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A simple-to-make, hot-burning, backcountry stove The Super Cat Alcohol Stove
Originally published
January 29, 2005 Last revised November 18, 2008 |
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Originally developed in early 2004, the Super Cat alcohol stove was first shared with the online backpacking community in January, 2005. Since then, it's become one of the most popular do-it-yourself alcohol stoves in the world for probably two reasons: it works exceptionally well and it's very easy to build. This update to the original article is intended to incorporate many of the design ideas that Super Cat users have contributed over the years, as well as to expand the scope to include new information about build techniques, fuels, windscreens, stands and other accessories. It's also being published concurrently with a new article entitled "The Fire Bucket Stove System", which describes a companion product that can serve either as a high performance windscreen for the Super Cat (and most other alcohol heaters), or a stand-alone stove that is capable of burning wood and solid fuel tablets—all at a weight (in its lightest implementations) of about two ounces. The Super Cat and Fire Bucket together form an efficient, lightweight, integrated stove system that's easy to make and fun to use. You can navigate directly to each chapter using the links below.
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Background More than a century after the venerable Svea kerosene stove first went into production, one might reasonably wonder why backcountry stoves, like so many other mature products, haven't coalesced around one or two successful design technologies. Instead, today's backpackers still burn white gas, kerosene, diesel, automotive gas, jet fuel, propane, solid fuel tablets, butane/propane mixes, wood, paraffin, mineral spirits, vegetable oils, methanol, ethanol, and isopropyl alcohol in a huge variety of stove types. So why still so many choices? I guess simply because no one has yet been able to come up with an all-in-one design that can meet the wide range of environmental, fuel availability, weight, cooking, and safety requirements of today's hikers. The future may belong to an atomic fusion stove that weighs two ounces and can run for a hundred years on one gram of fuel, but in the meantime, the top-mounted (iso)butane/propane canister stove probably comes closest to meeting the needs of most of today's three-season backpackers, particularly those who trek in Europe and North America. Ultralight models, often equipped with piezoelectric lighters, are made by MSR, Primus, Gaz, Snow Peak, Brunton, Optimus, Coleman and others, and are convenient, dependable, quiet, efficient, and though hot burning, can also simmer well. In addition, analysis has shown that even for gram-counting long-distance hikers, these stoves compare favorably with other types of stoves from a weight-to-performance point of view. Nonetheless, these stoves are not perfect. My own biggest complaint is that the fuel canisters themselves are not refillable by the user, which means that it’s often difficult carry just the amount of fuel that you think you’ll need for a given outing. In addition, compatible canisters are sometimes difficult to find outside the U.S. or when re-supplying on long hikes (canisters can’t be transported on commercial airliners). These stoves also typically perform poorly in cold weather and can be difficult to use with conventional windscreens (although the KiteScreen offers a great solution to this last problem). Alcohol Stoves Alcohol stoves are one of the most popular alternatives to canister stoves, especially for long-distance hikers, perhaps because they solve one key problem: fuel availability. Alcohol stoves can burn ethanol or methanol (and combinations thereof), or in a pinch, even isopropyl alcohol, which means that fuels at re-supply points are generally fairly easy to find at liquor, hardware, automotive or variety stores. Alcohol stoves also allow hikers to carry just the amount of fuel needed for a given outing, are super-quiet in operation, are usually very simple in design (no fuel jets to clog or other components to fail in the field), and are usually extremely light weight. Many green-spirited backpackers also like alcohol stoves because they use a renewable fuel source, unlike the majority of stoves now in use that burn fossil fuels (although alcohol stoves still emit carbon dioxide, a greenhouse gas). On the flip side, alcohol stoves present certain safety hazards, including a flame that's nearly invisible in sunlight (which means that it's sometimes difficult to detect when the stove is operating) and the use of an open fuel tank that can spill flaming alcohol on you and your surroundings if tipped over. These stoves are typically also more wind-sensitive than other types—and as a double whammy—use their fuel much more quickly than do fuel-metered stoves under windy conditions, at least if not properly protected. And while they excel at boiling water, alcohol stoves are typically difficult to control for simmering. All designs of which I'm aware must be also re-filled after every use. This single-shot-of-fuel per use operation means that it's often tricky to gage how much alcohol to add to your stove for any given cooking operation. As a result, backcountry cooks usually either run out of fuel before finishing, or end up with excess fuel that frequently burns wastefully away until it's finally consumed (although some of the accessories and techniques described below can reduce this waste). Likewise, the energy content of alcohol is considerably less than comparable petroleum-based fuels (typically about half), which means that significantly more fuel must be carried. The good news here is that alcohol fuels are safer to handle, can be transported in ordinary plastic bottles, and can be extinguished with water in an emergency. JIM'S STRATEGY Despite their disadvantages, I still like alcohol stoves. Sometimes I use one as a primary heater and sometimes as a backup or "fill-in" for my canister stove. Carrying an alcohol stove, which weighs almost nothing by itself, often allows me to avoid carrying a second weighty canister if I'm concerned about not having quite enough fuel. I can include just the right amount of alcohol to supplement the main butane/propane supply for the trip. For this type of use, a backup stove doesn't need to be a full-featured replacement. All it really needs to do well is boil water in a reasonable amount of time, which also allows it serve as a second burner in the preparation of some meals. I also now usually carry the Fire Bucket Stove System which can work as a windscreen for either the Super Cat or canister stove (see the Fire Bucket article for details), or in a pinch, burn wood if I run out of other types of fuel. Alcohol Stove Designs There are dozens of do-it-yourself alcohol stoves currently described in various spots around the Internet. Rather than duplicate information that's already available, I would instead refer you to the most