Keith Calton converts an under-seat locker into a bespoke boat fridge for under £450
Making a one-off boat fridge is well within the skill set of most DIYers.
It is not expensive either as the materials for the ‘plug’ are pretty cheap, and since you will want the smooth side inside you won’t need a mould!
Aboard my 31ft Macwester Wight the most suitable space for a ‘bespoke chiller’ was an under-seat locker so I started by stepping the interior to cope with the turn of the bilge.
The most important features of a boat fridge are: a well-fitting lid, very good insulation and a bonus is ‘top opening’.
Open fridge showing plate installation several years after initial use, the extra wire shown is to the thermostat sensor. Credit: Keith Calton
It also helps if you plan the height with the lid closed to accommodate a bottle of wine (or champagne which is taller).
The lid depth was 40mm in my case.
Heat rises and the heavier cool air runs out of front-opening versions, so an insulated well-fitting lid keeps the cool in and the thicker the insulation, the lower your power consumption.
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I initially built this fridge more than 15 years ago, and while it has worked well, recent improvements to add insulation thickness have greatly reduced battery consumption.
Power consumption is well under half of what it was – the compressor hardly ever seems to be on.
After a few days of use there is quite a bit of water condensed in the fridge which has to be mopped up, so something that I’d do now if starting from scratch would be to include a small plugged drain with a tube running from the lowest part of the boat fridge down into the bilges.
During construction of the lid, a tip is to put wavy lines with crayon on the lid sides, press it in place and see where it transfers to the bottom half so you can tell where it is tight – but bear in mind that no matter how accurately you make this, it will fit better just one way round!
Build a boat fridge: step by step
Credit: Keith Calton
1. Start by finding a suitable space. Bear in mind that the internal height will be reduced by the depth of the lid, 40mm in my case which still allows most wine and drink carton heights.
Credit: Keith Calton
2. The plug was constructed from cheap melamine-faced chipboard/MDF, screwed together. Be sure to countersink the screws and fill with car body filler before routing or sanding the exterior curves. A small taper on the sides will make the parts easier to disassemble when the component is cured. Bear in mind that all this inner wood is sacrificial and will be pulled apart to remove it, a slight taper will also be useful when making the lid.
Credit: Keith Calton
3. The whole box was mounted on a flat piece of board leaving 4in around the edges, plus a 1in strip of hardboard was laid around the edges of the box to create a slight lid recess for better sealing. This can be glued or held on by double sided tape, and it’s best to chamfer the outer edge beforehand. All ‘raw’ edges were sealed with a couple of coats of polyester resin and rubbed down, the odd small pit won’t matter as it will come out as a pimple when the component is made – easy to rub off.
Credit: Keith Calton
4. To get radii on the inner edges use pattern wax. I have a set of six balls for different diameters, you can use tubing or smooth dowel for a similar effect. Once every surface is smooth and sealed, apply three coats of silicone-free wax polish, then PVA release agent. To make the fridge body apply two coats of white gel coat and then laminate four layers of 1.5oz/450g chopped strand mat.
Credit: Keith Calton
5. After a good 24 hours at room temperature, the plug (inside bits) could be removed. This is a satisfying job but be careful to only use wood and plastic wedges so as not to mark the inside faces. Once all wood has been removed, remove all wax and wash off PVA release agent; use fine sandpaper to flatten blemishes then polish the interior with fine rubbing compound – clean this out after too!
Credit: Keith Calton
6. Use the new moulding to make a well-fitting lid. To create a lid thickness of 1.5in (40mm) I fitted a piece of melamine-faced MDF to the internal size of the fridge, mounted on upright MDF blocks, then put in the internal radius in pattern wax. For the top, glass sheet or melamine-faced MDF will get the required overlap. Add three coats of polish and PVA release, then one or two white gel coats (extra thickness will help if you need to rub it down later). Tape any rough edges. The idea is to laminate two layers of 1.5in/450g chopped strand mat to the internal and upper lid and squeeze together while wet. Add a weight on top to ensure good contact.
