Notes from Paul:

Great Idea

I use a 12 volt system and am very happy with it. It is an old Norcold unit
using a Danfoss compressor like most 12 volt fridges. I have a well
insulated box, minimum 6 inches of foam insulation and it is small, about 18
x 18 x 24 inches. All I did was buy a small used RV fridge from a junk
yard and mount the evaporator unit from it into the ice box. I bought some
refrigeration tubing the same diameter and length that was on the back of
the RV fridge and soldered it onto a copper plate. I then epoxied this to
the painted hull in the bilge. This condenser unit is used instead of the
fan and coil assembly that came with the RV fridge. The condenser unit and
the compressor are about 5 feet from the location of the evaporator unit.
With this method, there is no fan running and it acts like a water cooled
unit without the water pump or plumbing. It has worked great even in
tropical lagoons where the water gets over 29 C (84 F) and the air temp can
get over 35 C (95 F). A standard condenser unit with a fan mounted in a hot
locker somewhere will be far less efficient than a water cooled unit. The
only improvement I can think of would be a 12 volt system using a more
sophisticated electronic unit that senses when the engine or voltage is high
and runs extra hard to bring the temp down and take advantage of the running

By the way, when I thought of doing this, a refrigerator “expert” told me it
would never work. I had read a few books about refrigeration and couldn’t
see why it wouldn’t. I tried it anyway since I had little money and had
nothing to lose. It has now been maintenance free for over 15 years.

I previously had experience with an engine driven compressor unit and they
have to be topped up once in awhile since the shaft seals leak eventually.
They are also much more complicated (expensive) with valves and clutches
that can fail. With the engine driven unit you will have to run your engine
almost every day, even at the dock, unless you have a 12 volt unit as well.
In my opinion, a simple, hermetically sealed unit like a 12 volt system uses
is the way to go.

The fridge draws about 5 amps when its running and runs about 50 percent of
the time while in the tropics. I am not sure on that since I never really
timed it but if you figured 100 amp hours over 24 hours for the fridge, that
would probably be a good figure to go by and give you a good safety factor
for design. I noticed when I came to NZ and the water is cooler that it ran
a lot less. Because it runs depending on the setting of the thermostat, it
never gets shut off. I had two 75 watt panels. It’s a long story, but I
made a wind generator out of a tail rotor from a helicopter….it worked well
but was kind of scary. It disappeared one night in a storm in Suvarov
lagoon and smashed one of the panels on its way to Japan. After that, one
of the 75 watt panels was only putting out about 2 amps. They were never
enough for the fridge so I had to run the engine for and hour or so every
second day, on average. I have a 100 amp alternator.

I am refitting the boat now and not using it. My plan is to have a total of
3 panels on the rear arch where they get the sun but not the shadows. I
would get more panels if I could figure out where to put them. I am not a
fan of wind generators :).

I have more accurate measurements of the icebox now… is smaller than I
thought. Inside dimensions are 31 inch long, 14 inch wide and 17 inch
deep. 6 inch insulation on sides and bottom and 5 ½ inch on hatches. There
is a removal divider in the middle to make a freezer section and a fridge
section. The divider is just a piece of Styrofoam 2 inches thick. A gap of
about a half inch allows the colder air in the freezer section to spill over
into the fridge section. The condenser unit in the freezer section is the
aluminum box from the old RV fridge as described in my previous post. Its
dimensions are 6 inches by 12 inches by 8 inches deep. We usually throw any
meat into this box and it will stay frozen solid. The rest of the freezer
will keep things frozen, but only just.

Cheers, Paul

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