Refrigeration Installation

Having installed a purpose built fridge on our previous boat we quickly realised that the air-cooled fridge on the HR41 was starting to fail. We were able to improve on some aspects of our previous project.

The strategy for designing a fridge for a boat has not changed since our Maxi project (click here for details):

  • identify the space available
  • maximise the utilisation of that space
  • determine the minimal acceptable refrigerated volume
  • maximise the insulation to yield the target refrigerated volume
    Note that there is a 3-way trade-off between refrigerated volume, insulation and power consumption

The existing coolbox had 43mm thick walls consisting of polyurethane foam sandwiched between two fibreglass walls, the percentage of volume consumed by the insulation was 6%. The refrigerated compartment had functioned well for 35 years although a new air-cooled compressor had been fitted at some time, but we decided we wanted to reduce the energy consumption which meant re-thinking the insulation strategy and going to a water-cooled system.

Other than the specifics of the two boats there were not a lot of differences from our installation on the Maxi 10 years earlier. We were able to use generic polyurethane rather than food grade, also we made a different air seal on the lid. We weren’t sure at first but the two web-pages we wrote 10 years earlier still applied:

We have broken this page into parts: a discussion of strategy and design goals followed by the second part which is more on construction tactics.

Fridge Design Strategies and Goals

The space available was very large in total volume but as it was situated against the inside of the hull at a place where the hull is very curved, a lot of the overall space was not easily useable. Also it was so deep (i.e. top to bottom) that it would have been impossible to reach the bottom from the top without some tongs or other tools.



The diagram above shows the relative curve of the hull (both inner and outer) as well as the floorboard level. To most efficiently use the available space we fitted a shape that was:

  • the height was such that bottom could easily be reached through the counter-top level opening
  • maximised the depth (i.e. the distance forward aftward)
  • easy to manufacture (i.e. no complex curves)

In the diagram above the dimensions are labelled with:

H = height from top to bottom
D = depth, the distance forward aftward
W = width, the distance from outboard to inboard
Note: W = W1 + W2 where W1 is the width of the section with the flat floor and W2 is the width of the section where the floor is sloped upwards

Simple geometry gives a formula for the volume of D * ((W1*H) + (W2 * H * 0.5)) which will be in cubic centimetres (CCs) so we divide by 1,000 to convert to litres. Note that there is a slight error as the angled floor in W2 is not exactly 45 degrees but it is not worth getting into trigonometry for our purposes.


The diagram above shows the dimensions of available space, applying our formula gives us an available volume of 300 litres before insulation etc.

For insulation we used industrial slab polyurethane which is used for insulation in buildings. It comes in standard thicknesses, we used the 5cm thickness. When building the Maxi fridge we had used a plastic faced polyurethane but then ended up lining the fridge compartment with stainless steel for hygenic and cleaning purposes. As we intended to fit a stainless steel liner to this fridge and there was no advantage to using faced polyurethane as the stainless steel liner would protect them. There are 2 readily available options for polyurethane insulation: slab and foam. When filling a large volume the foam must be applied in situ and it is hard to control density and avoid air pockets and dense places where the foam does not fully cure. Consequently we used the slab foam as it is extruded by an industrial process and the density is controlled. We then used the spray foam to seal the ends and fill the slight gap between the joints and the lining box.

There are 2 points of strategy in using the slab foam: one is to dovetail the ends and the other is to vary the number of slabs.


The above diagram demonstrates both of these principles. The slabs of polyurethane are fitted so that any air gap between the ends of the slab tends to be cancelled out by the next level of the slab insulation. Also the number of slabs are varied so that there are more on the bottom, less on the sides and fewest on the top. The reason for this is that heat (the enemy of refrigeration) rises – which means that effectively cold falls. Therefore we put more slabs on the bottom to prevent the cold from “falling out the bottom” and the heat from the warm hull (especially in summer water) rising into the fridge. There is less heat from the sides although the stove is immediately forward of the fridge. And finally the top is of leats risk as any heat up there will rise (i.e. go away from the fridge) and and cold source will “fall” into the fridge. Our ratio of slabs for bottom : sides : top is 3:2:1. Perhaps a physicist would find a more optimal ration which saves material and yileds the optimum efficiency but 3:2:1 is easy for us to achieve with our materials and is better than having equal insulation on all aspects.

