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Ice Box Design


Howard

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Ray:

On my existing 17 footer, I've always used an Igloo or Coleman cooler. Standard 54 quart size. While the claim was made to hold ice for 5 days or so, I could never get much past 2 or 3. A 12 pack in the bottom, 20 pound bag of crushed on top of that and perishables in the small tray on top. All ice, not much food and the ice melted pretty fast (temps usually in the 80's to 90's). Generally fine for a weekend, but during extended trips, perishables were going to suffer. Not enough room in the small tray for what I brought and anything left on the ice was going to get soggy. Beyond that, there was no place dedicated in the boat for such a cooler. It stayed in the cockpit at anchor or tied to a marina and on the cabin sole while underway. I finally resorted to packing seat cushions and spare life jackets around it to keep it from flying around when the boat heeled. It was (and is) not very good and always in the way. I then tried a small, six pack sized styrofoam cooler. Fine for a day or so, takes up almost no room, but not much capacity...and fragile.

As an alternative, I explored no ice at all. You can do pretty well like that....red wine instead of beer.....olive oil instead of butter.....canned meats, etc. Fine for a short term, but long term....ice would be a good option. On a larger boat, it should be possible to dedicate some space to build in a really well insulated ice box. As you say, minimum size, but maximum holding capacity. Low / no maintenance, highly efficient, convenient and out of the way.

A big part of what I'm up to here is to take the bits and pieces of what I can find about good ice box design and test it. Build a nearly full scale mockup and see where all the trouble lies. What works and what doesn't. That way, when I get around to building the real thing, I've got a notion about what I"m doing. Find all the problems, work out the kinks.

Case in point, if you go back through this thread, where has it been discussed what to do with that large block of ice skating around on the bottom of the box? Without any melt water to slow it down, won't it pinball around, slamming into the sides with every bit of motion of the boat? Where does the food go? Trays? Where and how are those going in? What are they made of? If you build in slides for the trays, what are the slides made of? Wood or HDPE?

Lots of things to explore and consider and a lot of good information to sort through.

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crushed ice melts pretty quick compared to block.

What we do is cook all of our food ahead of time and freeze it in portion control sizes. Very little cooking done underway, Mostly reheating rather than cooking.

We pack the 5 day cooler full of our packed portion controlled frozen food. Fill in leftover space with frozen drinking water and frozen gatorade and frozen juices and milk etc.

Every day we access that once per day to take out next days food. That food keeps what few perishables we have in daily use cooler cold

Gets stored on port side Vberth of boat and packed in with stuff to keep form moving. Wife seeps in starboard v berth, I sleep in Port Pilot berth. Vberth on boat is a bit of a joke as both people need share toe space..

Daily use cooler goes under cockpit floor in companionway behind ladder.

Works well for us and I suspect will work nicely with Bellhaven as well. I can't personally see wanting a built in ice box for the way we go, but have no issue with other people having other needs.

Nice thing about portable cooler is they are a whole lot easier to clean and access.

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Did a two part foam test pour yesterday. It seems to me there is a learning curve associated with this stuff, but if mastered, holds good promise for this type of insulation need.

My test container was a cardboard box, just under a gallon in size. Expansion of the 2# foam is said to be 25-30x the initial volume. My calculated test pour using 25x came up short by about 15%. One reason might be that not all product left the measuring cups and it also appears to be temperature sensitive. All the literature suggests you need temps in the 80 degree plus range, or higher for optimum expansion. Ambient temperature in my test pour area was under that, but I warmed my test box and foams to about 85.

A couple thoughts.......the foam reaction itself is exothermic....it creates it's own heat, much like polyester or epoxy resins do. Like a wildfire, it appears to make it's own wind (heat)that adds quite a bit to the expansion process. Subsequent pours of flat, shallow fills to bring it on up to the top did not fill nearly as well as one in which the foam was contained in a tight containers. What may also play a part is stirring too long. More testing is in store for this one.

The foam doesn't rise level. Friction on the sides results in "mad caps" or mushroom shaped rise patterns. Literature says it will take the path of least resistance, and if an opening is present, will expand in that direction without undue pressure on the sides. My box was open on top and yet the sides bulged out. I fear any pour in a constricted area will be pushing hard on the sides. Best to have them blocked. Another way to avoid bulges is to use several small pours instead of one large massive one. I did that and it appears to stick to itself seamlessly. It sticks about the same (or not) to things as epoxy does.

