Revised by BoatUS editors in June 2012
The single most important fact about marine refrigeration is that it improves your quality of life aboard with such luxuries as cold drinks and fresh food. However, the amount of time required to provide this luxury is not to be overlooked. If you spend a lot of time at the dock where shorepower is readily available, if you have an onboard generator that you use frequently, or if you live aboard, your best choice may be a 110vAC refrigeration unit. They're relatively inexpensive, dependable, easy to install, and should give you years of trouble-free service.
Most boaters don't live at the dock and use 12vDC units to provide the same dependable refrigeration as they have in their homes. The simple installation of only two major components-compressor and evaporator-make these units very popular and there has been great improvement in many of these units in recent years. The compressor cycles on and off as the box warms and cools. The amount of time they run depends on the unit weather it is raw water cooled (with a raw water circulation pump which comes on with the compressor, and, very importantly, the amount and type of insulation in the cold compartment. While these units draw relatively little current, the total amperage draw over a 24-hour period can be more than the boat's electrical system can handle. A close evaluation of your boat's battery size and recharge capability is in order when installing a 12vDC refrigeration system.
Most 12vDC units come with an air-cooled condenser, although a water-cooled condenser is preferable, especially in warmer climates. Water cooling is more efficient, so you get greater cooling capacity with a smaller unit, and they can be installed almost anywhere. Air-cooled condensers must be mounted where they can be ventilated to dissipate the high heat removed from the ice box. For 12vDC running at dockside, simply plug in your battery charger and replace the battery power you're using. Away from the dock, your primary consideration is how often and how long you must run your engine to have ice and cold food. Having a dedicated battery bank for your refrigeration unit is a good idea, but remember that you still have to run your engine or generator to recharge the battery. You can increase your options with an AC/DC refrigerator or icemaker. The dual-voltage units switch from AC to DC automatically when the AC power source is shut off.
Thermoelectric refrigeration is also 12vDC, but uses no refrigerant and has no moving parts except a heat-dissipating fan. These quiet units can last a lifetime, and installation is simple-but they're not for every boat. They're less efficient than other types, requiring more amps daily to cool a two-cubic foot box. They'll cool the box to 40-50 degrees below ambient temperature, which is inadequate for the tropics and other hot weather areas and possibly not worth the power drain. Portable units use only half the power of full-size units. They're a smaller, lighter, and convenient alternative to hauling that heavy, bulky cooler of ice on every outing. If you plan to convert your existing ice box to a refrigerator and/or freezer using a conversion kit, it's critical that you pay special attention to the box's insulation. Heat loss through leakage can be cut in half by increasing the insulation from two to six inches. Most refrigeration manufacturers will be happy to provide you with good information for properly insulating and/or building an ice box for use with their units.
Engine-driven systems with holding plates are the most powerful system for those wanting deep freezes and lots of ice. The compressor runs only once or twice a day, for short periods of time. However, the initial cost is far more than the cost of a 12vDC unit, and is more complicated so there's a greater potential for problems. If you consider an engine driven compressor, be sure to factor in the mounting of the compressor on the engine itself rather than on a nearby stringer. This allows the compressor to vibrate and move with the engine, resulting in less wear and tear on bearings. Also consider that you will not want the compressor to block access to other components. Usually this type of unit is installed by qualified professionals while a typical 12 volt unit is easier to be installed by a “do-it-yourselfer.”
Look for marine-grade construction materials. Most holding plates are made of stainless steel; electropolished stainless steel is more corrosion resistant and more attractive. Water-cooled condensers exposed to seawater should be made of a noncorrosive material such as cupronickel. Marine-grade materials add to the initial cost of the unit, but if you're going to make this kind of investment, buy a dependable unit that will last. Check the manufacturer’s warranty-longer is better.