It’s not just useful for keeping food fresh during the day when it’s hot: a sleeping bag as a cool box. The other tricks you can use in hot weather work better
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When the sun is beating down on the boat, the cool box quickly becomes a power issue. With a few simple tricks, drinks and provisions will stay cool for longer, even without shore power, a running engine or a large battery bank.
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Summer heat on a boat is doubly troublesome for crews: drinks get warm, fresh food goes off more quickly, and the cool box draws power just when the service battery needs to be conserved for lights, the pump, the anchor light or navigation. Simple passive solutions are particularly helpful on small motorboats, pleasure craft and weekend boats, before the battery starts to suffer.
The key is to avoid letting food get unnecessarily warm in the first place. Anyone planning a day trip, a weekend at anchor or a holiday on a boat should consider their provisions, packaging, storage space and power supply as a whole. If more energy is needed after all, a portable power station for use on board could be an alternative to the permanently installed on-board electrical system.
What actually kept things cool in the mini-test
In a small series of experiments, the various methods were tested using a liquid at exactly 30 degrees Celsius; in each case, half a litre was to be cooled. 450 grams of ice cubes lowered the temperature in the bottle to 1.4 degrees within two hours; after 24 hours, it had returned to room temperature. 350 grams of ice mixed with 100 grams of salt cooled the liquid more quickly: to 0.6 degrees in an hour – but it also warmed up again more quickly. Standard ice packs cooled the liquid to 15 degrees; after 15 hours, they were on a par with the first two methods: a viable option. As predicted, the 440-gram Easy2Cool cooling pads reached their lowest temperature of 8.3 degrees after seven hours and maintained it for a long time; after 24 hours, the temperature had risen back to 19.3 degrees.
Whether it was a towel, a sock or a clay pot: all the evaporators struggled to cool the air; after 24 hours, the temperature was one to two degrees below the ambient temperature.
Key points at a glance
Ice with salt cooled the quickest, but the effect didn’t last long.
Cooling pads took longer to take effect, but kept the temperature low for longer during the test.
Evaporation using a towel, a sock or a clay pot resulted in only slight differences in temperature.
Good insulation is often more important on board than even the most spectacular cooling trick.
If you chill your food and drink before setting sail, you’ll save on electricity and ice later on.
Insulate first, then cool
Photo: HerstellerKeeping food cool on board is always a faff. It’s better – and easier – not to let it get warm in the first place. Cooler bags or standard cool boxes usually provide poor insulation; polyurethane insulation offers a more robust and durable solution. For example, in the Icetime range from Campinggaz (10, 23, 28 litres, from 40 euros) >> available here*.
Thick walls and a seal make all the difference
Photo: Photographer: Ata TaefiModels with rubber lip seals and thick polyurethane insulation are even more effective: Dometic (13–110 litres, from 69 euros) and Peli (19–51 litres, from 240 euros) offer a range of sizes. Despite having walls five centimetres thick, the food-safe Dometic products are quite light: the 33-litre box, for example, weighs just 5.7 kilograms.
If electricity is an option after all
Photo: YACHT/Lars BollePortable, compact compressor-based cool boxes are the first choice if there is no built-in fridge on board. They are essentially compact fridges; here is one from Dometic with a 40-litre capacity from the CFX-3 series (>> available here*). In testing, the PLB40 battery was able to maintain food and drinks at a constant temperature of seven degrees Celsius for just under four days at an outside temperature of 20 degrees.
Anyone wishing to remain self-sufficient for longer should keep an eye not only on the cool box but on their overall electricity consumption. This also applies to other electrical appliances on board, such as when electric cooking on board or for additional 230-volt appliances.
Cooling pads: slower, but more enduring
Photo: Nils TheurerYou can find them at the dentist’s, in chemists or as promotional gifts: Gel pads* consist mainly of water and propylene glycol. This is said to be harmless to health even if the sachet leaks. However, it is advisable to pack food in bags and tins simply because of the mess that can be caused by any leakage.
Cooling elements: leak-proof, robust and easy to stow away
Photo: HerstellerCooling packs* in rigid, flexible plastic containers are often easier to handle (and leak-proof). Those with handles are much easier to label – for example, with a tag for the communal freezer at the harbour – as well as to carry and stow (photo: Coleman/Campingaz pack, from 3 euros).
Evaporative cooling: only a minor effect
Photo: Nils TheurerThanks to evaporative cooling, the first few minutes after coming on board from a swim are cool, even in intense heat. It is only once you are dry that radiant heat regains the upper hand. This effect is utilised by wrapping a sock or towel around drinks – it is most effective when there is a breeze as well.
Clay pot: simple idea, poor test result
Photo: Nils TheurerThe clay pot method is also based on the principle of evaporation: two containers act as transpiration chambers; an intermediate layer of sand moistened with water regulates the supply of fresh moisture. Perhaps we did something wrong whilst making it: our amphora fridge was struggling.
Ice and salt: quick to cool, quick to melt
Photo: Nils TheurerAll you need for ‘black ice’ in a bottle is salt and ice cubes: in our mini-test, we replaced 100 grams of ice with salt – the endothermic effect cooled the contents to 0.6 degrees very quickly, but after four hours, ordinary ice cubes had caught up. The process cannot be repeated; the brine must be discarded
Lowering food: use cool water
Photo: Nils TheurerEven at a depth of ten metres, the Baltic Sea is often barely 15 degrees in summer. Whilst that is slightly warmer than a fridge, the temperature is sufficient for the butter to retain its shape. The food, packed in waterproof containers, is placed in a bag or net with a ballast weight and lowered into the water alongside the boat. It is important that the line is securely fastened and cannot get caught in the propeller, oars or swimming ladder.
Self-cooling barrel: a specialised solution for beer
Photo: HerstellerHow long does it take to chill a fresh Pils? 45 minutes with the self-cooling beer keg for 10 or 20 litres. Once a valve is opened, surface evaporation causes a layer inside the thick wall to freeze, cooling the beer. The 20-litre CoolKeg costs 40 euros. The Tucher brewery sells Pilsner, Helles and wheat beer throughout Germany.
Thermos flask: Cold drinks stay cold for longer
Photo: Gunkel, FridtjofIf it keeps things hot, it keeps them cold too: The DIY store’s Thermos flask* 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 flasks are suitable for use on board. They are also available with a wide neck, for example for ice cubes. A nice winter storage project: make a holder for several flasks.