complete backpacking stove site that I've found thus far: the Zen Stove site. NON-PRESSURIZED vs PRESSURIZED For purposes of classification, however, I will note that alcohol stove designs generally fall into one of two categories: non-pressurized and pressurized. Non-pressurized models work by simply housing an open flame that’s created by burning expanding alcohol gases. These gases exist anytime that liquid alcohol is warm enough to evaporate, where the warmer the liquid, the faster the rate of evaporation. Examples include Roy Robinson’s Cat Stove and the Robert Crowley Plumber Stove. Pressurized models restrict the expansion of this gas in some manner to create gas “jets” that are said to burn a little hotter than do non-pressurized flames. Since I’ve done limited testing myself, I can’t say for sure that it really makes any difference, but in either case, the Super Cat would be classified as a pressurized design. Also, because of slightly higher flame velocities, pressurized models are probably tend to be a bit less wind-sensitive. While I'm at it, I'll also note that the term “pressurized” may also be a bit overstated, since the amount of back-pressure created by restricting the gas flow is very small—just enough, in fact, to force the expanding gas out through a series of burner holes. It’s nothing like the pressure, for example, that’s created by manually pumping an MSR white gas fuel bottle. SIMPLICITY OF DESIGN Probably the most significant thing that sets the Super Cat apart from other pressurized stoves such as Scott Henderson’s Pepsi-G, the Anti-Gravity Gear Tin Man, or the Brasslite stoves, is the simplicity of design. Most pressurized stoves require multiple components that must be taped, epoxied, or welded together in order to create their pressure chambers. The Super Cat’s pressure chamber, on the other hand, is created simply by placing a pot on top of the stove, thereby blocking the ability of the expanding gas to escape through the top, forcing the flames out through the side vents. The Super Cat design is also simpler than most others because it doesn’t require the use of a separate pot stand. The stove itself serves as both the burner element and the stand, since the pot is placed directly over the top of the stove. Because of its uncomplicated design, the Super Cat is also very easy to build. So easy, in fact, that they're sometimes constructed on the trail (often in less than five minutes) using materials procured at re-supply stops. Incidentally, I should note that this stove is named “Super” because it burns hotter and faster than most other alcohol stoves whose specifications I’ve seen. With some implementations of the Super Cat, I've consistently (under ideal, no-wind conditions) brought two cups of water from cool room temperature to a full rolling boil in under four minutes, which is among the fastest times I’ve noted so far, though there's really no way to guarantee the consistency of the conditions among all those who conduct these tests. It’s also called “Super” since it’s super-easy to make and, at a weight of less than ¼ of an ounce, super-light weight. The “Cat” portion of the name was derived from its construction using a single, 3 ounce aluminum cat food can. WHY THE SUPER CAT WORKS Unfortunately, one can't punch a few holes in any old can and expect to turn it into a working pressurized stove. For starters, the can's volume needs to be just right for the alcohol vapors to pressurize in a way that allows them to expand through the side vent holes after lighting. If the internal space is too large, the flame will simply be extinguished when the pot is lowered into place. Alternatively, if it's too small, the same thing will probably happen, but even if it manages to pressurize, the stove's fuel capacity will likely be too low to complete most cooking operations without refilling. The diameter of the can is also important since the stove also serves as the pot stand. If the diameter is too small, the stove could become unstable when a pot is placed upon it. If the stove is too wide, the flames will probably miss most of the pot's bottom surface and some of the heat will be lost up the sides (though there are some interesting exceptions to the conventional wisdom regarding stove and pot width that are discussed below). The material and thickness of the can's walls are also critical. In order for the stove to work properly, some of its heat must be transferred back into the open pool of alcohol to keep it boiling ("thermal feedback"). If the the stove is made from a metal that's either too thick or that conducts heat poorly, the alcohol can stop boiling, killing the flame. Conversely, if the sidewalls are too thin, then the stove probably won't support the weight of a pot filled with two to four cups of water (which might weigh two pounds or more) and could collapse when heated to operating temperature. SO HERE'S THE MAGIC It turns out that the 3 ounce aluminum can that's recommended for Super Cat use is just about perfect for this task. The volume is such that the alcohol vapors pressurize properly under almost all altitude, temperature and other operating conditions. It's also large enough that, depending on hole configuration, it will hold up to 2 fluid ounces of fuel, which is usually more than enough for most cooking chores. Likewise, the can's diameter easily supports most commonly used pot sizes while maintaining a high degree of efficiency. The aluminum walls are likewise thin enough to efficiently conduct heat to the alcohol pool to keep it boiling, but are also thick enough to safely support the weight of a full pot of water (at least of the size range most likely to be used). Over the years, I've experimented a variety of other cans types that have been larger, smaller, and constructed from metals such as steel and brass, and I have yet to find anything that works as well as the 3 ounce aluminum can. the question of pot width One interesting thing I've learned from my development experiences is that the conventional wisdom about side-burner stoves and pot width is not necessarily correct. Many hikers believe that alcohol stoves like the Super Cat only work well with wide-bottom pots under the theory that narrow pots allow too much of the stove's heat to flow up the sides of the vessel, thereby significantly reducing efficiency. In truth, it's not that simple. Somewhat to my amazement, the fastest boil times I've ever observed with the Super Cat have been with tall, narrow cook pots. More specifically, I'm referring to those that are constructed from 24 or 25 ounce aluminum beer cans sold by Heineken, Fosters and others. These cans, which are quite popular with ultralight backpackers, have bases that are only about 3¼ inches wide, compared with a pot like the Snow Peak Trek 1400 (my personal favorite) whose base is a bit over 5 inches wide. With these beer can pots, I consistently clock boils times for two cups of cool, room temperature water at under 4 minutes with the Super Cat, while the best I've seen with the much wider Snow Peak 1400 is about 4½ minutes under the same conditions.
