Guide: Cooling food without on-board power in the heat - 12 tips

Nils Theurer

· 15.08.2025

Guide: Cooling food without on-board power in the heat - 12 tips

Photo: Nils Theurer

Can keep food fresh during the day, not only in hot weather: Sleeping bag as a cool box. The other tricks that can be used in the heat work better

There are various tricks and aids for keeping drinks and food at a pleasant temperature even without an on-board power supply - even in the summer heat. We show you 12 tricks

Topics in this article

Cool boxes I
Cool boxes II
Cool boxes III
Cooling pads
Cooling elements
Shivering bottle
Clay pot
Ice and salt
Keelhauling
Frost barrel
Thermos flask

The boat is hungry at anchor, the crew for cans from the cooler. Skipper's gaze hangs anxiously on the voltmeter of the supply battery: will there still be enough juice for the night trip when the newly stocked ice box has cooled everything down?

Either as a supplement to the existing cooling system or for purists: the following tips work completely without electricity. Especially on smaller boats, there would be no room for powerful battery banks anyway. So special tricks are needed in hot weather if you don't want the butter to float away.

In a small series of experiments, the various tricks were tested with a liquid at exactly 30 degrees Celsius; half a litre was to be cooled at a time. 450 grams of ice cubes lowered the temperature in the bottle to 1.4 degrees within two hours, and after 24 hours it had returned to air temperature. 350 grams of ice with 100 grams of salt cooled faster: to 0.6 degrees in one hour - but it also warmed up again more quickly. Ordinary cool packs cooled down to 15 degrees, after 15 hours they were on a par with the first two methods: practicable. As predicted, the 440-gram Easy2Cool cooling pads reached the lowest temperature of 8.3 degrees after seven hours and maintained it for a long time, returning to 19.3 degrees after 24 hours.

Whether towel, sock or clay pot: all evaporators had the greatest cooling difficulties; after 24 hours, one to two degrees below air temperature were registered.

Cool boxes I

Chilling food on board is always time-consuming. It is better and easier not to let it get warm in the first place. Cool bags or ordinary cool boxes usually provide poor insulation; polyurethane insulation is more stable and durable. For example in the Icetime series from Campinggaz (10, 23, 28 litres, from 40 euros) >> <a href="https://www.awin1.com/cread.php?awinmid=14102&awinaffid=471469&clickref=BO+K%C3%BChlbox+Icetime+Plus&ued=https%3A%2F%2Fwww.bergfreunde.de%2Fcampingaz-kuehlbox-icetime-plus-kuehlbox" target="_blank" rel="noopener noreferrer nofollow">available here</a>*.

Photo: HerstellerChilling food on board is always time-consuming. It is better and easier not to let it get warm in the first place. Cool bags or ordinary cool boxes usually provide poor insulation; polyurethane insulation is more stable and durable. For example in the Icetime series from Campinggaz (10, 23, 28 litres, from 40 euros) >> available here*.

Cool boxes II

Models with a rubber lip seal and thick polyurethane insulation are even more effective: Dometic (13-110 litres, from 69 euros) and Peli (19-51 litres, from 240 euros) have various sizes in their range. Despite a wall thickness of five centimetres, the food-safe Dometic products are quite light: the 33-litre box weighs just 5.7 kilograms.

Photo: Photographer: Ata TaefiModels with a rubber lip seal and thick polyurethane insulation are even more effective: Dometic (13-110 litres, from 69 euros) and Peli (19-51 litres, from 240 euros) have various sizes in their range. Despite a wall thickness of five centimetres, the food-safe Dometic products are quite light: the 33-litre box weighs just 5.7 kilograms.

Cool boxes III

Portable, compact compressor coolers are the first choice when there is no permanently installed refrigerator on board. They are nothing more than compact fridges, here one from Dometic with a 40-litre capacity from the CFX-3 series (>> <a href="https://www.awin1.com/cread.php?awinmid=11657&awinaffid=471469&clickref=BO+Dometic+CFX3&ued=https%3A%2F%2Fwww.cyberport.de%2Fhaushalt%2Fgarten-balkon%2Fkuehlboxen%2Fdometic%2Fpdp%2Fke02-010%2Fdometic-cfx3-45-kompressorkuehlbox-40l-12-24v-100-240v-app-steuerung.html" target="_blank" rel="noopener noreferrer nofollow">available here</a>*). In the test, the PLB40 battery was able to keep food and drinks at a constant temperature of seven degrees Celsius for almost four days at an outside temperature of 20 degrees.

Photo: YACHT/Lars BollePortable, compact compressor coolers are the first choice when there is no permanently installed refrigerator on board. They are nothing more than compact fridges, here one from Dometic with a 40-litre capacity from the CFX-3 series (>> available here*). In the test, the PLB40 battery was able to keep food and drinks at a constant temperature of seven degrees Celsius for almost four days at an outside temperature of 20 degrees.

