CASE STUDY 2: BUILDING A UNIT FROM SCRATCH
When we decided to refit our 38ft sailboat for full-time cruising, my wife, Tara, insisted that we add pressurized hot water, a cabin heater and a refrigeration/freezer system. The first two I felt comfortable installing, but I had no experience with marine refrigeration and had read enough articles on the subject to know that there is no single right way to go about installing such a system on a boat. Refrigeration is one of the top energy draws on a small boat, and I did not want to get this wrong.
The first thing we had to decide was whether to install a self-contained pre-made unit or build our own refrigerator/freezer box. After taking measurements where we wanted the system to go, we decided building our own box would make the most efficient use of the space. Once that decision was made, we now had to find the best 12-volt refrigeration components for our needs. Much online research ensued.
We soon zoomed in on the Sea Frost BD system. Owner Cleve Horton became an invaluable resource while we built the box, helping to determine the dimensions and amount of insulation to make the system as efficient as possible. We chose the BD system because it would be sufficient for our box size and can be air cooled. Water cooling is more efficient than air cooling, but if the box is well insulated and not too large, the difference in efficiency is not large and air cooling simplifies the system and lowers maintenance.
In addition to the basic BD system, we chose a freezer bin and an electronic thermostat with compressor speed control options. The freezer bin consists of two direct evaporator cold plates set on opposing sides of the bin, creating a small freezer. The rest of the box becomes the refrigerator as cold air spills over from the freezer bin. The electronic controller allows us to set different compressor speeds for when we are in different climates. It also allows us to monitor refrigerator temperatures as well as cold plate temperatures, which makes it easy to adjust plate temperature for a desired refrigerator temperature.
The hardest part of the job was building the box and then deciding where in the boat all the other components would go. This needed to be done so Cleve would know how long to make the coolant lines and wiring harnesses. We had enough room
3 to build a 4.4ft top-loading box, which would allow us to use 6in of Dupont TUFF-r insulation around all four sides and bottom. We used 4in of the same material for the top and hatch doors. This would give us an “R” value of 39 on the sides and bottom and 26 on the top. The freezer bin set in the back of the box would be just under one cubic foot. This sounds small, but it has worked well for us living aboard full-time.
I used marine plywood to form the box and laid four layers of fiberglass cloth on the inside, wetted out with WEST System epoxy resin. As I did this, I also mixed a white dye with the epoxy so the inside of box would not need to be painted. It is critical to make the box waterproof to keep moisture from getting to the insulation and keep out any air circulating outside the box. Next step was building mockups of the freezer bin and compressor/condenser box. After I had the three major components of the system, the box and mockups, I cut the insulation and
set everything in place, so I could make the required measurements for Cleve.
After receiving the mechanical components from Sea Frost, it was just a matter of putting all the parts together. The whole system came fully charged with refrigerant; the coolant lines between compressor and evaporator plates also remain sealed until the connectors are torqued together, so assembly was easy.
It took a few days of adjusting the thermostat to get the refrigerator compartment to our desired temperature, but once we got a feel for the controller and a baseline of cold-plate temperatures it became a simple procedure. We have found no need for a circulator fan inside the box. Power draw varies, but the maximum is approximately 40 Ah per day.
This refrigeration unit is simple and has run continually for almost three years without a hiccup. The only upkeep is defrosting the freezer based on the humidity and ice build-up, about once a month. But at the end of the day, we always have ice for our sundowners!