Beer
can cook pots with Super Cat stoves (+) Much of the reason for the speedier beer can boil times is related, of course, to the material from which these vessels are made. The very thin aluminum walls of the cans conduct heat more efficiently than thicker titanium walls of the Snow Peak pot, partly offsetting the effects of their less-than-optimal shapes. And to be fair, in a titanium-to-titanium comparison, the Snow Peak 600 mug, whose base is about the same with as the beer can pots, requires 15% to 20% more time to boil two cups of water than does the wider Snow Peak 1400, whose walls are of about the same thickness.
Snow
Peak 1400 (L) and Snow Peak 600 (R) But my point here is even when using fairly narrow pots, a significant amount of energy transfer occurs when a stove's flame wraps around the pot bottom and travels up the sides. Accordingly, I think you'll be happy with the performance with the standard 3 ounce can when used with just about any reasonably-sized cook pot. Just to be sure, I've constructing stoves from narrower aluminum cans like those used for Red Bull energy drinks. Those cans are about 2 inches in diameter versus 2.4 inches for aluminum pet food cans (i.e., are about 83% as wide) and I have seen no significant difference in boil times.
Red Bull sized stove with Snow Peak 600 titanium mug (+) Disclaimers and Safety Notes Disclaimers Before proceeding, I should point out that I am not a chemist, nor an expert in stove technology. I am just a backpacker who struck upon something interesting a few years ago that I felt was worth sharing with my fellow hikers. If you decide to build your own Super Cat, you must assume all risks. I obviously can’t guarantee your safety nor indemnify you against accidents. While there are a number of hazards associated with any backpacking stove, an alcohol stove like the Super Cat probably has more than its fair share, as I discuss in the next section. That said, as long as you're careful, building alcohol stoves can be safe and a lot of fun, perhaps even bordering on addictive for some. I should also mention that when researching existing stoves prior to developing the Super Cat, I found many clever and well-tested designs available. Nonetheless, I had an idea for a type of stove that didn’t seem to be represented by any of the models I read about, though it’s certainly possible that someone has employed this design before. If so, I apologize for the lack of attribution, but note that I did arrive independently at all of my conclusions. Safety Notes Experienced outdoors people already understand that any backcountry stove is potentially dangerous and should be handled with great care, especially when operated in the vicinity of a tent or tarp. Alcohol stoves like the Super Cat, however, are probably even more hazardous than some other types for reasons that are discussed below. CARBON MONOXIDE All backcountry stoves can emit fair amounts of carbon monoxide (CO) which can be deadly if concentrated in closed spaces. The best review I've seen of hiking stoves and CO was prepared by Roger Caffin, an expert who writes for backpackinglight.com. In Part 4 of his excellent series of research articles, Roger studied the emissions of ten commercial alcohol stoves and concluded that: "...each of these alcohol stoves emits more CO than the best canister stoves... all should be considered extremely dangerous in any confined space." To my knowledge, the Super Cat has never been tested for CO emissions, a task that requires a unique laboratory setup. I have no reason to believe, however, that the carbon monoxide generated by the Super Cat would be much different from any of the stoves Roger tested. Accordingly, you need to be especially careful when operating the Super Cat indoors or inside a tent vestibule. Without plenty of fresh air ventilation, carbon monoxide can kill you. By the way, Roger's article can be found here. To read the full text, you'll need to be a BPL online subscriber (currently $24.99 per year); otherwise, only the abstract will be available. FIRE HAZARDS Because fuel is burned in an open container, an alcohol stove like the Super Cat can present a significant fire hazard. Unless the stove is anchored to the ground or to a windscreen like the Fire Bucket, it's fairly easy for the stove to tip or blow over during operation. And as you might imagine, spilling flaming alcohol on you and your gear is a great way to ruin your day. Likewise, when using the Super Cat, make sure that all combustibles are positioned well away from the vicinity of the stove and that there's a water source available if things go seriously wrong. Unlike grease or petroleum fires, which are often spread when water is applied, alcohol fires can usually be safely doused by drowning. Other less drastic methods of stopping the Super Cat, such as depriving it of oxygen, are discussed below. Fires can sometimes also start with alcohol stoves because they operate so quietly and burn with a flame that's nearly invisible in daylight. You need to be especially careful to keep flammables (like synthetic clothing) away from your stove if there's any chance it could be running. Probably the best method to confirm operation if you're uncertain is to place your hand near the stove or its windscreen to feel for warmth. Petroleum-Based
Fuels And in case you're tempted to try a higher energy content fuel (like white gas) in the Super Cat, you should also know that I've already tried many of them and they just plain don't work. They typically burn with a low-temperature, yellowish, sooty flame that won't pressure in this stove. These liquid fuels only work effectively when vaporized under fairly high pressures and temperatures in stoves like the MSR WhisperLite. Silnylon
