Features August 2018 Issue

Making Ice Last

A few simple steps can get you an extra day.

Does block or bottled ice last longer than cube ice? What about dry ice? Will wrapping a blanket around the ice chest really help? What about moving the ice into a smaller cooler?

Seeing so much confusing—and flat-out wrong—advice from dockside experts on how to best manage our ice box this summer, we decided to do our own testing.

36-quart Coleman cooler
Photos by Drew Frye

We used a 36-quart Coleman cooler for testing, shown here with loose ice. The purpose of the black pipe on the left was to allow testers to use an infrared sensor to take temperature measurements at different levels.

How We Tested

First, we tested a 36-quart Coleman cooler loaded with 8 pounds of ice in several different forms, measuring meltwater generation over a 24-hour period. We also measured the temperature inside the cooler (two locations) after 24 hours. We also tested the effect of retaining versus draining melt water.

We then tested the common suggestion that wrapping the cooler in a sleeping bag (40F rated) or fleece blanket preserves ice.

Finally, we tested two compact, inexpensive alternatives to a cooler—the Wonderbag (a retained heat cooking devise) and a styrofoam mailing cooler.

The USDA recommends a maximum temperature of 40F for refrigerated food storage, so for the purposes of our test, the object was to maintain sub-40F for the longest period. Anything else was a failure.



A doubled fleece blanket can add R-4 insulation to a portable cooler. It is also important to wrap it around the bottom.

Opening and Closing

Assuming the entire air volume of our 36-quart cooler was exchanged when the lid was opened and this happened 10 times each day, only about 30-40 BTUs of heat enter the cooler, consuming ¼-pound of ice.

Bottom line: The overall impact is probably not worth sacrificing domestic tranquility.

Filler Blocks & Meltwater

A chest full of food takes a little longer to warm up because there is more cold mass inside. However, adding room-temperature filler blocks to replace cold food or drinks as they are removed doesn’t help keep things cooler. As for meltwater, keeping it will help ice last slightly longer, but if you insulate the bottom of your ice chest (place it on a blanket) the difference disappears.

Bottom line: Filler blocks don’t work. A thin layer of meltwater helps, but a deep pool usually isn’t worth the mess.

Insulation vs. Bigger Cooler

The coolers in our test ranged from R4 to R6, pretty miserable compared to the R30 in the typical attic. In our tests, a 20-pound bag of ice in a 36-quart cooler wrapped in a sleeping bag kept temperatures below 40F degrees for four days. A 35-pound bag of ice in a 46-quart cooler (no sleeping bag) matched this.

Bottom line: More insulation or a bigger cooler (with more ice) are both good approaches to prolonging chill.

Smaller Coolers

Reducing cooler size reduces heat loss. We tested both a 4-quart disposable mailing cooler (1.5-inch thick styrofoam, about R-10) and the Wonder Bag (about R-20, see “Slower Cooking Onboard,” Practical Sailor, May 2016.).

Bottom line: Both of these are better insulated than the typical cooler, fit a smaller amount of food, and proved very effective at stretching ice.

Frozen bottles

For 2- to 4-day trips we love the convenience of grabbing pre-frozen bottles and stuffing them in the cooler. You need to be just as generous with them as you would with ice. If you normally pack 16-pounds of ice, you’ll need two gallon jugs, or a gallon jug and eight sports bottles.

Bottom line: Heavier-duty sports drink bottles and ice tea jugs hold up best. Pack the jugs on top of the most critical food items.

Loose Ice

The old standby, loose ice packs well around food and cools food quickly, but the melt water is a mess and it doesn’t last as long as other alternatives.

Bottom line: Readily available, but less practical than other options.

Block ice

Blocks pack well in rectangular boxes and lasts a little longer than loose ice. It also makes it easier to access the food. However, melt water is still troublesome, though not as messy as cube ice.

Bottom line: Recommended if the ice box has a drain.

Dry ice

With nearly three times the cooling capacity per unit volume, this is the obvious choice for longer life and keeping meat frozen. At -109F (compared to about 0F in the typical home freezer) serious burns are possible. Some of the food will freeze, which might not be desirable.

There is no melt water from the ice; if the ice box has a drain, it should be plugged to prevent the cold from flowing down the drain. Water-tight coolers (Pelican, Yeti) can build pressure and should be left unlatched.

Although more expensive than normal ice, dry ice might be a better value if it reduces food loss. Be aware that dry ice outside of a well-insulated cooler can generate CO2 at dangerous levels inside a tightly sealed boat.

Bottom line: Best Choice for frozen foods, if used carefully.

Sleeping bag vs. Fleece Blanket

Sleeping bag or doubled fleece blanket? The light sleeping bag (40F rated, about R-5) was effective but awkward and bulky. A doubled fleece blanket was nearly as effective and is more likely to serve dual use on board in the summer.

Bottom line: A cooler should be kept out of the sun (obviously) and we recommend wrapping it with a fleece blanket to preserve cold.

Wonderbag

As the ice and food dwindle, the large cooler or ice box becomes progressively less efficient. The Wonderbag (R-20) only holds a few gallons of volume, but that is enough for critical food items.

