We cruised with a family
that did exactly what you are describing.
They had a very well insulated box that was their Refrigerator
and then an Engle 40qt portable. They rotated frozen water bottles each morning and evening from the Engle to the refrigerator
box. They cruised for years like that and it gave them a refrigerator and some small freezer space in-between the cycling in/out of the water bottles. They did have pretty big swings of their refrigerator temp, but managed to live with it.
If I was going on a short term cruise
was a huge concern in getting me AWAY from the Dock
...heck...I'd do it myself. Will it work? Sure, they did it and in essence you are using a "portable and rechargeable eutectic holding plate"...but...isn't their always a "But".
The negative here is that you will most likely be asking your portable unit to run 24hrs/day. Sure 2.5A is small...but at 24hrs per day that's a 60AH per day load. For half that power you could have a 6-7CF Box with an installed CoolBlue freezer/refrigerator and "set it and forget it". (but yes more cruising kitty money)
Don't be tricked by the "low" real time amp usage of a portable. When I keep my Engle (Yes...even the owner of CoolBlue has an Engle on his boat) set to a hard freeze it never shut down when we were in Mexico
...never...it just ran 24hrs per day at 2.7A. Here in Morro Bay where the boat temp is between 60-75degs it still runs from 60% to 80%% of the time when I keep it in hard freeze mode. So you are not going to save any power and if anything you will use more than a built-in system. The built in system will also give you more of a true single
digit temp Freezer and True 35-38deg refrigerator.
Next, the hang-up I would have a hard time getting over would be the convenience issue. Having to add the water bottle cycle game
to the daily to do list on a cruising boat may not sound like a big deal and perhaps if I was younger and more into camping I wouldn't care a much, but you could be setting yourself up for a hassle that sounds good at first, but then irks you by losing food when you forget to do the bottle swap out.
The important thing at the end of the day is that you cast off.
So I say if saving some money
on the ships refrigeration
system makes it more likely that you will be one of the few that really makes it off the dock
...then by all means...do it! Sure you will use more power, sure it won't be as convenient, but I'd rather use more power and not be as comfortable than to never get off the dock!
Now if you do go the frozen bottle route
, don't use water. Beer
maybe...but not water.
Fill your liquid containers with a 25% Glycol 75% water mixture (or maybe 30%/70%). That will lower the freezing point of your eutectic solution allowing you to store more energy absorption BTU's than plain water. This could require some testing/trial and error because you may find that your portable unit won't get cold enough to freeze the bottles if you lower the freezing point too much and get greedy on the amount of BTUs you want to "store" in the eutectic solution.
Tech Note 2:
When you do a Amp Hour per Cubic Foot volume comparison of the small portable Engle and Dometic units compared to say a 7CF box 1/3 Freezer and 2/3 refrigerator with a CoolBlue system in it. That's when the "efficiency myth" of the small portable units falls apart. What makes these small 35 and 40Qt portable units appear to be efficient is their small cubic foot size. 40Qts is a 1.33CF Box that's nothing when you compare it to a set-up that has a 2.1CF Freezer plus a 4.9CF Refrigerator and can run on less daily power with good insulation