Shelters FUEL TOXICITY When compared with (iso)butane/propane canister stoves, liquid fuel stoves can present the additional hazard of direct exposure to toxic chemicals. Denatured alcohol and yellow Heet are the most commonly burned fuels in alcohol stoves, at least here in the United States. While a more detailed discussion of these substances is included in the "Super Cat Fuels" section below, I'll just concentrate here on the potentially harmful effects of one of the key ingredients in these fuels: Methanol. Most denatured alcohols contain some amount of methanol, which a toxic form of alcohol that's intentionally added to ethanol to render it undrinkable. Methanol, which is used in a wide range of applications, is also known as methyl alcohol, wood alcohol, carbinol, wood naphtha and wood spirits. Poisoning
Hazards This form of poisoning is perhaps best known in connection with drinking liquor (such as moonshine) that's produced under unsafe conditions. Unless properly distilled, methanol can be generated as a byproduct of whiskey production. Most backpackers probably don't need to worry about accidentally drinking methanol, however. Instead, exposure is more likely to occur through inhalation or absorption through the skin. After plenty of Internet research, it's still not clear to me at what exposure levels methanol becomes dangerous through these two mechanisms. Perhaps the studies have been done, but I've not found them. There is universal agreement, however, that the risks are real. As noted in the Wikipedia methanol article: "...Dangerous doses will build up if a person is regularly exposed to vapors or handles liquid without skin protection." The truth may be that nobody knows for sure at what point inhalation and skin exposure becomes unsafe. I would simply offer the following obvious advice to any backpacker who might elect to use methanol-based fuels:
I will finally note that denatured alcohol products often contain other toxic ingredients besides methanol. Methyl isobutyl ketone, acetone, ethyl acetate, rubber solvent and other hydrocarbons are frequently added to ethanol as denaturants, each of which is potentially dangerous. Because their concentrations are usually small, however, these other ingredients may not represent quite the same magnitude of risk as does methanol. SUPER CAT-SPECIFIC ISSUES In addition to the general risks described above, the Super Cat presents a few unique challenges that are discussed in the "Starting and Using the Stove" section below. Fortunately, all of these Super Cat-specific risks can be easily minimized or eliminated by using either the techniques or accessories that are likewise discussed in that section. LIVING HAPPILY IN SPITE OF THE ODDS While I feel it's important to inform you about potential dangers, I hope I haven't discouraged you from building and using a Super Cat. The truth is that alcohol-based stoves are generally accepted as safe by most knowledgeable backpackers, as long as the user exercises a fair measure of common sense. I've been working with and testing these stoves for many years without serious incident and accordingly, would encourage you to give the Super Cat a try. Materials CAN SELECTION At the core of the Super Cat project, of course, is an empty, clean aluminum can. The 3 ounce size that's widely found at most grocery stores and used for products such as cat food and potted meat sandwich spreads is probably the best all-around choice for the reasons discussed above. Examples of this type of can are shown below.
Samples of products packaged in 3 ounce aluminum cans (+) You'll find that even among similar can types, there will be some variations in dimensions and weights. For example, the Hormel potted meat can shown above is slightly taller than the Fancy Feast cat food can, even though both contain 3 ounces of product. Most of these minor variances will have little or no impact on stove performance, however. Aluminum
vs. Steel
Samples of 3 ounce steel cans (+) While some of these steel cans are perhaps useable for Super Cat stoves, there are some noteworthy issues:
The performance results from tests conducted recently in a head-to-head comparison of aluminum and steel stoves is shown below. Both stoves were fueled with one fluid ounce of SLX denatured alcohol and used to heat two cups of cool room temperature water in a Snow Peak Trek 1400 titanium cook pot to a rolling boil. The fuel in both stoves was also ignited without external priming mechanisms (such as a priming pan).
Bottom line: unless you have compelling reasons to use steel, I'd suggest sticking with aluminum. But when shopping for a suitable can, how does one distinguish between the two, since their appearances can be very similar? One way to tell the difference is to gently press on the sidewall of the can with your thumb. Aluminum cans will flex fairly easily, while steel cans have much less "give". Another way is to take a scale with you to the grocery store. An aluminum can that contains 3 ounces (net weight) of product will probably weigh about 3.3 ounces in total. A similar steel can will likely weigh over 4 ounces. You can also, of course, select products that are known to be packaged in aluminum. The list below includes a few brands distributed regionally or nationally in the United States that I've verified (at least as of the date of publication) are sold in aluminum cans.