Lastly, the final product is both light (very light) and tough. I was able to stand on my small box sample and didn't even dent it. I have no doubt it will be able to support a fully loaded ice box with no fear of it moving around or crushing the insulation.

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Personally I think you are way over-engineering. How hard is it to get ice every week? What is the effect of adding warm drinks to your box on a daily basis and how do your super efficiency design factors stand up under this condition? I am guessing they go down the toilet in real benefit. I don't know about you, but when I am on vacation I want to have good food and cold drinks every day and all day long. I don't care what I spend on ice. If I have to buy a couple blocks every 5 days then I will. I am going to enjoy myself. In the grand scheme of things this ice expense is minimal. I like technology as much as the next guy, but some efforts are just not worth it.

It might be amusing. And if this is so then research and go for that 2 or 3 week box. In the mean time I will be wasting another $8 on a couple blocks and adding more beer and soft drinks to my box.

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Over engineered? Perhaps. I prefer to think of it as "informed".

Over the years, I have found it frustrating to attempt projects like this with only vague and sometimes conflicting information. Ideas on tin foil and insulation types for example. So the normal pattern for me is to research it all to inform myself as best I can, then test it myself. Yes, the effort is both informative and fun, by reporting the results here, I can pick the brains of the best and brightest, and in doing so help inform those heading off in the same direction, facing the same frustrations, and about to make the same mistakes. (Guys like Calder do something similar, then write books about it).

As for why go to the trouble, sssuming two boats have ice boxes.....one good (1 week or more) and one not (days), given a choice, the good one would always be preferred. (Recently spent a few days on a 28 footer with an ice box that had poor access and held ice for a day at most....the owner has lashed a refrigeration cooler on the cabin sole and likes to head for a marina and shore power each night).

There probably isn't more than 10% difference in the cost of building the two of them. The difference in performance will be in how they are designed and put together. If I'm going to the trouble of building one at all, why not go for the good one? It isn't the cost of the ice.....it's the freedom the extra days of not having to worry about it buys you.

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I'm not talking about insulation. It is always worth the effort to use the best and do it well. I am talking about the layout and such factors to keep melt water and keep food and ice out of it, and pumps and all the other frills. You have limited space on a boat for an ice box. You have very limited space in it. That space is going to be filled completely all the time if the goal is to keep not just food, but a good supply of cold drinks. I know from years of cruising my Renegade, which has a large ice box, that there is no room for anything except ice, food and drinks. And when you factor in the cold loss (technically, heat gain) associated with adding warm drinks on a daily basis, anything except super insulation becomes a space waster and with minimal gain. If you were packing the ice box once, and just drawing from it, and adding ice was not an option, and longevity were crucial, then all the added ideas might add a couple days. But is that what any of us would really do? I am not saying don't do your research. But let's be realistic about how this thing is going to be used.

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If you were packing the ice box once, and just drawing from it, and adding ice was not an option, and longevity were crucial, then all the added ideas might add a couple days. But is that what any of us would really do? I am not saying don't do your research. But let's be realistic about how this thing is going to be used.

I think you have described my intended use better than I could. Except I'm hoping to stretch it from a couple days to several days...up to a week or even longer....minimum. In short, as much longevity as possible from a modest amount of ice in volume about the same size as a standard 54 quart cooler.

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Well, Howard, I'm with you... I'm a perfectionist with regard to anything that will be permanently installed on my boat. If I were going to install a permanent cooler, I'd take the same route and test what works best. In part, because I enjoy the intellectual challenge and in part because I've become cynical about how much you can trust others' advice, which often proves unreliable or exactly wrong.

I don't disagree with Hirlonde's pragmatism... sometimes there is no good reason to fuss over the details for little actual difference, but in this case, I'd be fussing too. My boat is my baby, and it's my hobby to perfect the details to make every aspect of using it easier and more satisfying. If you have the time and materials to kill, why not?