Credit: Keith Calton
7. After waiting for a day, I taped in plastic wedges to release the upper surface and the inner lid, then finished the lid ready for cleaning and trimming. It was then a case of trimming the lid and base part to size, leaving enough flange for both the insulation material and an overlap to rivet/screw the fridge in place. If the lid is a bit tight, relieve the sides with fine abrasive paper.
Credit: Keith Calton
8. To insulate the lid, I drilled two holes in it and squirted in builders’ foam from the bottom up. It’s best do this when it’s not too cold as the foam will work better warm and don’t put in too much as you can blow the lid apart. As it was, I did get a slight bulge in the underside.
Credit: Keith Calton
To insulate the base I decided on two part polyurethane foam for the insulation as I had various batches from different projects over the years and I reckoned about 1.5in/40mm to 2in/50mm would be OK (wrong), the thicker the better but I was constrained by the overlap when fitting the fridge in the space cut out for it.
Credit: Keith Calton
9. I made a box lined with clingfilm and securely fixed it over the upturned fridge and poured the various batches of foam over it, and ended up with a solid insulated fridge.
Top tips for building your own boat fridge
- Buy plastic ice ball sheets and fill them with water and hang over the fridge plate which should freeze them to pop into your drinks.
- Something I didn’t do which is useful is to put a small plugged drain with a tube running from the lowest part of the fridge into the bilges or a container therein, after a few days there is quite a bit of condensed water to be mopped up.
- Always keep your fridge as full as possible, just replace food with containers of water and if you have ice available, start the trip off with a bag of ice which will take the temperature right down quickly, save on batteries and give you ice for drinks for days!
Price breakdown
The cost of an Isotherm external fridge cooling unit, plate and thermostat is about £390 – it wasn’t much less than that in 2006 when I built and installed my fridge.
I have just topped up the polystyrene balls after a year of use and settlement so there is no air gap.
Without my fridge unit plugged in a bag of ice was not defrosted totally after five days so insulation is pretty good.
The wood and fibreglass used in the construction were all from what I had in stock for my old business, but I wouldn’t think they’d cost more than £50-£100 at today’s prices.
Most people who would undertake this sort of project would probably have some of the wood needed already… or am I the only hoarder in this world?
Total estimated cost: £440
Adding extra insulation
This boat fridge has worked well for about 15 plus years but I always felt I needed more insulation to cut battery consumption, I really hadn’t planned the original thickness that well.
I considered foaming the entire space between the hull and seat but that would seal the whole fridge in so it could never be replaced or the cooling plate serviced; eventually, as the space the fridge is in is sealed, I thought I might be able to get small polystyrene beads and get them in there but I wasn’t sure of availability.
My pal Les suggested his daughter’s old bean bag, which was the perfect size and price (free if we didn’t tell her!) so that only left the method to get the beads in there.
Inserting beads was easier with a DIY detergent bottle funnel. Credit: Keith Calton
Using beads, you can, in the event of needing to remove the boat fridge, suck out most of the beads into a vacuum cleaner with a bag and not make too much of a mess.
I cut three holes – two for 4in inspection hatches in the compartment sides and one vacuum cleaner nozzle-sized at the back at the top – then covered one of the lower hatches and made a polystyrene block with the vacuum cleaner nozzle in place and taped that over the open hatch cover and started my trusty ‘Henry’ vacuum cleaner.
The partly-filled insulation space. Credit: Keith Calton
You must place a couple of layers of muslin over the nozzle of the vacuum before you start sucking or you’ll just pull the beads straight through into the hoover.
A low-mess method of getting beads in turned out to be a funnel made from a detergent bottle.
It didn’t take too long and with a bit of poking around and putting the nozzle in from other holes, all the space around the fridge was full.
If needed in the future I can top it up. I countersank a screw in each of the inspection hatches to deter any inquisitive finger from undoing them in the future as it would be quite a mess if they were accidentally opened!
Using a scew as a security feature to prevent accidental opening. Credit: Keith Calton
All this fiddly effort has been worth it, as battery consumption has reduced by more than 50% – the compressor hardly ever seems to be on.
There may be many situations where this will be useful to others – you just need the compartment where your fridge is to be sealed and this can even be done with masking or gaffer tape.
Enjoyed reading How to make a boat fridge: step by step? 
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