PREPARING THE CAN Once a suitable can is obtained, you'll want to first remove the label and clean the interior. I'd also recommend that you remove the gummy label adhesive using a solvent such as Goo Gone or Goof Off (by the way, the lubricant WD-40 also does a great job of dissolving many adhesives and is probably less toxic than most other solvents). While some of the residue will eventually burn away if you choose not to bother, the remainder will tend to stay somewhat sticky. If you build an an optional stand (more below) that uses a "docking socket" to hold the stove, the residue will often melt between the stove and the holder, effectively gluing the two together. This remaining goop will also sometimes transfer to other items in your pack, so it's best to remove as much as possible. Tools The tools you'll need will depend on how you build your Super Cat, but are mostly quite simple. For the basic Super Cat described below, all you'll need are a flexible measuring tape and a felt tipped pen for measuring and marking ventilation hole positions, and a standard office punch for making those holes.
Simple Super Cat tools (+) MAKING HOLES There are many ways to create vent holes in the walls of the can, but one of the easiest (and neatest) is with a handheld office paper punch such as that shown above. Most standard punches are designed to make holes that are about ¼ inch in diameter, which is about the largest size you'll want to consider for the Super Cat. Available at most office supply stores, these punches are also inexpensive and can easily puncture the soft aluminum. Not all of these punches are created equal, however. For example The $0.97 punch I recently bought at Wal-Mart does not work very well, nor do any of the dollar store versions I've tried, all of which failed quickly. On the other hand, the $1.29 model from Staples that's shown in the photo above has performed very well. Online craft stores are another source for quality paper punches in sizes other than ¼ inch. I actually prefer vent holes that are slightly smaller at 3/16 inch in diameter because the resultant stove flame seems to be a little less wind-sensitive than with ¼ inch holes. Mister Art, for example, offers such punches in a range of sizes. Another option is an inexpensive sheet metal punch that is much more durable than a paper punch. Also known as "Whitney" punches, they can be purchased for as little as $20 plus shipping from online sources such as Harbor Freight, that offers both standard and deep-throated models. These punch kits include multiple die sets that create holes in up to 16 or 20 gauge (depending on model) steel sheet metal in a variety of sizes. I own the standard Harbor Freight model and have found that it's ideal for building Super Cat stoves.
Sheet
metal ("Whitney")
punch An electric drill is another obvious choice, though drilled holes tend to be a little messier than punched holes. One exception are those that are made with a variable width bit called a "Unibit" that allows for creating fairly smooth edges. Made by Irwin and Klein, these bits are available from a variety of online sources and sell for about $20.00. I haven't tried a one myself, but some Super Cat builders have reported excellent results with Unibits. Finally, as described in the original Super Cat article, a sharp awl or similar tool works well and allows for easily making holes of multiple sizes. The downside is that the holes are not very pretty, but I still like this method in part because of its simplicity, but also because it can be easily improvised on the trail using a nail or pocket knife.
Alternative tools for punching holes and flattening collars (+) If you employ the awl method, you'll probably also want a pair of pliers to flatten the ragged "collars" created by the punctures on the insides of the can. This process is described in more detail further below. Super Cat Build Instructions Note: There are many tools and techniques that can be used to build Super Cat stoves. The build instructions below are designed to serve as a starting point, but I'd encourage you to experiment with alternative construction methods (some are discussed later on). HOLE SIZES and PATTERNS The number, sizes and positions of the vent holes will determine how well (or whether) your Super Cat works. They will control the fuel / air mixture, the burn intensity and will also affect how much weight the stove can support. The good news is that a wide range of sizes and patterns will work to some degree, so the question is selecting the combination that will optimize the stove for a particular purpose. Most of the time, backpackers want a stove that simply boils water quickly and efficiently. By varying the hole sizes and patterns, however, it's also possible to build a stove that burns with reduced heat output, useful perhaps for extended simmering. Within limits, increasing the size and number of holes in the sidewall produces a stove that burns hotter, while smaller and fewer holes will cause the stove to burn cooler. The prototype stoves below show a few of the hole patterns with which I've experimented.
Stove samples show some of the hole patterns tested (+) The first hole pattern discussed below will create a stove that burns hot and fast. The stove also burns with a mostly blue flame (with only occasional bursts of yellow) that flows smoothly from the vents without the "pumping" action that is sometimes observed when the hole pattern is not quite right. The holes in this design are also made with a standard office paper punch and are ¼ inch in diameter. As noted above, slightly smaller holes (I like 3/16") will produce a flame that seems a little less wind-sensitive, but ¼ inch hole punches are much more widely available and so will be used for these instructions. I'd suggest proceeding as follows:
Note that the dimensions shown in the diagram below assume the use of a Fancy Feast gourmet cat food can and are delineated in English units (inches). If you'd prefer to work in metric units (millimeters), you can click on the link under the caption.