The down side is that the bag is not waterproof. This can be remedied by placing the food and loose ice in your largest pot and placing that in the bag, or using frozen bottles to cool.

Bottom line: If you have a retained heat cooking devise on-board, remember that it keeps things cold too.

Mailing cooler (R-10)

Used to mail frozen meats and medical supplies, our test cooler was insulated with 1 ½ inches of styrofoam and weighed a few ounces, making it easy to justify keeping one in the back of a locker. When the loose ice is down to just a few pounds, pour it into the mini-cooler. Add food, and wrap in four layers of fleece blanket (R-4) and the melting will slow to less than 1-pound per day, stretching critical foods for a few days.

Bottom line: Recommended, if you have some extra space handy.

Conclusions

For short trips, frozen bottles, ranging from sport bottles to gallon jugs, have proven most versatile. In a cooler, they can be place on top of critical items, concentrating the cold and keeping them cool or even frozen for days, without risk of melt water contamination.

For longer trips using an on-board fridge, home-frozen bottles can be transferred to the fridge to accelerate cool-down. You can also do this with ice, but it can get messier if the ice melts faster than the fridge cools down.

Dry ice is best for keeping meats frozen, but it is sometimes hard to find often requires a special trip.

Cube, block, or dry, when your ice dwindles, transfer the remains to a small cooler or Wonderbag. These can stretch a few pounds of ice for days.

Comments (4)

The real cooling power of ice is in the melting.

The scenario posed of ice melting at 38 degr F instead of 32, with all other factors being constant would mean -less- energy is needed to reach 40 degr F. It takes 1 calorie to raise the temperature of 1 gram of liquid water by 1 degree Celsius. But it takes .49 calorie to raise 1 gram of ice (solid water) by 1 degree Celsius. It takes 79.72 calories to transition (melt) 1 gram of water from solid to liquid.

4 degr C is close to 40 degr F; 3 degr C is close to 38 degr F; -10 C = 14 F; 0 C = 32 F.

It takes 88,620 cal to raise 1 kg of ice at -10 degr C to a liquid state at 4 degr C.
1000 gr*(10 degr*.49 cal/degr/gr + 79.72cal/gr + 4 degr * 1 cal/degr/gr)= (4900 cal + 79720 cal + 4000 cal) = 88,620 cal = 351 BTU

But, it would take only 87,090 cal to raise 1 kg of water from -10 C to 4 C if the melting temperature was at 3 degr C.
1000 gr*(13 degr*.49 cal/degr/gr + 79.72cal/gr + 1 degr * 1 cal/degr/gr)= (6370 cal + 79720 cal + 1000 cal) = 87,090 cal = 345 BTU

So, if you could change the melting temperature of ice to 38 F, all other factors being constant, it would take 6 BTU less energy to bring 1 kg of water from -10 C ice to 4 C liquid.

Posted by: Eric B | August 15, 2018 7:36 PM    Report this comment

The real cooling power of ice is in the melting.

The scenario posed of ice melting at 38 degr F instead of 32, with all other factors being constant would mean -less- energy is needed to reach 40 degr F. It takes 1 calorie to raise the temperature of 1 gram of liquid water by 1 degree Celsius. But it takes .49 calorie to raise 1 gram of ice (solid water) by 1 degree Celsius. It takes 79.72 calories to transition (melt) 1 gram of water from solid to liquid.

4 degr C is close to 40 degr F; 3 degr C is close to 38 degr F; -10 C = 14 F; 0 C = 32 F.

It takes 88,620 cal to raise 1 kg of ice at -10 degr C to a liquid state at 4 degr C.
1000 gr*(10 degr*.49 cal/degr/gr + 79.72cal/gr + 4 degr * 1 cal/degr/gr)= (4900 cal + 79720 cal + 4000 cal) = 88,620 cal = 351 BTU

But, it would take only 87,090 cal to raise 1 kg of water from -10 C to 4 C if the melting temperature was at 3 degr C.
1000 gr*(13 degr*.49 cal/degr/gr + 79.72cal/gr + 1 degr * 1 cal/degr/gr)= (6370 cal + 79720 cal + 1000 cal) = 87,090 cal = 345 BTU

So, if you could change the melting temperature of ice to 38 F, all other factors being constant, it would take 6 BTU less energy to bring 1 kg of water from -10 C ice to 4 C liquid.

Posted by: Eric B | August 15, 2018 7:36 PM    Report this comment

Yes, it would melt more slowly, and the box would be warmer, most likely above safe food storage temperatures (USDA says 40F), which is already a challenge with frozen bottles and cooler packs. Always a trade-off.

Posted by: Drew Frye | July 21, 2018 12:26 AM    Report this comment

Since one of the drivers of how fast Ice melts is the Delta between the ice temperature and outside temperature, could you make the Ice last longer by increasing the Freezing Temperature of Ice.
i.e. raise the freezing point to 38 degrees

Posted by: ColdInTheSummer | July 17, 2018 12:24 PM    Report this comment

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