D W1 W2 W H Volume
space available 54 57 38 95 73 300 litres
volume for insulation (5cm slabs at 3:2:1) 20 10 10 20 20
final volume 34 47 28 75 53 110 litres

Initial calculations showed that we were looking at a refrigerated volume of approximately 110 litres. Given that we started with an available volume of 300 litres, we have allocated about 65% to insulation (compared to 6% on the factory installation).


Once the dimensions of the final refrigerated volume and insulation are determined then it is time to turn to some structural issues. Polyurethane has a very good rating for thermal insulation but is useless for strength. Hence we place it between a 5mm plywood liner and the stainless steel liner which will become the “inside” of the fridge. A cross section of the insulation looks like:


The plywood is there mainly to hold everything together, protect the polyurethane slabs and provide a surface for mounting the cool box (i.e. the refrigerated are as well as the insulation and plywood outer liner) into the joinery. The space blanket is there to reflect radiant heat back outwards. The slab polyurethane is there to stop conductive heat loss.

At this point we made one change in the hardware from the previous project – we opted for the bigger model of Isotherm refrigerator. There are two Isotherm models available, one rated for a capacity of 125 litres and the larger model rated for 175 litres. Both models have the same Danfoss compressor, the difference being in the size of the cooling plate. The cooling plate is a container filled with an amonia solution that freezes at temperatures between 0 and 10 degrees Centigrade. When the compressor runs, it freezes the solution in the cooling plate, then once the compressor is switched off the cooling plate slowly thaws as it absorbs heat from the refrigeration compartment. Eventually the compressor will come back on, chill the cooling plate so that it’s solution is frozen again, and the cycle starts again. By putting in the larger cooling plate we got a longer period before the compressor came on again which meant the fridge could go longer between taking electricity from the batteries. Of course when the compressor came on there would be a larger volume of solution to freeze and it would use more power but on a sailing boat when you have wind and solar power, what you can not store in the batteries is lost. So if our cold plate could last until we had wind and/or sun power and the charging voltage went up, it would take power directly from them, power which might otherwise be lost once our batteries were fully charged. The end result being that our batteries would take less of a load from the fridge compressor and hopefully more of its power would come from the wind and solar.


Of course not all space in the refrigerated volume is useable – unless you are storing a liquid and able to completely fill the volume. Some space is consumed by:

  • the cold plate which should be mounted as high up as possible in the refrigerated volume
  • if a vertical divider is placed near the cold plate then colder air will be trapped which yields a colder section to the fridge which will not necessarily freeze things but will keep things colder
  • one or more shelves, preferably with slide-able segments
  • a grating on the floor as there will be liquid accumulation and by having a slightly raised floor the foodstuffs can remain clean

Something that consumes no volume but should be fitted is a drain point. This means the fridge floor should be slightly sloped so any liquids will run to the drain point. Unless you have a collection container there should be a valve fitted on the drain. Not fitting a valve means that chilled air will “fall” down the drain and out of the refrigerated volume. That is the same as warm as getting in. A valve will stop any air from moving except when the fridge is being drained.

The access lid to the refrigerator is the last area of special concern. Once again it must be insured that cold air can not leak out or hot air leak in. Two techniques are available: gaskets and stepped or sloped surfaces surfaces.


Obviously different techniques can be combined – the main thing is to stop any air exchange. On the Maxi fridge we used the sloped surfaces and decided to use the stepped surfaces this time.

Fridge Construction Tactics

As the fridge on the HR41 was physically much bigger than on the Maxi (both volume and insulation), and seeing both companionways were about the same width, we were not able to assemble the fridge outside the HR41 and then lift it into place. We had to build it, disassemble it and then re-assemble it in place. Note that the fridge can not be installed with the counters holding the sink in place.