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I don't disagree with Hirlonde's pragmatism... sometimes there is no good reason to fuss over the details for little actual difference, but in this case, I'd be fussing too. My boat is my baby, and it's my hobby to perfect the details to make every aspect of using it easier and more satisfying. If you have the time and materials to kill, why not?

I don't think you understand. I am considering the details as well. I am not willing to give up the space that these added details consume. Even my 27 foot Renegade does not have it to spare. Anyway, have fun Howard, just be aware of what the details really cost. If less food and drink cooled longer is the goal, then so be it.

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I do understand that. But I believe Howard is experimenting to confirm first, to his own satisfaction, whether the thickness of insulation and the design of various elements (such as different styles of drain) make a significant impact on the convenience of his ice box or not. He may indeed find, as you suggested, that he is over-engineering, but then he'll have some experience to draw from to decide more accurately what will suit his needs. I think that's a good way to proceed. What works for one does not always work for another, but that's a humbling and painful lesson to learn after you've permanently bonded an ice box to your boat in a way that is not easy to undo.

You might not be willing to give up the space these materials consume, but Howard is doing a test run so that he can discover for himself whether or not he feels the same way. But to be clear, I didn't mean to criticize your advice... I agree with your pragmatism wholeheartedly and respect your opinion. I just meant to say that I also fully understand why Howard might prefer to test these issues to his own satisfaction by trying them out. When it comes to committing to a permanent installation, I'd want to test the advice out for myself also.

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Wes has correctly described my motivation and intent. I now have 3 different books on boat design and construction, each describing an ice box design and while all are similar, all are also different and a lot of it is conflicting. Beyond that, as is often the case with this and a lot of other things, I find out stuff when I actually try to build the thing that is not mentioned. That learning curve again. I'm sure the same will be the case in trying to use it. Inform yourself as best you can, then make the best choice for you and your situation.

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Granted, but as a practical issue, you don't want the foil in contact with anything, so in my built-in I faced it out but tried to leave an air gap, but on the top I faced it in. One thing to watch out for: the foil is a good reflector but also an excellent heat conductor: don't let it form a path for heat to be conducted right through the rest of the insulation by (for example) leaving inside-facing foil on at the edges where a piece of foam will butt its edges against the outside wall. Paul, what would be the point of a coil? Anything that allows that water to absorb heat exposed to the outside would defeat the purpose. Leaving the water in place inside the insulation can't be beat.

[Whups! Just realized I was looking at bottom of page 1, not seeing the next page!]

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Had a chance to work on this some over the weekend. Because you don't want anything touching the ice box but insulation, and since poured foam is structural (supposed to support 40 psi) the goal was to suspend the ice box inside the containment box and pour foam beneath it and around it on all four sides. Came up with this idea to suspend it:

Picture003.jpg

Cross pieces were placed on top of ice box rim, then clamped into place. Cross pieces and ice box were then placed inside containment box. That registered the top of the ice box level with the top of the containment box. Once positioned where I wanted them, deck screws were used to keep the box from moving around or being lifted out of place by expanding foam. Inside bottom and sides of containment box were lined with padded foil type insulation. More on that later.

This is the foam ready to mix and pour:

Picture001.jpg

I had calculated that 2 quarts (one each of Part A and Part B) would fill the bottom below the ice box (2 cubic feet) and then some, and it did. To get the ambient temp up to the recommended 80 to 85 plus, I had to turn my heat on in the house and along with supplemental heat cranked up, got the room up to 90 degrees. Warmed the foam to that or more. On the larger pours, I got a very agressive rise. More on that later.

Not exactly sure what to expect, I went with the recommendation of multiple pours, vs. one big one. If things go wrong with the one big pour, things can bust and blow apart. It happens fast and there is no stopping it once it gets going. At those temps, 25 seconds is all you have to get it mixed and pouring and most of that was spent getting it mixed. Five pours total did the job:

Picture012.jpg

This stuff really sticks...... to itself and almost anything else it touches. Very similar to epoxy...maybe more so. While initially fragile and sticky, within 30 minutes a pour is firmed up and will support weight. I gave it a day to set and cleaned it up:

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Pours of this size confirmed the exothermic reaction. Inside of ice box walls were very hot to the touch for several minutes after the rise. Outside walls were cool, confirming the foil was doing it's job.....in this case, reflecting heat back in. Hopefully, later on it will reflect radient heat back out. I fit the one piece foil liner loose and allowed the expanding foam to press it out to the sides, which it did. Edges of foil are inside the box and insulated. Should not get any conductive heat gain coming inside.

