A fridge is considered essential for a cruising yacht these days, but what is the most efficient system to install? Duncan Kent investigates the options
Keeping your food fresh when sailing means having a decent boat fridge.
An old, leaky, inefficient one will increase the risk of your food going off and suck the life out of your batteries.
Insulation is key to an efficient boat fridge
Problems with boat fridges are almost always caused by the lack of decent insulation.
Early models often had thin (usually no more than 12mm) layers of cheap, low-density porous foam all around the cool box, topped off by a poorly fitting, often uninsulated lid with no seals.
It’s no good fitting the latest high-efficiency compressor cooling system to the old cool box if your hard-earned cold air leaks out through the seams or is quickly conducted away by saturated polystyrene foam insulation.
When upgrading your boat fridge, rule number one is ‘Don’t skimp on foam thickness’.
When planning the task carry out your calculations allowing for at least 50mm-thick sidewall insulation, 75mm on the bottom and 25mm in the lid.
Add 50% again to these dimensions for a freezer compartment or if you’re intending to spend long periods in hot climates.
Insulation that is too thin can double the compressor operating time, and consequently the power consumption, of the boat fridge.
The type of foam you use is also very important.
It needs to be high-density, polyisocyanurate closed-cell foam, which is highly energy efficient and waterproof, and available from most builder’s merchants in flat sheets (Cellotex, Kingspan or similar).
Cheap, open-cell foam sheets such as polystyrene will quickly absorb water making them useless as an insulator, along with most DIY spray foams,
which are rarely up to the job.
Ideally, you would buy, or already have, an existing polyethylene inner liner, to which you can glue the foam onto the outside.
Alternatively, you can fabricate a box from the foam sheets, seal the seams with foil tape, and then either coat the inside with GRP, finishing with a smooth gelcoat or use prefabricated glass-fibre wall panels cut to size then glued and sealed at the joins.
Although proprietary insulated boxes are widely available and even stocked by some chandlers, many do not meet the recommended insulation standards as they are primarily intended for vehicle use where power consumption is not necessarily an issue.
You could, however, buy one of these and simply add to or replace the insulation.
It’s important to realise that the insulation needs to be completely sealed on the outside by an air and watertight outer covering otherwise a great deal of condensation will be created – possibly waterlogging the foam and thereby destroying its insulation properties.
Your choice of cooling system will make the difference between ‘vaguely cool’ and ‘properly cold’ storage, so a clear understanding of which system offers what is vital.
Cheap 12V car cool boxes nearly always use fan-assisted thermo-electric (Peltier-effect) cooling systems, which work by blowing air over a solid-state thermocouple.
While they might be fine for those who have an endless supply of power, they are not ideal for sailing yachts as they draw a constant current, usually between 6-10A.
They’re also seldom available above 35L capacity and rarely keep their contents sufficiently cool in ambient temperatures above 20ºC.
If you only go weekending or are happy to throw in a frozen water bottle or block of ice, and you kick start things by loading food straight from your home fridge, a thermo-electric cool box might suffice, but it will consume an inordinate amount of battery power on warm summer days.
For a proper boat fridge or freezer you will need a compressor-driven cooling unit in which an inert gas is cycled through an evaporator plate inside the fridge box, collecting heat on the way.
The compressor then pumps the gas to a condenser outside the fridge (the equivalent of a car radiator), which disperses the heat either by air or water cooling, and re-liquefies the gas before returning it to the evaporator.
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For small- to medium-sized boats, the condensers are more commonly air-cooled, so it is important to ventilate the area around the cooler adequately
as they can lose up to 50% of their efficiency when installed into a small, unvented locker.
Furthermore, the heat build-up in the boat can be unbearable in a hot climate, so boat owners intending to cruise the tropics would be well advised to ventilate the area to the outside or, better still, install a water-cooled condenser on the hull.
Dissipation of heat through water is some 20 times more effective than in air.
Some fridge cooling units utilise a heat exchanger through which cold sea water is circulated.
