Refrigeration Installation

After spending the summer of 2000 putting cartons of milk into the ice box and taking out cartons of yogurt, we decided that we wanted to install a refrigerator when we redid the galley. Had we not been replacing the galley we would probably not have installed a fridge.

Note: we installed much the same model all over again on our second Maringret, described here.

We deliberated for months on our refrigerator purchase and finally selected an Isotherm 3251 ASU SP which utilizes the galley sink drain pipe and external sea water for cooling. Our reasons included the information presented on our Refrigeration Research page but also what price we were able to find and also that a sister ship had fitted the same fridge and was very happy with it. This last step seemed as important as any as by talking to an owner with a usage profile similar to ours we were best able to decide before buying anything whether the model would fit our needs.

Note: all pictures on this page will expand when clicked

The through-hull has been fitted and can been seen sticking up above the moldings; the compressor is mounted in the forward end of the port side cockpit locker and can be seen through what will become the garbage hatch. The fitting of the through-hull is a good example of where simple line drawings were not sufficient to determine if all the parts would fit together.

The Box

A major part of the refrigeration is designing and building a box to fit the space available and maximise the usable chilled volume. There are a number of different insulation materials, foams, vacuum panels and polyurethane (PU) sheets. For insulation against conductive heat loss we decided to use prefabricated PU sheets which are supplied by Isotherm and have food grade plastic surfaces laminated to each side. The thickness is 50mm and the material can be easily cut with a hand saw of bread knife. The common wisdom seems to be to first insulate the bottom of the cooled space, then the sides, finally the top (as warm air rises and cold air “falls). We purchased some 25mm PU sheets which did not have plastic facing and combined the two makes to make up our desired thicknesses. Our bottom thickness was 100mm of PU, the sides were 75mm and the top was 50mm.

We started by making a plywood box to enclose the chilled volume, this does not need to be overly strong so we used 4mm plywood and joined the edges with both tacks and West Epoxy soaked fabric. This box was sized to utilise the largest volume while still keeping a relatively simple shape (complex corners vastly increase the amount of fitting required during assembly).

So as to have a gradient on the floor of the cooled volume we built up the floor using 4mm plywood pieces so that the forward end was approximately 12mm (3 layers of plywood) higher than the rear where we intended to put a drain hole. The markings for these pieces of plywood can be seen in the previous picture. Although the theory was OK, in practice the trim of the boat can change much more and cause the gradient on the floor to run forward so that condensed water etc. actually run and pool in the forward end. This could have been addressed while we were mounting the finished box to the boat was unfortunately we did not notice it until much later.

As an insulator against radiant heat loss we bought one of the reflective foil blankets sold for survival use to hiker and backpackers and used this to line the box prior to fitting the PU slabs.

We then fitted the non food grade PU slabs and joined them with SikaFlex (actually we used two layers of the 50mm food grade laminated PU on the floor of the chill box as we had enough material on hand).

The food grade PU slabs finished off the box and provided an attractive although soft surface on the interior.

Christer, our metal worker, convinced us to then line the refrigerated box with stainless steel. Although not adding anything thermally, the stainless is highly impervious to cuts and scratches that the food grade plastic would be marked and scored by. Certainly if we had initially decided to line the box with stainless steel we would not have gone to the added expense of the food grade lining PU slabs as the normal PU slabs have the same insulation qualities and are much cheaper.

With a stainless steel liner mounting the accumulator was much easier and more solid as mounting bolts could be welded right onto the stainless lining.

We used a single layer of the 50mm food grade lined PU slab for the top (the top is not lined with stainless steel). The kits from Isotherm include some strips of the food grade plastic which can be used to line any cut outs made such as the access hole above. In fitting the lid we made sure we had a lip for the countertop lid to sit on so that there were no straight channels for warm exterior air to manage to enter the closed chill box.

This is the cool box as it appeared prior to the countertop and sides being fitted.

To see how all this fitted into the galley as a whole click here.

Of the 169 litres of the plywood box, 115 litres (or 68%) of that volume was used up for insulation. Meaning that for every chilled litre, two litres of insulation were needed! We should point out that not all of the 54 litres is usable as things never fit in perfectly and with a top loading fridge there is always the question of how much can be stacked (don’t put the eggs on the bottom).

First Experiences

After calculations based on our insulation thickness, our intended cruising areas, and the physical space available for batteries we installed one 60 amp-hour and one 80 amp-hour battery in parallel giving us 140 amp-hours. Using the wind generator, 50 watts of solar panels and charging from the Yanmar we have never been below 12.1 volts (which equates to 50% of charge remaining). Admittedly the first season in Norway we motored 95% of the time so the engine charger was running for an average of 5 hours every day. Once we reached our winter berth and ceased using the engine, we actually switched the refrigerator from automatic to manual so we could set it to a warmer temperature which meant reduced power consumption and ice free liquids. Obviously being in a warmer location would possibly have meant using the manual setting to force a greater degree of cooling.

With the compressor mounted in the cockpit locker we can barely hear any operational noise. At an absolutely silent anchorage we can hear a faint hum when it runs but this is much quieter than water boiling.

Mediterranean Experiences

In the Mediterranean we encountered anchorages with a water temperature of 29° C: at anchor and with little wind (for the wind generator), our 50 watts of solar panels were able to keep up with the cooling requirements (we always had the temperature control set on Automatic). We have been told that Mediterranean waters are warmer than Carribean waters and that in the Carribean the winds are more dependable due to the Trade Winds. Consequently we have concluded that if our cooling requirements are met in the Mediterranean then we can assume that they will be in other non-tropical cruising grounds. In all our time in the Mediterranean we had no problems with our Isotherm refrigerator.


Every 240 hours the accumulator defrosts itself by stopping the cooling process until it reaches 4 degrees Centigrade. This ensures that any solid ice on the accumulator melts and the accumulator is left bare. This is not really described in the documentation and so gave us a bit of a worry the first time. Now we get used to the Isotherm and it’s 10 day cycle. Once we got into the Mediterranean we wished we could defer the defrosting so it would defrost in the night when there was no sunshine for the solar panels. When it defrosts in the day it has to rechill the accumulator during the night when there is no sunshine so the batteries must provide all the power. The lowest this took our batteries to was 11.9 volts one time. Of course if the wind is blowing then daytime defrosting would not be a problem.

  • RParts for refrigeration parts

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