Super
Cat dimensions using a Click here to see dimensions in metric units When you've finished, your new Super Cat should look the the sample shown below. Other
Hole
Creation Methods
Vent holes
made with an awl
Flattening vent hole collars
Completed stove (+) ALTERNATIVE HOLE PATTERNS Reducing
Heat Output for Simmering The first way to reduce heat output, which offers the finer level of control, involves limiting the amount of fuel that reaches the flame. This technique is used, of course, in all canister and commercial liquid fuel stoves, usually by employing a screw-type fuel valve that's typically located either at the stove or on a remote fuel bottle. With almost all alcohol stoves, however, the entire fuel supply is always available for the duration of the burn, so there's no practical way to limit its exposure to the flame. The second method, which is the only choice available for most alcohol stoves, involves reducing the amount of oxygen that reaches the flame. This option, however, is usually much more difficult to control since even a slight breeze can radically alter the airflow in and around the stove. One technique that's used by a fair number of alcohol stove users is to add some type of air-restriction shield around either the vent holes of the windscreen or around the stove itself. Some Super Cat users have, for example, built "simmer rings" that can be temporarily attached to the stove in such a way as to block one row of vent holes in order to reduce heat output. Creating airtight seals with these kinds of shields is difficult, however, and so they often don't work very well. In addition, most alcohol stove designs don't allow this type of airflow adjustment to be made in real time while the stove is operating. Before proceeding, I should mention that one of the best approaches to simmering is not to simmer at all, but rather, to cook with retained heat using either single or double pot cozies as described in August, 2008 article "Three Mods for Your Mug". As many backpackers already know, this method can save a lot of fuel by preserving the energy that's already been added to a cook pot during the initial heating process. For meals that need extended cook times, I'd strongly suggest that you try this method first. Building
a "Simmer Cat" You can build a Simmer Cat in many ways, but probably the easiest is to construct the same stove that's described above, but without the bottom row of ventilation holes. Based upon my tests, this single-row stove will operate with a bit less than half the heat output as a comparable model that uses a double row of holes.
"Simmer Cat" with a single row of vent holes (+) A performance comparison between similar Super Cat and Simmer Cat models is shown below. Both stoves were fueled with one fluid ounce of SLX denatured alcohol and used to heat two cups of cool room temperature water in a Snow Peak Trek 1400 titanium cook pot to a rolling boil. The fuel in both stoves was also ignited without external priming mechanisms (such as a priming pan).
The heat output of a Simmer Cat can be further adjusted by either adding or deleting vent holes. Obviously, adding a hole is easy, while deleting a hole usually requires starting over with a new stove, so it's best to proceed slowly when experimenting. Likewise, hole sizes can be reduced, though I've found that when using a single row of vents, holes that are much smaller than 3/16" in diameter will probably lead to stove that doesn't work at all. By the way, if you really think you need real-time control over your stove's heat output, a completely different approach to simmering is offered below in the "Accessories" section. Called the "Swivel Cat", this stove requires a separate pot stand, but allows you to make flame adjustments while the stove is operating. Using
a Simmer Cat on the Trail Other
Hole Configurations The first is that the fuel capacity is (obviously) defined by the height of the bottom edge of the lower holes above the base of the can. The higher these holes are positioned up the wall of the can, the greater the potential fuel volume. The tradeoff, however, is that if the flame jets that emanate from these bottom holes are too far away from the top surface of the alcohol, there may not be enough heat transferred to the alcohol pool to keep it boiling and the stove could extinguish itself, especially in chilly weather. The range of distances, as measured from the bottom of the can to the bottom edge of the lowest hole, that seem to work are 1/2" to 5/8" (13mm to 16mm) for double row stoves and 7/8" (22 mm) for a single row Simmer Cat stove. The approximate fuel capacities of each of those hole heights is shown below.
Another variable to keep in mind is that the larger the vent holes, the more wind-sensitive the stove is likely to be. The largest hole size I've found that works well is about ¼ inch in diameter. On the other hand, vent holes that are too small may not work at all. A couple of Super Cat builders have reported making stoves that use three rows of very small holes, but I've never been able to get this arrangement to work (perhaps I'm missing something). It's also not mandatory that all vent holes be of the same size. Some of my earliest Super Cat prototypes (that worked very well) were constructed using a row of relatively large top holes, along with slightly smaller holes in the bottom row. Also remember not to create so many holes that the structural integrity of the can is compromised. If you remove too much aluminum, the stove might not be able to safely support a pot full of water. While the stove may not actually collapse during operation, the walls might slowly warp under heat stresses, shortening the Super Cat's life. Irrespective of the hole configuration you select, you'll want a mostly blue alcohol flame (a few short yellow bursts are OK) that flows smoothly from the vent holes without the "pumping" action that usually indicates that the fuel / air mixture is less than optimal. Super Cat Fuels Fuels that are appropriate for use in the Super Cat, as well as in most other alcohol stoves, have been widely discussed on the Internet, so I don't want to simply regurgitate what others have written. One of the best reviews is available on the Zen Stove website. Likewise, the names, availabilities and even colors of these fuels can vary from country to country. A good resource for international fuel information can be found here. The discussion below is a brief summary of the fuels that are either used, or might be considered for use, in alcohol stoves and reflects many of my own experiences. DENATURED ALCOHOL Over the years, I've tried a wide variety of fuels in the Super Cat. The best results have consistently come from denatured alcohols, which usually burn hot and clean with virtually no odor or soot production. Denatured alcohols are widely available in the United States, though there is no standard formula for these products among its various manufacturers. Ethanol To render the ethanol undrinkable (and therefore not subject to liquor taxes), a variety of "denaturants" or toxic chemicals are mixed into the ethanol to convert it into denatured alcohol. In some countries, dyes (often purple) are also added to help distinguish the product from clear nontoxic beverages such as water. Methanol Aside from its use as an ethanol denaturant, methanol is also widely employed as an industrial and marine solvent, a paint remover, a car racing fuel, and as a component in shellacs, photocopying compounds and windshield-washing fluids. The biggest problem for backpackers is that unlike ethanol, methanol is poisonous when ingested, inhaled or absorbed through the skin (see the "Safety Notes" section above). Methanol does, however, have combustion properties that are similar to ethanol. A comparison of ethanol and methanol, along with the petroleum-based Coleman fuel that's used in traditional backpacking stoves (like the MSR WhisperLite) is shown below.