Confined pours really trap the heat and get the rise going. This does matter as the difference between a "hot" rise and one less aggressive was evident in the density of the pour. The slower ones don't expand nearly as much and cure hard as a rock......the fastest have bigger bubbles and are borderline spongy. You can see this in some areas. The nickel and dime sized gaps and holes are large air bubbles that were trapped in the mix. Because this was several smaller pours, it looked to me like I got a near 100% fill, with no voids. Also, with multiple pours, the foam always had a pathway to rise into and despite no blocking of either ice box or containment box walls, neither one bulged out on me.

The foam seems to rise the fastest and fullest when the mixed foam is confined into a small area. It rises and expands from the mass. Spread it out and the rise isn't nearly as great. As the rise peaks out, mushrooming is evident. The foam peak that forms will pull away from the walls. If there is an advantage to larger pours, it would be expand the mushroom cap well beyond the edge of your container to get a complete fill below the cap.

Picture005.jpg

Lastly, get the support braces off before the final pour. If you let the foam expand into them, you will play hell getting them off once the foam cures. Also, cured foam is easy to work. You can trim it with a hand saw. I flushed it up with a long board and coarse sand paper.

What all this means is there is an art to working with this type of poured foam. Expect a learning curve. If in doubt as to the amount of product, order a bit extra. You may need it.

Next up is a temporary insulated lid and if curiosity gets the best of me, a test with ice.

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Ice box math:

This really is swerving into engineering, which is not my strong suit, but I figure there may be some following this thread who can either confirm my notions, or set me straight.

I'll again refer to the two text book references......Calder and Pardeys (the latter reference Calder as one of their technical sources, so they listen to him too). Interesting how they differ in their conclusions, but again, they are viewing things from a different perspective. Calder is describing mechanical refrigeration systems and two of his recommendations are good insulation and a small volume. Pardeys use an ice box as an ice box and also go with good insulation, but recommend as large a volume as possible. Again, that contradiction. Since I'm in the ice in the ice box camp, I'm heading up the "as large a volume as possible" avenue. To a point.

Now for the math part. Overall shape does appear to affect efficiency. As an example, assuming you have an 8' sheet of plywood and want to construct an ice box, you could build one that is 6 inches x 3.5 feet x a maximum of 4 feet high (likely something less). You would have 8 linear feet of exterior surface area, but looking down from the top, only 1.75 square feet to cube from. A ratio of 4.6 : 1. A rectangular box of 1' x 3' still has 8 linear feet of exterior wall area, but now has 3 square feet of area to cube or a ratio of 2.67: 1. A square box of 2' x 2' still has 8 linear feet of wall area, but now has 4 square feet of area to cube for a ratio of 2:1. With 8' to work with, you can't improve on that. Bottom line is a square shape gives you the most internal volume in comparison to the surface area of exterior wall. A cube shape is an extension of this.

Next comes the part I've struggled with and that relates to overall size. Assume you have a cube that is 6 inches square. It has 6 sides of 36 square inches each, so 6" x 6" x 6 sides or 216 square inches of internal facing surface area. It also has 6 x 6 x 6 or 216 cubic inches of volume. A ratio of 1 square inch of surface area per 1 cubic inch of volume (1:1). All this inside the surrounding foam insulation, be it 3, 4, 5, 6 or more inches. More on this later.

A 12" cube has 12" x12" x 6 sides or 864 square inches of inside facing container surface area but now has 12 x 12 x 12 or 1,728 cubic inches of internal volume, for a ratio of 2:1

A 16" cube has 16" x 16" x 6 sides or 1,536 square inches in inside facing container surface area, but has 16 x 16 x 16 or 4,096 cubic inches of internal volume, for a ratio of 2.5:1. Assuming heat gain through the external surface area is constant, it would then follow that the larger box, containing more ice volume in proportion to potential heat gain from surface exposure, would last longer as it simply has more volume of ice to absorb the BTU of heat gain through the walls.

