 1 illustrates relative positioning of cells for timing, juxtaposition of cells for symmetry or sequence, and the use of different cell types to support an overall theme or dramatic effect.
Note that changing tides, and especially storm surges, can be used to effect a complete change in artistic effect. Some cells might be buried and thus inactive at high tide and others might be out of the water and thus inactive at low tide. Careful planning and simulation can take advantage of this effect to great advantage.
Figure 2. A Composite Wave Sculpture
Figure 2 illustrates another composite shape consisting of two promontories similar to that of Figure 1. These could be matched for symmetry or differentiated for variety. It may at first appear that this configuration is simply twoseparate wave sculptures, but in fact it is three. The space between the two promontories has been shaped to be a large converging channel designed to produce a variety of effects or one huge central effect. For instance the central effect might be a scoop that fills a descending fountain as in figure 3, or it might be a spectacular jet that is only active during high tides and severe weather.
Figure 3. A scoop feeding a trough with cascading falls
Some Basic CellsThe vocabulary of interesting wave-shaping cells is infinite, but many interesting effects can be obtained from four basic shapes and their simple variants.
Figure 4. Four basic wave-shaping cell types
Jet - fig. 4a The jet is designed to convert the forward momentum of a breaking wave into pulse of high pressure that shoots out a jet of water at high velocity.
Scoop - fig. 4b The scoop is a non-turbulent effect. In its simplest form, it is designed to shape a wave to run up a slope without ever breaking over. The water just runs up the slope and back down. However this shape can be combined with a converging outer channel to deliver a high volume of water to an elevated feature such as the top pool of a cascading fountain as in fig. 3a.
Blow hole - fig. 4c The blow hole is similar to the jet in that its best effect is produced with high impact pressure, but in this case the configuration is designed to trap air that is then expelled in a swirling blast of air and spray.
Cannon - fig 4d The cannon is similar to the jet, except that it is designed to shoot a small slug of water at an even higher velocity. One can think of the jet as a gun, with the entire gun barrel full of water. While a high impact pressure may shoot the water quite far, the mass in the barrel is large, and therefore resistant to acceleration. The cannon is shaped so that, when idle, there is pool of water in the exit barrel, but the chamber is mostly full of air. When a wave impacts the chamber, its first effect is to compress the air and shoot the pooled water out the barrel.
Convergence Almost all interesting wave shapes derive their impressive effects from some degree of convergence or focus of the incoming wave. Focus increases velocity, which increases momentum and impact pressure. Almost every cell can benefit from a converging shape at its entrance, but huge converging shapes are also possible as illustrated in the overall design of Figure 2. Here the two promontories, in addition to being active wave sculptures themselves, serve to focus the entire wave volume between them on a narrow space at the shore, affording a great concentration of wave volume and momentum.
Ram A ram constrains moving water to impact a barrier generating high instantaneous pressures. The simplest version of this feature is a pipe leading to an active cell. Because the pipe is closed, the momentum of all the water in the pipe will act in creating a pulse of high pressure at the end. A ram pipe may also be somewhat convergent to increase velocity toward the end. Help Wanted Artists: I'm in the process of building up a credible presentation to fund a real wave sculpture. I would love to work with an artist to produce a few motivating images of an actual wave sculpture in action. Can you see my sketches as real life? Computational Fluid Dynamicists: At the same time, I would like to work with people who are capable of simulating some of these effects so that we can understand how to improve them and combine them to best effect. I don't think we can expect any patron to support a wave sculpture installation without a credible simulation of its action. Environmental Artists: There are all sorts of challenges involved in funding and permitting environmental art. If you know people with experience in this area, please put me in touch.
I have always been fascinated by the power of water and by ways to harness it. I watered my garden in Menlo Park for over a decade by a hydraulic ram that I placed in the creek behind our house. I generated power for my cabin in Mendocino with a Pelton Wheel that I built from a Honda Civic alternator and a bunch of stainless soup ladles. I have designed numerous forms of wave power generators (maybe another blog entry), though I have never built one.
