How to keep things cold on a boat

Why a boat needs a fridge can vary widely, from storing the day's catch to providing food and drinks over a number of days.

Most fridges suitable for smaller boats use are top-opening chest types, but there are also small front-opening types for larger craft. There are also ones with pull-out drawers.

If you're seeking a powered unit, it’s wise to buy a fridge that will work just as happily with 12 volts as it will a 240-volt input.

It’s necessary to know something about the ebbs and flows of the cooling business if you want to be sure of buying the right unit for your needs.

The simplest and by far the cheapest type of food and drink cooler is the faithful Esky: an insulated box with an insulated lid, into which you put ice that someone else has created.

When you buy bagged ice, you’re actually buying a "slug" of pre-packaged refrigeration.

One of the big problems with an icebox is that the ice melts. It means everything stored in the 'box will get wet, and the water will "dissolve" flavour and cross-contaminate it with everything else it comes in contact with.

It can be a bit messy, too, as the best way to store ice is to drain the liquid melt from it.

Likes: Low price; relatively portable (some have built-in wheels); doubles as a seat; can be used for general storage when not cooling; zero maintenance
Dislikes: Bagged ice is not always available; everything gets wet; ice can taint; can be messy

These days there are many thermoelectric cooler boxes on the market, filling the price gap between an icebox and a true fridge.

Thermoelectric fridges employ the Peltier effect that occurs between two closely-coupled, dissimilar materials.

When an electrical current is passed through the materials, a temperature variation occurs. This can be used to either heat or cool the interior of a container.

Unlike true fridges that can achieve a target internal temperature, thermoelectric coolers are rated to cool relative to the ambient temperature.

Claims for their performance usually quote an internal temperature: “22 degrees below ambient” or similar. In that respect they’re not reliable for storing temperature-critical, perishable contents.

However, the thermoelectric effect works both ways, so these units can be set to heat their contents, up to around 65 degrees Celsius.

At this stage of development, thermoelectric coolers work as compact means of cooling or heating non-critical consumables.

They’re not very efficient, with 12V current draws around 4-5 amps, but they’re cheap and have no moving parts that can break.

Likes: Low cost; noiseless; compact; able to heat or cool contents
Dislikes: Relatively inefficient; doesn't handle hot days

The most popular form of portable refrigeration is the compressor-type fridge. Sizes vary from small models with single lifting handles to chest-type large units with handles on each end.

The operating principle is simple enough and similar to your household fridge. Pipes contain a gas that liquefies at low temperatures and is pressurised through fan-cooled condenser coils that are located outside the food storage area.

As the gas sheds its heat through the coils it condenses into pressurised liquid and is forced through an expansion valve (a technical term for a tiny hole) into a low-pressure area created by the suction effect of the compressor. This action causes the liquid to vaporise and drop its temperature to around minus 250 degrees.

Compressor suction pulls this cold gas through the evaporator coils that are inside the food storage area, lowering the temperature of the contents and warming the gas, which then passes through the compressor, continuing the cycle.

These fridges can also be used as a freezer.

Most portable compressor-type fridges are 12V/240V compatible, so can run off a 12V power outlet, an inverter or a mains socket. Most compressor-type fridges use a reciprocating compressor, while Engel uses a highly efficient "swing" motor with only one moving part.

A compressor-type fridge can run off the alternator while the boat is running, but it needs auxiliary electrical power, such as a battery, when the outboard engine is switched off.

A deep-cycle auxiliary battery is a common source of 12-volt power and can normally power a fridge for between one and three days, without re-charging, depending on ambient temperature and battery capacity.

A solar panel can run a fridge when sunlight is available, or it can be powered at any time by a generator.

Likes: Can operate as a fridge or a freezer and some models can do both; median pricing; highly to relatively portable (depending on size)
Dislikes: Needs to cycle on and off periodically, so electrical power always needs to be available while cooling; fridge noise can be a problem at night

Eutectic or ice-bank fridges use the compressor refrigeration cycle but add a significant extra component.

Instead of evaporator coils (the coils that get cold) being located inside the food compartment, a eutectic fridge has them running through a tank that forms the inside of the fridge.

The compressor freezes the liquid in the tank, and the ice bank it forms is what cools the interior of the fridge.

The advantage, then, of a eutectic fridge is that it can work off battery power while the outboard engine is running, but continue to cool long after the outboard engine is switched off.

If you're a light sleeper, the fridge won't cut in and out all night like a normal compressor-driven fridge.

As with a straight compressor-type fridge, a eutectic unit can function as a freezer, or a combination fridge/freezer.

Testing in real-world conditions by boatsales.com.au's sister site trucksales.com.au indicates a eutectic fridge draws less electrical load when working on the same cooling cycle as a straight compressor-type fridge.

However, you get what you pay for, so a eutectic fridge costs a lot more than a conventional portable fridge and is heavier.

Likes: Efficient cooling; power-free overnight potential
Dislikes: More expensive to buy; bulkier and heavier than a straight compressor-type fridge for its internal capacity.

An absorption-type fridge is also known as a "three-way" fridge. It uses heat rather than an electrically-operated compressor to pump refrigerant around the system.

As the name suggests, the refrigerant is released from solution and 'absorbed' again during the process.

Heat, sourced from a 12 or 240-volt electrical supplpy, or by burning LPG gas, is used to raise the temperature of a strong ammonia/water solution so that most of the ammonia evaporates.

From there the ammonia passes through a fan-cooled condenser, where it becomes liquid again before passing through a restrictor that causes it to atomise.

This action cools the gas as it passes through tubes in the food storage area, drawing heat from the contents.

The warmed gases then flow into an absorber filled with the weak ammonia solution where they’re reabsorbed to form a strong solution and the cycle repeats itself.

Absorption-type fridges aren’t as energy-efficient as compressor-type fridges, but they have advantages that made them popular.

The most obvious one is silence. With no moving parts, an absorption-type fridge won’t disturb your slumber.

Another big advantage is the wider choice of ways to power the fridge. However, our experience over many years of travel is that 12-volt operation is much less efficient than 240V or LPG.

The reason for this is simple; electrical energy is least efficient when it is used for radiant-heat applications such as the heating element for an absorption fridge, but is more than 90 per cent efficient when powering an electric motor such as a fridge compressor.

To supply current for an absorption-type portable fridge in a boat it’s necessary to have a reliable and guaranteed 12V power delivery of around 10 amps.

A compressor-type fridge draws about half that. An alternative is to run the fridge via a 240V inverter, but you’ll still need plenty of electrical grunt.

As with a compressor-type fridge, an absorption-type can function as a freezer or a combination fridge/freezer.

However, very high ambient temperature can upset the absorption-cycle efficiency and it operates optimally only when level.

Likes: Silent operation; can utilise three different types of power input
Dislikes: Slow to cool down from room temperature; doesn't like high ambient temperature or not being level; greater electrical draw; needs to be vented when running on LPG