When all the new insulation was glued in place it was time for a third test using another 16Lb bag of ice and solid block ice below the shelf. This time the temperature inside the box sank to 39 degrees F, but stayed there two extra days. Better still, I could not feel any cold on the outside of the new insulation.
I felt some satisfaction in achieving this small advance, because I had evidently improved the insulation significantly for only a small cash outlay. So even if we decided to keep it only as an ice box it would be more efficient. I asked myself why the builders didn’t do this when the boat was built.
The temperature graph shows the differences between the three tests, But I remained mystified why, after all the new insulation was fitted, the temperature did not drop much below 40F. I was told that even in a well insulated box the holding plate temperature must be about 10 degrees lower than the temperature required in the freezer. Therefore, in order to maintain freezing inside the box, the plates needed to go down to minus 22 degrees F. Assuming the temperature of the bag of ice was at freezing, 32 degrees F, I was actually doing quite well to read 39 degrees F, on my test.
I now had to decide whether to bite the bullet and buy a freezer unit. These are not cheap, starting around $1900 to upwards of $4,000, for different specifications.
Refrigerators and freezers all work on the same principal, and with an abundance of detailed explanations on the web there is no need to repeat that much technical detail here. Simply put: a compressor pumps freon gas through plates mounted inside the freezer box. The freon absorbs heat and causes the inside of the box to get cold. There are basically two types of freezer plates: evaporator plates and holding plates. Evaporator plates are like those in a household fridge/freezer—thin corrugated wrap round plates usually in the top of the fridge. These freeze quickly, but because they are so thin they also de-freeze quickly, so the compressor needs to be running almost continuously to keep them frozen. Holding plates are much thicker, up to 3” inches if there is space inside the cabinet. Holding plates take longer to freeze, but they stay frozen longer, so once they are frozen it takes less power to keep them there. For any boat, particularly a sailboat, holding plates are the best solution for a freezer.
BUYING THE EQUIPMENT
I sent a drawing to four suppliers of marine refrigeration equipment, asking for recommendations, specifications and prices. One didn’t reply at all, another was so difficult to drag a price out of I gave up. Finally I choose Sea Frost in Barrington New Hampshire, who were the most helpful. (www.seafrost.com) They could supply all the equipment, along with custom sized holding plates. The owner, Cleave Horton, is extremely knowledgeable and recommended two holding plates, one each side inside the box, connected together to the compressor. These would be 15” inches square x 1” inch thick and would just fit through the 12” inch square lid of the box. I would therefore loose 2” inches in the upper box width. I asked Cleave if it wouldn’t be better and cheaper to have one 2” inch thick holding plate? His reply was, “Think of it as standing in front of a fire in your house. Only your back gets warm, but if you were between two fires you would toast evenly.” I thought this was a good analogy, and I’m sure it is the sort of thing an inexperienced buyer likes to hear.
Another important item to consider in any fridge/freezer system is the compressor and associated equipment. There are electric compressors, of which by far the most popular make is Danforth. Then there are engine driven compressors, which are more powerful and don’t deplete batteries. There are also air cooled compressors and water cooled compressors. Difficult choices indeed for an apprentice.
As usual on boats, there are pros and cons for each type. My boat has a very large battery bank of over 1300 amp hours, and numerous ways to keep them charged. I have a wind generator, solar panels, a 6.5kw generator with both a 12 volt alternator and 120 volt outputs, which also powers the 40 amp battery charger. This is not counting the main engine alternator. I therefore decided to buy a 12 volt air cooled system.
The compressor, along with it’s condenser, (the radiator part which dissipates the hot air), and the control electronics are particularly well made on the Sea Frost units. All the items are enclosed in a strong metal box, which keeps dust and dirt out and allows for other things to be stored in the same locker, without resting on the equipment. All the other manufacturers I looked at had open equipment. The control unit can be mounted anywhere and is connected to the holding plates with pipes, pre-loaded with liquid freon and employing self sealing couplings. My pipes were made exactly the right length to my drawing and fitted a holding plate on each side of the box.
Sea Frost’s instruction manual is meticulous, which was very helpful to a novice. They even include photographs of how to bolt the pipes together using two wrenches so as not to over tighten the connectors.
I had ordered the latest Danforth BD80XP compressor, which they say uses less power, along with a fully programmable remote electronic thermostat. Sea Frost built and shipped my order in one week. The cost was $2312.00
Before installing the holding plates I decided to repair a few chips in the fiberglass and paint the box with two-pot epoxy paint. This made the inside look new.
I found the installation to be quite straightforward. I screwed the plates to both sides of the box, using the special fasteners supplied, which hold them clear of the wall of the box about 1/4” inch, to permit all round circulation. Then I drilled a 1” inch hole through the side of the box and passed the pipes and thermostat wires though. The trickiest pipes actually connect the holding plates together inside the box. I followed the detailed pipe connecting instructions and managed it with no loss of freon. The other end of the pipes connect to the compressor unit.
I mounted the compressor cabinet under a saloon seat next to the freezer. A small fan circulates air through the condenser radiator inside the cabinet and blows it into a cupboard under the sinks, which had a louvered door. I therefore had a good circulation of ambient air through the unit.
I then wired the electronics into my 12 volt circuit board, with a 30 amp breaker marked “freezer.”
Finally, on further advice from my trusty Cruising Forum friends, I lined the inside of the box with 1/4” inch thick bubble wrap with foil on both sides. This is used to line freezers and ice boxes in RV’s and easily removable for cleaning. I also fitted a ball valve on the water drain pipe in the bottom of the box to prevent even the slightest amount of “cold” escaping down the pipe.
Amazingly, when the moment of reckoning arrived, everything started to hum, with no leaks and amazingly quiet. The holding plates immediately started to get cold to the touch, and after about two hours we found ourselves with a superb little freezer, which stays at 26 degrees F, all the time.
It is certainly one of my better modifications to our good old boat