Note that option 2 can be contained inside a 2' x 2' containment box (my box) with 6 inches of foam all around. Option 3 can also be contained inside my 2' x 2' containment box, with 4 inches of foam all around.

Or consider Option 4, an 18" cube, with 3 inches of surrounding insulation and ratio of 3:1. So size does matter?

Interesting note about insulation from Calder. As the thickness of insulation is increased, the insulation value goes down in proportion. Example.......an insulation with a value of R5 per inch might have a heat loss of 2,000 Btu. Double it to 2 inches and heat loss becomes (2,000 / 2) = 1,000 Btu. Double that to 4 inches and it becomes (2,000 / 4) = 500 Btu. To 6 inches, it becomes (2,000 / 6) = 333 Btu heat loss. You can quickly see there is a point of diminishing returns. As per Calder, the optimum benefit tops out at around 4 to 6 inches of insulation. Beyond that, you lose internal volume, but gain hardly anything in terms of insulation value.

So here is the engineering question. Assuming all other heat gain (such as through the access door) is constant, what is the optimum mix of insulation to internal volume to give an ice box user the maximum amount of cooling time? (Assumption here is it all needs to fit inside the 2 cubic foot containment box.

Related to that is the question of how many pounds of ice does one really want to lug around? Option 2 has 1,728 cubic inches of volume or roughly 7.5 gallons, which would weigh roughly 60 pounds if filled completely with water. Forgetting the ice to water weight difference, cut that in half and you could have about 30 pounds of solid block ice.

Option 3 would have 2.37 cubic feet of volume, hold 17.5 gallons and weigh roughly 150 pounds if filled completely with water. Cut that in half and you get 75 pounds of solid block ice.

Option 4 would have 3.375 cubic feet of volume, hold 25 gallons, weighing nearly 210 pounds if filled completely with water. Again, cut that in half and you get about 100 pounds of solid block ice capacity.

What is the optimal mix / ratio? Engineering again.

By accident, I'm pretty close to being in the middle of all this. My exterior containment box measures 2' on all sides, my ice box is 14 x 15 x 19. Should have done all this math before. If I were to do it over again, knowing I have about 30 inches of vertical space to work with, I might opt for 16 x 16 x 20 inches internal ice box dimensions (strictly a shoot from the hip guess). That would seem to be a good fit for my goal of holding longevity, internal capacity and tolerance for weight and space dedicated to an ice box.

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Arrrrghhhhhh... nightmares of the SAT and ACT exams are coming back to me! :)

I don't know if there is a mathematically precise answer because the other variable here is what you intend to put in it. You're right that there are diminishing returns the thicker you go with insulation, but more insulation is still better than less. Internal volume needs to be ideally exactly the size of the contents, plus room for ice. Any extra space is placing additional cooling load (though not a lot more) on the ice and is also creating more space for the warm atmosphere to enter into each time the lid is opened.

So, the correct answer as I understand it is to go with 3 to 6" of insulation (I'd probably go 4" and some reflective foil) and then to size the internal compartment that is *just* large enough for the amount of supplies you would need to have on hand for a desired number of days PLUS the desired load of ice. The problem, of course, is that you first need to experiment with various ice amounts to see what is the ideal ratio of ice to food supplies. That's where your prototype will come in handy, I think. But keep in mind that the food itself can also be a source of cooling... so, for example, I would probably pre-freeze about two-thirds of the beverages I intended to carry along on a week-long trip. Then I might also pre-freeze a liter bottle (or two) of water to serve as the main ice supply. Or I might buy a large block of ice, or perhaps a 5- or 10-pound bag of ice. It depends on how much of the food will be pre-frozen versus merely cold.

Don't know if that helps, but to me the solution cannot be mathematically calculated without the variable of how much food (and how much pre-frozen) will be included.

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Yup. The size issue is a surface area:volume:problem. Neglecting the lid, heat enters through the surface so that, all other things being equal, the bigger the suface area, the faster heat enters. Of course, we're assuming the same shape box (A cube like yours is better than any other straight-sided, right-angle shape). HOWEVER, as the length of a cube increases, its surface area goes up as the square but the volume goes up with the cube. For example, doubling the length increases the surface area four times and the volume 8 times. Compare the volume:area ratios of the two and you'll see this double-dimensioned cooler will keep its stuff cold twice as long--if it does indeed start with eight times as much ice, though!