The steps for designing and fitting the fridge are:

  1. Determine the available volume
  2. Allow about 1 cm all around for the liner box which will contain the  polyurethane insulation and facilitate the mounting to the existing joinery
  3. Determine the amount of insulation for top, bottom and the sides
  4. Your stainless steel liner (which is highly recomended) will be the inside face of the polyurethane insulation
  5. Have your stainless steel liner fabricated, don’t forget:
    – the top of the liner will be the thickness of the insulation below the level of the countertop
    – the drain hole
    – the fitting to mount the cold plate on
    – the hole for the coolant hose from the compressor to the cold plate to pass into the refrigerated area
    – the slope on floor so liquids will run to the drain point
    – fittings for sliding shelves
  6. Fit the polyurethane slabs around the stainless steel liner
  7. Fit the plywood containing box around the polyurethane insulation – we used 5mm plywood
  8. You are now ready to finally fit and close in the fridge

We followed the steps above and built our fridge outside the boat as it was easier for us and the mess was left outside. Once we had completed the assembly we started to prepare the installation location by removing the factory installed fridge and cleaning back to the hull. We then put on an epoxy paint to seal the hull while we had access to it:


Notice the shelf that is fitted, this will bear the weight of the fridge. The hole is for the drain to fit through.

Now the previously assembled liner box can be fitted. We This is where the assembly is too large to fit through the companionway if you have done the construction outside the boat. We solved this by making a curved cut down the outer side, across the bottom and up the inner side, by using a curved line it is easier to fit back together as if the cut was straight then the two pieces can easily move with respect to each other:


At this point the lining box is in place (although in 2 pieces), it is being supported by the shelf below, fitting tolerances may now be verified on each side.

The lining box can now be fibreglassed in place which will restore its structural integrity. We varnished the plywood on the inside of the box and gel-coated the outside as we wanted no ingress of moisture into the plywood.


The plywood lining box is now back to its original size and shape and is actually captive in it’s location. It can not be removed without breaking it into pieces again.

Next the space blanket can be placed – we held it in place with pieces of masking tape:


Next the pre-cut polyurethane insulation slabs can be assembled, on the inside of the space blanket. The following picture is looking down into where the fridge will be. The bottom most polyurethane insulation slabs can be seen, the cutout is the footprint of the bottom of the stainless steel liner. If you look carefully you can see that the polyurethane has been sloped down to match the sloped floor of the stainless steel liner. Such carving can be done by either knife or sand paper as the polyurethane is very soft. Also a crack in the bottom slab of polyurethane can be seen, although not serious it would be better if it had not happened. Finally the hole for the drain can also be seen:


The stainless steel liner can now be lowered down to fit into the bottom insulation. If everything was dry fitted previously then the pre-cut polyurethane slabs should simply slide down the sides of the stainless steel liner, between it and the plywood lining box:


Then the polyurethane top piece can be put in place. We fitted a fibreglass sheet under it to keep moisture from the refrigerated compartment away from the polyurethane:


Then the countertop can be fitted over:


Now it is time to deal with the lid. It also must be insulated as well as some construction at the edge to prevent air from moving in and out of the refrigerated compartment. Assuming you are going to edge the lid and cut-out this is a good time to do that:


Now is the time to fashion the lid closure. We decided to use the step approach to sealing the air in/out. We got some aluminum extrusion of the right profile, made a 45 mitre cut so we could have a “L”-shaped mould. We made two “L”-shaped pieces on the mould and then joined them to each other, making a 45 degree mitre cut at the end of each leg so they fit together and snugly fitted in the access hole:


The following picture is of a completed “L”. Two of these were used for the air lock on the countertop, another two were used for the airlock on the lid. All with the same aluminum mould.


Fitting the lid with the airlock, insulation and hinges is finicky, fiddly work but if taken slowly will eventually finish. Leaving a highly attractive refrigerator installation:


Inside the fridge:


the fittings can be seen:

  • the drain hole
  • the runners on the side for the shelves
  • a grating which allows liquid to collect below where the foodstuffs sit
  • the divider to the left which separates the normal fridge volume from the colder section which is immediately under the cold plate


  • compared to our previous power consumption, the new installation is much more efficient
  • fitting the larger cooling plate has reduced the load on the batteries
  • the stainless steel liner has earned its price every time we spill something in the fridge or need to clean it, with the stainless steel liner we simply empty the fridge, pour in hot water and wash it out, draining through the fitted drain

  • IsoTherm
  • slab polyurethane insulation is available from builder’s supplies
  • RParts for refrigeration parts

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