A few years ago I moved near the ocean (Rio Del Mar, CA, just below Santa Cruz), and this has revived my fascination with waves. Also I live in view of the ugliest landmark on the California coast -- the dead hull of the SS Palo Alto, a concrete ship that never went to sea, but was towed here to be a docked casino on the 20's. The business failed, the hull broke, and the ship sits at the end of a pier, gated off from public access, and stinking of dead fish and seagull excrement.
Inspiration
But... in storms, it occasionally does interesting things with the waves, and this got me thinking...
I am not good at drawing, but I see endless possibilities in my mind, and maybe I could sketch a few of them later, but here is how I think about it...
Water is nearly incompressible, and this causes great shock forces when it hits a rock in the right way. It is not uncommon for spray to rise more than 10 times the height of incoming waves. Unfortunately, the same forces break up the walls and cliffs along the shore, so that the best shapes for wave bursts never last very long (except for some famous large blow holes).
But what if the "rock" were engineered to serve just this purpose, with reinforced concrete and durable coatings? And what if the shapes were perfect curves for compressing and redirecting the force of the wave? A series of cavities could be placed at intervals along the path of the waves, leading to sequences of jets as waves come in. Some shapes could be perfect nozzles leading to narrow, far-reaching jets. Others could be shaped to mix air, making clouds of spray. Different cavities could be located in range of low and high tides so that behavior would change constantly with the changing tides. Then there could be one or two really large cavities reached only during storms. These would be designed to make colossal explosions of spray -- the kind of thing that would bring TV stations out to the shore whenever the weather is really "bad" (good ;-).
I have focused so far on the physics of the problem but, when I imagine the result, it is always a piece of art, a joyful celebration of the power of the ocean and the beauty of water in all its forms. I want to release the power of waves in the most fascinating, thrilling and beautiful possible manner.
Intuitively I have a sense of what would make good effects. Beginning with a simple vertical wall, clearly the shape could be deformed to do an increasingly good job at sending all the water straight up in the air. Or at sending a small amount of the water very high in the air. And clearly it is possible to focus the energy of a wave by directing it into a converging channel. And since water is nearly incompressible, very large pressures can be produced by directing waves into a "dead end" chamber.
Simulation
I have wanted some convenient way to simulate these effects and to produce a library of useful shapes and interesting effects. I wrote to various professionals in the field of hydrodynamic simulation and they have mostly been uninterested, or they have told me this problem is too intractable. I don't believe that, and I began to study the Navier-Stokes equations, cellular simulations and the complicated situation where water meets air. Now while I still don't believe it's intractable, I did decide that I would rather work with actual waves than spend even more time on my computer.
So I built a wave tank in my driveway, 18 inches wide and 12 feet long, with a paddle at one end and a sloping "beach" at the other. I made a bunch of shapes out of modelling clay. This was the most fun part. Modelling clay is about as far from computers as you can get.
Here is a video that shows how a converging shape can quite easily send water up in the air more than ten times the height of the incoming waves...
This was very gratifying of course, but the funny thing about this project is that I have known from the beginning that it will all work. It is reality in my mind, and I just have to stick to the vision to make it happen.
Engagement
The next step in motivation was to engage the ocean. There are all sorts of rules about the beach where I live. You need permits from the Park Service and the Coastal Commission if you're going to do anything interesting, and these are almost impossible to obtain. So I figured I would just do it and ask for forgiveness instead of permission. I set about to design something we could carry out to the surf, and then remove when we were done.
Modelling clay was out of the question, and so was concrete, but I figured I could at least get my toe in the water, so to speak, with a plywood scupture, as long as I made a place to shovel sand in to keep it in place in the waves.
Scupting with a Skil saw is another good vacation from programming. I thought about this at night and built it all the next day...
It's a very simple converging shape to give some concentration effect, and some ram effect. It had to sit in the driveway for about a week because we needed low tide in the morning and evening so that we could install it on the sand in the morning, observe it in the waves during the day, and then remove it in the evening. The thing was heavy, even without sand, so I put a pipe on each side for handles because we had to carry it a fair distance. Here we are, schlepping it to the water's edge...