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In the nature of 'over-engineering'...

It may be a little late to comment on this topic, but I have a keen interest in the subject. I am in the beginning stages of building a live-aboard boat, and the idea of not having to pull into port every two or three days for ice is somewhat intriguing. So let's go ahead and take this to the next level....

Very often we over-think things. The basic concepts are usually too obvious, and I know I am as guilty as anyone for overlooking them. I spent a lot of time in college years ago learning how to predict where a sphere will land, given the mass of the sphere, the angle of trajectory, velocity, and arc of travel. However, if I throw my dog a ball, he will catch it. The mass of the ball, trajectory, or arc are of little concern to him :) . I prefer his physics to ours...

I may also include that I have been referred to as a nerd on occasion....

As with anything these days, conservation of energy is the key. The opening and closing of the cooler is your biggest source of energy loss. More so than the insulation on the sides, or the evaporated waste water trapped within. One only needs to go back about a hundred years or so, and look at what was done with the electric ice box.

Other than the opening and closing of the cooler, the 'variables' are the consistency of the cooling source, and the variation in temperature of the contents.

Concerning the variation in temperature:

Coolers have a general fallacy of the size of the opening required to access. The interior contents need access for loading and cleaning, but general access requires an opening no more than the size of the largest item needing to be removed. So, a door big enough to maintain the cooler is necessary, but a smaller door for grabbing a beer or a sandwich is beneficial, and, in reality, all that is required. I might also suggest that you consider making the access from a vertical surface, rather than a horizontal one. If the secondary access is located higher, the cooler air stays in the lower portion of the cooler.

For the consistency of the cooling source:

The 5" or so thickness in insulation needs to be kept, at least until we figure out a better insulating material. However, as I understand it, cold air has an acknowledged tendency to travel downward. Placing the cooling source above makes a great deal of sense, for your example, in the lid. A better application might be a cabinet-mounted unit, with the ice above, and the cooled items in an area below. Freezing items that are expected to be used at a later date can beneficially be added to our cooling energy source. A couple of bottles of water for a short trip, or some steaks or a roast for a longer cruise. Either way, consolidating the cooling energy is a good idea. I wouldn't recommend freezing beer, though. Unless it were cans.....and not too solid...

Let me take the aluminum foil theory mentioned earlier a little farther...

If the cooling source were placed above, on an expanded metal surface perhaps, it would be good to have a 'membrane' of sorts below that could allow for the transfer of cold, and also help direct the shed of moisture due to melt. Aluminum foil would not make a good 'membrane' for this, but some other 'very-conductive' material would be awesome. A very thin, micro-corrugated material, angled to drain away melt water....the drainage could be made through the insulating walls easily enough and collected below, or sent to another waste area, be it waste water or 'bilge'.

Since the cooling is no longer being made through direct contact, as in the cooler theory, circulation would be beneficial. Drawing air from the bottom of the cooling area to the top would help keep the entire cooling area at the same temperature. A fan located under a drip tray at the bottom of the cooler would work. Since there is now a barrier between the cooling source and the fan, this will not increase the evaporation of the cooling source, but will, admittedly, add somewhat to its decay. The fan would need to spin at no more than 30 RPM, so a solar-powered version of this idea would be rather inexpensive, and a DC version would not be power-exclusive. If the fan speed were any higher, you would definitely increase your melt-rate...

I am not a betting man, but I dare wager that a cooler built with these aspects in mind would keep a standard block of ice for at least 5 days in 90F heat, shaded. My rules, of course :)

I hope this is beneficial, but most importantly I hope this does not offend. I'm sure that Alexander Graham Bell received some criticism for his efforts along the way, but I am really glad that he decided not to just walk down the street to talk to his neighbor...

I can supply some CAD drawings of my ideas, if you would like. No charge ;)

Godspeed, my friend.