Although more efficient, the additional pump increases both the power consumption and the chances of something going wrong.
Units that circulate the coolant itself through a hull heat exchanger are more efficient and economical.
Modern 12V fridge compressors draw around 3.5A-4.5A while running.
However, this is usually for only 15-20 minutes every hour in a well-insulated fridge as the thermostat turns the pump on and off as required.
Thus, a 48W/12V fridge is likely to consume a very reasonable 24-32Ah over a 24-hour period with an ambient temperature below 20ºC.
A common alternative, particularly in larger yachts and those that run their engines often, is to fit a device that senses the alternator charge when the engine is running and switches a variable speed electric compressor to high speed to bring the marine fridge temperature down quickly.
The compressor then drops back to low speed when the engine is shut down to save power.
The addition of a ‘eutectic’ holdover plate (a metal ‘box’ filled with a fluid that freezes at a temperature above that of water) will allow it to remain cool for long periods in between engine cycles.
Cooling elements running through a matrix within the plate freeze the fluid solid, turning it into a ‘cold reservoir’.
This is especially useful at night when the noise from an electric compressor can be disturbing.
Finally, the greater the surface area of the evaporator plate, the more heat it will absorb and the quicker it will cool the box down so, if building your own, choose the largest size you can sensibly fit into your box.
Some cooling units incorporate a device that electronically monitors the battery voltage and controls the fridge accordingly.
When the engine is running the device senses the extra power available and turns the compressor to full speed.
When the engine is off it reduces power. This method works well when combined with a holding plate.
Top-opening, rather than front-opening boat fridges, are more energy efficient as the cold air tends to ‘fall out’ of the latter when opened.
As cold air sinks, it mostly remains inside the box with the former.
The bottom is always the coldest area where items susceptible to warming should be stored.
This is why front opening freezers tend to have drawers – they reduce but do not eliminate this ‘fall out’ of cold air every time the door is opened.
Many fridges will have a drain at the bottom, which is necessary for keeping water out.
It shouldn’t be permanently open, though, or it will allow the cold air to leak out.
It should either have a U-bend in the drainpipe or include a tap or pump.
Try to keep your boat fridge as full as possible but allow enough air space for the cold air to circulate.
Remove all cardboard packaging from food as this retains heat and often becomes a soggy mess that blocks the drain.
Finally, if weekending, load your boat’s fridge with food and drink that is already cool if possible and plan your meals and drinks carefully, opening the lid as few times as possible.
Saving money on boat fridges
Some may be tempted to install a domestic fridge instead of an expensive marine model.
However, they’re really not designed to (and may well not) operate when heeled over.
Besides, many of the steel screws, nuts, bolts and fittings will rust in no time.
Caravan and camper fridges look to be better value and often run on AC, DC and/or bottled gas.
But once again these won’t work on an incline and using gas only when moored on the level would still require a proper flue arrangement, which is hard to waterproof from the sea.
Portable cool boxes and fridges are very popular for camping.
These can be thermo-electric or compressor types and often come with a cigar lighter-type lead so that they can be connected to a vehicle’s auxiliary power socket.
While this might be fine for the occasional weekend when initially filled with cold provisions, the non-compressor types will use a huge amount of power over 2-3 days.
How to convert an existing galley locker to a top-and front-opening fridge
- Location: Choose a location low in the boat, ideally below the waterline. The box ideally should not be installed too close to the engine compartment or boat cooker.
- Insulation: Insulation should be waterproof and water tight inside and out and at least 50mm thick. If you install an air-cooled fridge in a locker, it should be ventilated to enable a natural flow of air both in (lower) and out (higher). Alternatively, installing a small computer fan to assist the flow of air from the surrounding locker will increase efficiency, especially when cruising in hot climates.
- Lining: A watertight GRP interior keeps the insulation dry.
- Drawers: Drawers help front- opening fridges retain cold air.
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Source: Yachting Monthly