If this looks like a funeral procession to you, it apparently also did to a couple of the seaside residents, and we were soon visited by the local gendarmes. We almost had to abort the mission, except that I took some liberties in quoting a discussion I had had with the district park ranger (he didn't say a flat "no"), and then a reporter from the local newspaper happened by, and quickly took the side that this would be a very cool thing to try. The ranger left with "Get it gone by sundown", and we filled the outer hull so it wouldn't wash away.
Not much happened at first. It just sat there, with only the occasional wave touching it, but after an hour we could tell the tide was rising, and in another hour we had our first spout. Things just got better and better. At high tide we were getting wavelets about a foot high entering the converger, and we had spouts over fifteen feet high...
As you can see, this was all without ever getting out into the real waves that were bigger than our box. We shot some video, drank some beer, got sunburned, and made the front page of the Santa Cruz Sentinel...
Anticlimax
The next step is more complicated. We need permission to do something big and, ideally, the funding to do it as well. I thought these videos would let us breeze through the regulatory commissions, and get some easy funding from Santa Cruz and the Park Service, but not everyone can see how cool this thing could be. Fact is I have gotten nowhere locally.
It would also be great to do some serious simulations, but I have not been able to get attention from companies that sell hydrodynamic simulation systems, and not even from the fluid dynamics people at Stanford.
Where there's life there's hope...
I've been meditating on the difficulty of enrolling the local officials, software companies, and academics, and I think they are simply the wrong people with the wrong perspective. While riding my bike the other day, I think i've come up with a "perfect storm" of potential receptiveness to this project; a small community that encompasses ocean technology, shoreline authority, likely artistic entusiasm, and possibly even some funding.
There are several wave power projects around the world and, for the most part, they are sites that you can go visit, and look at. You can imagine that they have little lookout platforms with explanatory plaques and PR about the companies and funding agencies. Wouldn't it be cool, and soooo appropriate, to have a wave sculpture installation in view of the observation deck? A true celebration of the ocean's power and man's creative involvement in it. These people know how to simulate ocean waves in action, and they probably even have a small fund set aside for PR.
This is the kind of waves we should be working with (note the SUV for scale)...
So, if you know anyone involved in a wave power project, please send them a link to this modest proposal, and maybe we can take it to the next level.
You may know I am fascinated with all manner of steam machines, from pop-pop boats to locomotives and rockets. I've had a number of inventions in this area, but today I'm going to write about two designs for a boilerless steam engine.
This all began when, in a moment of weakness, I came into possession of a seventeen-foot dory powered by a steam engine. The boat needed work and I also wanted to convert the boiler from kerosene to gas in order to make it a little less messy and a bit easier to fire up. As I began to rework the firebox and fix other things around the boat, it began to bother me having a box full of high pressure steam in the boat with me. This became a meditation and led to my first design for a boilerless steam engine.
Design #1: The Boilerless Steam Engine
One area of technology that was missing in the golden age of steam was fast-acting solenoid valves and electronic controls that are used to provide electronic fuel injection in modern gasoline and diesel engines. The idea is simple: a fuel pump delivers fuel to the injector valve under pressure, and a computer commands the solenoid inside the injector to open a valve and spritz a certain amount of fuel into the airstream entering the engine. With today's technology the amount of fuel delivered can be carefully controlled even at speeds to match the an engine at 6000 rpm (that's 10 milliseconds pre revolution).
So my idea was is: instead of heating up a whole lot of water in a boiler and storing lots of dangerous steam, heat up a block of metal in the cylinder head, and then use a fuel injector to spritz water into that hot metal on the way to the cylinder. Only two controls are needed: a coarse one to keep the cylinder head at a heat of 300-500 degrees C (by throttling a gas burner), and a fine one to fire the injector at the right time and for the right duration.