Many experienced alcohol stove users prefer denatured alcohol blends that contain as much ethanol as possible, since it has a higher energy content than methanol and is also less toxic. On the other hand, the boiling point of methanol is lower than
ethanol which means that it will vaporize more easily in cold weather (but
still not as well as Coleman fuel). The brand of denatured alcohol I've used
the most is SLX from WM Barr, which contains roughly 50% ethanol and 50%
methanol. At that mix, its blended energy content is about 20.7 megajoules per liter or 58% of the petroleum-based Coleman fuel. On a volume basis, that blend is also about 113% of the weight of Coleman fuel, which means that on a weight-for-weight basis, denatured alcohol contains about half the energy content of Coleman fuel. In other words, to boil the same quantity of water on a backpacking trip, I'd need to carry twice the weight of denatured alcohol as I would Coleman fuel. With priming, a little waste, and lots of morning coffee, I typically use about 1½ ounces (by weight) of petroleum-based fuels a day, so a 7-day trip generally would require a total of about 10½ ounces. If I carried SLX instead, I'd need roughly 21 ounces of denatured alcohol for the same trip. Of course, there are many other variables, like the weight differences of the associated stoves and accessories that must figure into a final weight-efficiency calculation. Determining
the Ingredients The
MSDS For your convenience, I've also collected the MSDS's for a few popular brands of denatured alcohol as well as for the two kinds of Heet that are discussed below. You can find them here. No
Water, Please One example is Parks brand denatured alcohol. According to its 2002 MSDS, it contained 87% to 92% ethanol but also 5% to 10% water, a fact that might account for some of the negative backpacker comments that I've read about this product when used as a stove fuel. In fairness, I should also point out that in an updated MSDS released in December, 2006, water is longer listed as an ingredient, so perhaps their formula has changed. HEET FUEL-LINE ANTIFREEZE After denatured alcohol, the second most popular fuel for alcohol stoves is probably a product called Heet. Manufactured by Chicago-based Gold Eagle Company, Heet is an automotive and small engine fuel additive that is marketed as a fuel-line antifreeze and water remover. I suspect that its popularity among hikers is largely related to its wide availability, especially in the northern U.S. Sold in 12 fluid ounce plastic bottles at automotive stores, gas stations, and variety stores such as Wal-Mart, Heet is available in two varieties. Regular
Heet ("Yellow Heet") Compared with a denatured alcohol product like SLX (which contains about 50% each of ethanol and methanol), Yellow Heet has a slightly lower energy content, but also a slightly lower boiling point, so it should ignite a bit more easily in cold weather. Because it's almost pure methanol, however, Yellow Heet is also more toxic than most denatured alcohol fuels. The toxicity of methanol is discussed at some length above in the "Safety Notes" section.
Methanol-based
Heet (yellow) and Iso-Heet
("Red Heet") Also known as isopropyl alcohol, rubbing alcohol, 2-propanol or IPA, this is a third type of alcohol (after ethanol and methanol), that hikers sometimes burn in alcohol stoves. A 12 fluid ounce bottle usually costs $2.00 to $3.00. Although it's an alcohol, isopropanol has about the same energy density as petroleum-based fuels like white gas at 30.4 megajoules per liter, yet weighs about the same as ethanol and methanol. Unfortunately, it also burns much like petroleum-based fuels in alcohol stoves, which is to say, not very well. When ignited, Red Heet burns with a low temperature, yellow flame that will quickly deposit a coating of black soot on cook pots. If allowed to burn long enough, however, it will usually produce enough heat to bring a couple of cups of water to a boil. Most experienced users will chose Red Heet only if there's no better alcohol fuel available.