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Latest issue of "Good Old Boat" arrived over the weekend and had a short article on, of all things, ice box design. Not much new, in fact read a lot like the crib notes version of Calder, but with a few new wrinkles. Once again, the recommendation was to use rigid foam insulation board, but this time, the "blue board" was defined as extruded polystyrene. I have looked for "blue board" in the box stores and didn't find it. But they do have the pink stuff, which, it turns out, is extruded polystyrene too. Blue Board is a Dow Chemical product. Pink stuff is by Owens Corning. Appear to be very much the same thing.

http://foamular.com/foam/products/foamular-250.aspx

One store had numerous choices of the Foamular, but in the larger thicknesses, the choices were 150 or 250. The numbers correlate to the compression strength in psi. One being 15 psi vs. 25 psi. They make higher compression rated stuff such as 40 and 60 (2# two part poured foam is said to have a psi value of 40), but those are not normally available in the box stores for home use. Web site above has several nice spec sheets to download. One in particular had some good engineering data on how much thickness you need, and also mentioned that as the thickness is increased, the live load psi numbers are divided by the number of layers. But as near as I can tell, the 250 product is easily strong enough to handle the load of a normal ice box, even with a "live load". So it is structural along with having good insulation properties.

Having now fitted a lid to the top of my box, I have a few observations about working with the rigid foam vs. the poured stuff. Despite having secured the interior ice box in place, and using smaller, multiple foam pours, it appears that not only did my interior box shift some, but the entire exterior box looks like it racked a bit from the expansion as the top is now out of square. (Don't use that cheap luan underlayment for anything that matters........ever!).

In retrospect, I think it would be far easier to build a box using the ridid foam. It is easy to cut (mark it with a drywall square, score it with a utility knife and snap it) and the square insulation panels would make it easy to keep the whole product square inside the containment box and to build to carefully measured dimensions. When I needed to glue panels together, I used "Gorilla Glue", which holds tight and since it foams up (resembles the two part poured foam) would appear to have a good R value. Looks to me like it would be easier to measure and fit a tight lid on top. If you have the room, 3 layers of 2" (6") or 3 layers of 1.5" (4.5") or 2 layers of 2" (4"). One spec sheet I downloaded has a nice graph that details the amount of insulation needed based on expected temperature differential. For our use, typical range is going to be min of 4" and max of 6", which dovetails with what most references suggest, only it gives some scientific basis for it.

Once again, suggestion is a bottom drain for the melt water, with an air lock loop. Not only does the loop prevent cold air from sinking out, but if cold air sinks out, it has to be replaced with warm air from the outside. The GOB article mentioned radient heat, but said it was insignificant, amounting to only 2% or so of heat gain. Conduction and convection are the big losers. Once again pointing to insulation and good air tight seals.

For a new twist, something I've been thinking about, but don't know what to do about, is condensation. As per Calder, in a warm, humid marine environment, it's almost impossible to keep water vapor out. So condensation forming between the insulation and the ice box is nearly a foregone conclusion (exterior of ice box has to be epoxied coated too). One benefit of the rigid foam board is it doesn't absorb moisture.......all the articles say two part poured foam does (although the literature from those who make says it doesn't). Anyway, regardless of which insulation is used, what happens to the condensation that forms inside the containment box? If the drain pipe is allowed to drop out of the insulation and a loop is formed below it, condensation may wick out on it, but condensation would then form on the pipe itself and would drip below. I can now see a puddle of water forming......not from melt water, but from condensation. If condensation is allowed to remain in the insulation it would seem to be a heat / cold conductor? But to open up a drain pathway through the insulation would defeat the purpose of the insulation. Anyway, been thinking about it, but not sure what to do about it.

Lastly, my experimental box is very nearly ready to start some melt tests. Unfortunately, it's now 50 degrees outside, so the melt tests will have to be conducted inside. Good in that the temps will be steady for reliable comparisons, but with the differential only being 30 degrees or less, it won't be much of a test.

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Mrthethird:

You raise a number of good points, one of them the recurring issue of opening the box all the time for access. Believe it or not, it has occurred to me to put a small (six pack sized) secondary insulated box downstream of the air loop to catch the cold melt water. You could put a beverage or two in that and get all the benefit there is to be had from the cold melt water. That way the big box need not be opened all the time for single beverages and such. Might also serve as a simple cooler for day trips, etc.

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