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Refrigeration for Coastal Cruising Boats

Refrigeration adds a civilized feeling to an otherwise minimalist lifestyle aboard an ocean-going vessel. Sipping an ice-cold soft drink or cocktail while watching stunning sunsets at anchor or underway a thousand miles from the nearest shore enhances the magic and romance of being on the ocean. Of course, an efficiently functioning, permanently installed refrigeration system drawing a conservative amount of amperage from the house battery requires picking the right system and installing it properly. When shopping for a fridge, you will find three types of refrigeration, each one depending on a different cooling mass in the surrounding environment: air-cooled, water-cooled or keel-cooled.


Air-Cooled Systems

Along the Southern California coast, a standard, air-cooled, 4-cubic-foot, top-loading sailboat refrigerator should stay at or below 40 degrees F, safe enough to store milk and raw meat, while cycling roughly 10 minutes out of every hour. At an average 5 amps during cycling, that comes to 20 amp hours per day. A single 50-watt solar panel generating half its rated power over a 10-hour period produces 21 amp hours, just enough to power the refrigerator for a 24-hour period.

One way to reduce electrical power usage and maintain a more constant level of cooling is to install an air-cooled cold plate, or holding plate, which is a thin, flat compartment with an evaporator tube in a bath of coolant that freezes at a point below the freezing point of water. When the coolant rises to a certain temperature, the compressor automatically kicks on to freeze the solution again. Even in a warm, tropical climate, a cold plate can keep the fridge cold while cycling only a few hours a day, which is a huge advantage over fridges with standard evaporators.

If there is a drawback to the cold plate, it’s the greater amount of amperage used while cycling. Over a 24-hour period, though, you can expect to cut total fridge amperage by half. Technautics of Escondido, Calif., claims their CoolBlue holding plate system can chill a seven-cubic-foot freezer-refrigerator at 24-amp hours per day, significantly less than conventional refrigeration. With the holding plate installed in a typical sailboat fridge measuring 3 or 4 cubic feet, we can expect even greater amperage savings.


Water-Cooled Systems

An alternative to the standard-issue, air-cooled fridge that comes stock in many new yachts is a water-cooled system, which is more efficient at removing heat from the condenser tubes. Like the radiator on an automobile engine, applying a comparatively cool liquid to a hot surface draws out heat far faster than air. A water-cooled system involves the installation of a water pump, a raw water filter, input and discharge hoses, an above-waterline discharge thru-hull fitting, various plumbing parts and a 12-volt switch. The pump is usually quite small, using only an amp or so of power, but the overall savings in daily amperage can be in excess of 25%, more than offsetting its meager amperage draw and offering a significant advantage over a conventional air-cooled boat fridge. Reducing the 90 amp hours of the air-cooled system by 25% gives us roughly 68 amp hours per day, a savings of 22 amp hours. Let’s not kid ourselves, though. A diet of 68 amp hours per day is still a huge demand on a typical coastal boat’s reserve power, far greater than that of any other single system on the vessel.


Keel-Cooled Refrigeration

An even more efficient water-cooled fridge may be achieved through a keel-cooled system, which is also used for some engine installations. Keel cooling uses fresh water to cool the condenser coils in the same manner as a standard water-cooled system, except that coolant passes through a heat exchanger attached to the outside of the hull. It is called a “keel” cooler because of its location close to the keel to guarantee constant submersion. Frigoboat and Isotherm both offer keel-cooled refrigerator kits with hydrodynamically shaped heat exchangers to reduce drag.

A self-contained fresh water cooling system eliminates the need for a strainer, since no seawater passes through the hull. One downside to a keel cooler is the need to inspect and replace zincs regularly on the bronze heat exchangers. Also, while sitting in an anchorage without a constant current of water running under the hull, the water immediately surrounding the heat exchanger can heat up and reduce its cooling capacity. Owners of keel-cooled fridges, though, are generally quite pleased with the superior cooling of this straightforward system, even in tropical anchorages.

Another potential problem is the growth of barnacles and algae on the bronze heat exchanger, and some boaters claim external heat exchangers cannot be painted with antifouling. However, Rob Warren of Coastal Climate Control in Annapolis, Md. the main U.S. distributor of Frigoboat, pointed out, “You will lose a little refrigeration performance by painting [a heat exchanger], but not enough that you will notice. I made some tests several years ago, and I found that it was better to have one painted than for it to be really heavily fouled.”


Portable Ice Makers

If all you need is ice for margaritas, consider buying a portable ice maker. Imagine plugging a machine into the AC inverter and drawing less than 3 amps to produce 2.5 pounds of ice in less than 15 minutes – in the middle of the ocean. As long as the ambient temperature is below 90 degrees F, you can make enough ice to chill several cans or bottles of liquid in the Igloo.

The current standard portable ice maker is manufactured in China and sold through such brand names as Dometic, Emerson, Magic Chef, NewAir, EdgeStar and MaxiMatic. The ice maker should be used only in settled conditions or while at anchor to keep it as level as possible. Also, avoid using an extension cord, which in extreme cases can overheat and cause a fire.

You should have little trouble finding a refrigeration system meeting your needs and your boat’s available space and electrical system. Consider the options, compare notes with fellow boaters and enjoy cold drinks and chilled food on your future boating adventures.

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