Cooling pads

They are available from dentists, chemists or as promotional gifts: <a href="https://amzn.to/43UOVos" target="_blank" rel="noopener noreferrer nofollow">gel cushions</a>* consist mainly of water with propylene glycol. This is said to be harmless to health even if the bag leaks. However, it is advisable to pack food in bags and tins because of the grease that can escape.

Photo: Nils TheurerThey are available from dentists, chemists or as promotional gifts: gel cushions* consist mainly of water with propylene glycol. This is said to be harmless to health even if the bag leaks. However, it is advisable to pack food in bags and tins because of the grease that can escape.

Cooling elements

<a href="https://www.awin1.com/cread.php?awinmid=14102&awinaffid=471469&clickref=BO+K%C3%BChlelemente&ued=https%3A%2F%2Fwww.bergfreunde.de%2Fcampingaz-kuehlakku-freezpack" target="_blank" rel="noopener noreferrer nofollow">Cooling elements</a>* in solid, stretchy plastic containers are often easier to handle (and leak-proof). Those with a handle are much simpler to mark, carry and stow, for example with straps for the public freezer in the harbour (photo: Coleman/Campinggaz element, from 3 euros).

Photo: HerstellerCooling elements* in solid, stretchy plastic containers are often easier to handle (and leak-proof). Those with a handle are much simpler to mark, carry and stow, for example with straps for the public freezer in the harbour (photo: Coleman/Campinggaz element, from 3 euros).

Shivering bottle

Due to the evaporative cooling, the first few minutes after coming on board from bathing are cool, even in very hot weather. Only when you are dry does the radiant heat regain the upper hand. This effect is utilised with a sock or towel around drinks - it is most effective when there is also wind.

Photo: Nils TheurerDue to the evaporative cooling, the first few minutes after coming on board from bathing are cool, even in very hot weather. Only when you are dry does the radiant heat regain the upper hand. This effect is utilised with a sock or towel around drinks - it is most effective when there is also wind.

Clay pot

Photo: Nils TheurerThe clay pot method is also based on the evaporation principle: two containers serve as transpirators; an intermediate layer of sand moistened with water regulates the supply of new moisture. Perhaps we did something wrong when we were tinkering: our amphora refrigerator was weak.

Ice and salt

Nothing more than salt and ice cubes are needed for flash ice in a bottle: in our mini test, we replaced 100 grams of ice with salt - the cooling to 0.6 degrees due to the endothermic effect was rapid, but after four hours, ordinary ice cubes were on a par. The process cannot be repeated; the brine must be removed from the boat

Photo: Nils TheurerNothing more than salt and ice cubes are needed for flash ice in a bottle: in our mini test, we replaced 100 grams of ice with salt - the cooling to 0.6 degrees due to the endothermic effect was rapid, but after four hours, ordinary ice cubes were on a par. The process cannot be repeated; the brine must be removed from the boat

Keelhauling

Even at a depth of ten metres, the Baltic Sea is often just 15 degrees in summer. Although this is slightly more than in the fridge, the temperature is sufficient for the butter to retain its shape. Simply pack the food in watertight packaging with a ballast weight in a bag or net and lower it down.

Photo: Nils TheurerEven at a depth of ten metres, the Baltic Sea is often just 15 degrees in summer. Although this is slightly more than in the fridge, the temperature is sufficient for the butter to retain its shape. Simply pack the food in watertight packaging with a ballast weight in a bag or net and lower it down.

Frost barrel

How long does a fresh pilsner take? 45 minutes with the self-cooling beer keg for 10 or 20 litres. Surface evaporation after opening a valve causes a layer inside the thick wall to freeze and cool the beer. The 20-litre CoolKeg costs 40 euros. In addition to pilsner, the Tucher brewery also sells lager and wheat beer nationwide.

Photo: HerstellerHow long does a fresh pilsner take? 45 minutes with the self-cooling beer keg for 10 or 20 litres. Surface evaporation after opening a valve causes a layer inside the thick wall to freeze and cool the beer. The 20-litre CoolKeg costs 40 euros. In addition to pilsner, the Tucher brewery also sells lager and wheat beer nationwide.

Thermos flask

What keeps hot also keeps cold: The DIY store <a href="https://www.awin1.com/cread.php?awinmid=12557&awinaffid=471469&clickref=BO+Thermoskanne&ued=https%3A%2F%2Fwww.bergzeit.de%2Fp%2Fthermos-everyday-isolierflasche%2F5027967%2F" target="_blank" rel="noopener noreferrer nofollow">Thermoskanne</a>* performed well in the test. The liquid, which was 6 degrees cold at the start, was only 13 degrees warm after 24 hours. Stainless steel jugs are suitable for on-board use. They are also available with a wide neck, for example for ice cubes. Nice winter storage job: making holders for several jugs.

Photo: Gunkel, FridtjofWhat keeps hot also keeps cold: The DIY store Thermoskanne* performed well in the test. The liquid, which was 6 degrees cold at the start, was only 13 degrees warm after 24 hours. Stainless steel jugs are suitable for on-board use. They are also available with a wide neck, for example for ice cubes. Nice winter storage job: making holders for several jugs.

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