Once again, it depends
The size of the reefer compartment and if it has a freezer or not and the amount of insulation it has determine how big an evaporator to use. That will drive the size of the compressor which will have to be balanced against the number of AH you have lying around extra in your battery which will depend on how you plan to use the reefer on the boat. I'm not up on my 68 H 34 iceboxes but here are some things that are always true. I assume that you are not just using shore power and want to actually use the reefer for a weekend or longer by doing the recharg the batteries once a day for an hour scheem. More insulation is always better, even if you don't have active refrigeration. Before you do anything else add insulation and see if an ice box works for you. 4" min, 6+" is better. If you don't put on insulation you will just be inviting a power hungry monster into your boat and your stuff will not stay cold anyway. Go with an air cooled compressor not a water cooled one if at all possable. Water cooling is great but has many more maintenance problems. Plan on at least 3 AH/hour or 72 AH/day to disapear into the reefer. If you recharg once a day for 1 hour that means you need have at least 144 AH (50% discharge) lying around in your batteries that you are not using right now and add at least 80 Amps to the charging system. When you figure out that: 1)you don't have 144 AH lying around and realize that you don't have a place to put 2 more 70 AH batteries, 2)it is going to take a 100 amp alternator and you only have to add another pully to the crankshaft to get it to work. 3)You can't add any insulation because there are 3 bulkheads right up next to the outside of the box and you have to add insulation to the inside which will make the whole thing moot cause now the box is too small for all the stuff that has to fit into it so you have to recalculate and now you need 3 batterise and a 135 amp alternator, halling ice begins to sound mighty appealing! I tried this on a Catalina 30 w/icebox. After looking at the $$ it would cost and the pain of running the engine for 2 hours each day not to mention installing the thing, we opted for 1" more insulation (it had none in some places and only 1/2" in the rest!!!) and my ice consumption went from 5 bags a day to 2 bags a day. This allowed us to do a long weekend overnight on the hook on just 6 bags of ice if we precooled all the stuff that went into the icebox. Course my smart slipmate informed me that he uses his icebox to store stuff like bread and just uses a big cooler stashed under the table. I know I sound pesemistic, but sailing is supposted to be fun and fighting with a power hungery monster all weekend long is just not part of what I'd recommend. Save your $$$ for that new mainsail or bruce anchor.