[schematic image of a boiler and steam engine yet to be supplied]
[schematic image of Boilerless Steam Engine yet to be supplied]
While I've never actually built one of these engines, there can be no doubt that it would work. However, I think I've convinced myself that it wouldn't work very well. The problem is being able to transfer enough heat to turn a pea-sized drop of water completely to steam in, say, 25 milliseconds (corresponding to 1/4 revolution of an engine at 600 rpm) I don't know that this won't work, but I've seen hot water dance on a griddle, and I think that sort of surface flash effect would keep the water droplets from making good contact unless you forced them through very small holes, and then that might well dissipate much of the energy that was supposed to do work on the piston. I'm sure it could work, but I just don't think it would work well. It would be fairly easy to test the idea by simply experimenting with injecting water drops into a channel in a ver hot piece of metal. But now I have an even better idea...
A little applied physics
We pause here for a lesson on superheated water and the critical point. You know that if you put a pot of water on the stove and heat it up, it starts to boil when it gets to 100 degrees C. The more heat you supply, the more water boils, but it never gets hotter than 100 degrees. In a pressure cooker, the container holds a pressure at 15psi above atmospheric pressure, so the water gets up to 120 degrees or so before it boils, and a pressure relief valve prevents the pressure from getting any higher for safety reasons.
But what if you have a really strong tank, and *don't* release the pressure, but just keep heating the water up? Well it just gets hotter and hotter and the pressure goes up and up. If you let some of this water out, you will find that it boils when it is released, because it already has enough extra heat content to boil it [this is what happens when you take the radiator cap off an overheated automobile engine]. Time out for two useful facts:
It takes 1 calorie of heat to raise 1cc of water by one degree C
It takes 540 calories (!) to turn 1cc of 100 degree water into steam
[bonus fact] that same 1 cc of water becomes 1675 cc(!) of steam
Now if you keep heating the water up to 374 degrees C (it will be at a pressure of 3200 psi) you get to what is called the critical point. At this temperature if you release that water into the air it will spontaneously turn completely into steam. Recalling that this involves an expansion by a factor of over 1600, you can see that this will be an explosive change.
Steam rockets (off topic, but inspriational...)
If you made it this far, you will probably enjoy seeing what some people have done by just putting a nozzle on a tank of superheated water to build a rocket...
While it may seem like an uncontrollable thing, the steam rocket is actually remarkable for its controllability. It is as simple as a solid fuel rocket to start, and yet it can be throttled down or stopped at any moment, unlike solid fuels. This is why Evel Knevel used it for his Snake River crossing attempt (it failed due to premature parachute deployment).
Design #2: The Superhot Water Engine
If you think like me, you are probably anticipating the second design. Here, instead of injecting cold water into a very hot cylinder head, hoping that it will flash to steam, we inject super heated water into a normal cylinder where it will flash spontaneously from its internal heat content. It may sound scary to be dealing with water that is hotter than molten lead at a pressure of 3000 psi but, there will be no large reservoir of this water. Here is the the setup:
The pressure is generated by a water pump (no tank involved), the heating takes place in a relatively small pipe (again, no large accumulation of hot water), the pipe goes straight to an injector valve into the engine's cylinder.
[schematic image of Superhot Water Engine yet to be supplied]
Baby steps
In order to conduct a few experiments, we need a pump, a heater, and an injector. We can do the first experiment just shooting steam out of the injector into the air. Then as a next step, we can shoot it into the spark plug hole of a leaf blower motor (I happen to have several leaf blowers left from an earlier experiment ;-).
Parts list
Pump: AR383 pressure washer - 1900 psi at up to 1.5gpm
available for $165 from Wallmart
Pipe: 1/4-inch copper tubing - small diameter gives both high rating
and good thermal transfer, and minimizes the amount of stored fluid.
Thermometer: Pyle PIRT30 high temperature IR thermometer
50 tp +550 degrees C, available for $70 from Sears
Heater: Bernzomatic TS4000T trigger-start torch
available for $35 from Walmart
Injector valve: ?
The injector valve is the one missing ingredient, and affordable pumps fall short of the desired 3200 psi. We should be able to observe plenty of the spontaneous injection effect at 2000 psi, though, with the only downside being that we will produce somewhat wet steam and somewhat less power from the engine.
Collaboration?
I'm eager to start experiments and build one of these engines. However because of a heavy work schedule and family obligations, I won't get much done in the next few months. If you would like to work with me, or even try some of this on your own, please get in touch with me and share the fun!