Red Iso-Heet burns with a yellow, sooty, low-temperature flame (+) OTHER FUELS Rubbing
Alcohol The ethyl alcohol version is composed of mostly ethanol that's been denatured (made undrinkable) by adding a combination of acetone and methyl isobutyl ketone. Usually sold in concentrations of 70% by volume, the remaining 30% is primarily water. Because of its high water content, this version does not work very well in alcohol stoves. With the Super Cat, in particular, its unlikely that the stove will pressurize with this fuel. The isopropanol (or isopropyl) version of rubbing alcohol is the same compound chemically as the Red Heet discussed above, except just with more water. Usually sold in 70% and 91% (or sometimes even 99%) concentrations by volume, neither is a very good stove fuel. The 70% concentration, in fact, hardly burns at all, while the 90%+ concentrations will generally work, but with the sooty flame of Red Heet. Distilled
(Drinkable) Spirits One of the most popular brands of grain alcohol is Everclear, which is sold in both 151-proof and 190-proof varieties ("proof" = 2 times the alcohol concentration by volume). A 750 milliliter bottle of 190-proof Everclear typically sells for $18 to $25 (usually plus sales tax), which makes it an expensive stove fuel. One 750ml bottle would fuel approximately 25 normal burns in an alcohol stove. Or in other words, if you include sales tax, it would cost about $1.00 every time you boiled two cups of water (I can think of better uses for Everclear). Because Everclear is not available in my area, I've not tried it myself, but others have said that in spite of its ~5% water component, it burns well in most alcohol stoves. During my early Super Cat testing, I did try Bacardi 151-proof rum as a fuel (75.5% ethanol by volume). The Bacardi burned cleanly, but probably because it still contains almost 25% water, the internal vapor pressure was not quite sufficient to allow the stove to pressurize (i.e., the flames could not switch to the outside of the stove). Bacardi 151 could therefore be used as a fuel for the Super Cat, but only in non-pressurized mode using a separate pot stand. * According to the Wikipedia "...It is illegal to sell the 190-proof variety [of grain alcohol] in some states of the United States, viz.: California, Florida, Hawaii, Maine, Massachusetts, Michigan, Minnesota, Nevada, New York, Ohio, Pennsylvania, Virginia, and Washington. In some of these states, the 151-proof variety may be sold. In Canada, it is sold in Alberta but not in Saskatchewan, Manitoba and most other provinces. Esbit
Tablets
Some of the fuels tested during early development (+) Laboratory
(Reagent) Grade Ethanol To produce the kind of waterless ethanol that's required in many automotive and laboratory applications, the remaining H20 must be removed through a chemical drying process that often involves the compound benzene. The resultant "laboratory" or "reagent" grade product is a kind of denatured ethanol that contains virtually no water, but which is still not drinkable, either because there are trace amounts of toxic benzene remaining, or because small amounts of denaturants such as methyl isobutyl ketone or methanol have been intentionally added. Nonetheless, this 95%+ pure, waterless ethanol is probably the best alcohol stove fuel available. Though it doesn't vaporize in cold weather quite as well as pure methanol, its high energy content (31% greater than methanol) and low toxicity (relatively speaking) make it very appealing. And at $6.00 to $7.00 per quart, the cost is also roughly comparable to many denatured alcohol products. The bad news is that in the United States, distribution of laboratory grade ethanol is restricted to businesses, governments and educational institutions. However, if you're in a position to obtain some from your school or workplace, you might want to give it a try in your Super Cat. Distilling
Your Own Ethanol The U.S. Department of the Treasury's Alcohol and Tobacco Tax and Trade Bureau (TTB), is charged with licensing private ethanol "plants", a process that starts by filing a 5-page form with the agency. Once the no-fee document has been received, the TTB must make a determination about whether the proposed use falls within the agency's interpretation of the term "fuel". While it might seem that use in an alcohol stove would qualify, my guess is that it probably does not. The following statement appears on the current TTB website: "...TTB has received requests to use fuel alcohol in the manufacture of products such as charcoal lighter fluid, firelighter gel, and chafing dish “fuel.” We must turn these requests down because these products are not within the intent of the law restricting the alcohol to “fuel use.” While it's unlikely that most backpackers would bother attempting to make their own alcohol fuel, there are almost certainly a hardcore few that have tried it (legally or not). There are a great many Internet resources available that offer both the instruction and the equipment necessary to distill ethanol at home. Ethanol
85% ("E85") According to the Wikipedia, there are currently "...1500 public E85 fueling stations available in the United States (out of 176,000 worldwide), at prices over 30% less than regular gasoline (when discounting the reduced fuel economy of E85), primarily in the corn-growing Midwest..." I've not personally tried E85 in a Super Cat stove, but I've heard from those who have. They've reported that although the fuel contains only about 15% gasoline, it burns with substantially the same properties as regular gasoline. The resultant low-temperature, sooty flame apparently burns mostly yellow and fails to pressurize in the stove, making E85 largely unsuitable for use in a Super Cat. If you have any different experiences with E85, I'd very much like to hear from you. Petroleum-Based
Fuels CARRYING AND DISPENSING YOUR FUEL Suitable
Fuel Bottles The good news is that almost any plastic or metal container will work just fine. Soda or water bottles made from common high-density polyethylene (HDPE) or PET (polyethylene terephthalate) are chemically stable enough that they won't react with most alcohol fuels. Some lightweight backpackers prefer collapsible plastic containers such as those made by Platypus or Evernew, but I personally find it's more difficult to manage a "floppy", soft-sided container than one with rigid sidewalls. Soda bottles (i.e., those used to package carbonated beverages) are a popular choice because they're lightweight, come in a large variety of capacities and are free. Because they're designed to contain beverages that are under a fair bit of internal pressure, they're also quite strong. One source, in fact, suggests that soda bottle manufacturers typically design their products to withstand internal pressures of at least 10 atm | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
