....So wouldn't a 100 amp alternator replace the 60 amp hours for the fridge in a little less than an hour at a fast idle or am I missing something? .
One thing that is overlooked is that the battery is not going to charge at the same rate and the amps it is able to use from the alternator will depend not on the alternator's output but the batteries ability to accept.
On the Mac we have 2 12 volt group 27 batteries at the moment. We were out day after day and kept them at over 50% charge. A few times if I felt they were getting discharged more than I liked before night fall or were discharged more than I liked in the morning we would run the generator with the 12 volt car alternator. it is a GM 3 wire, I have a 1 wire on my hot rod.
With the 3 wire you can use a switch on the field. It is important if you are hand starting the engine that is powering the alternator as it makes it possible to pull the starter rope without the alternator putting a load on at the time trying to produce electricity.
Used with the batteries at or above 50% the most I ever saw it putting out was about 24 amps, way below the 60 amp rating. That was only for the first few minutes of charge and they it would continue downwards from that point and within 30 minutes or less it would only be putting about 10-12 amps into the batteries. At this point we are at diminishing returns so I would stop and let the solar top the batteries off while they also supplied the power for the fridge. The fridge didn't discharge the batteries much since it was only on during the day for about 9-10 hours and the panels were working very well for about 6 of those hours.
A lager alternator would do nothing to increase the charge time with our batteries. Now there are other charge circuits that can be made that can force a charger to charge at a higher rate than the batteries acceptance rate. I didn't feel the need to go that way as you need to be very careful doing this.
The lager alternators come into their own when cruisers with lager boats have larger battery banks that can accept more and when at times they draw them down below 50%.
On the amp/hrs per day for the fridge remember this is the numbers I feel good about for our 63 quart Edge Star in Florida with mid 80 degree cabin temps and adding 2-3 warm drinks a day to the fridge and other food when we did get ashore. Different fridge or different use will mean more or less than the 35 amp/hrs. I feel ours is using during those time. It was less in Nov./Dec. when the cabin was in the 40's at night and 60'd during the day but also there was less daylight hours for the panels and the sun was lower in the sky.
I've lived in the northwest two different times and both times from mid-June to almost Sept. there was a lot of sun and long days. I think we would be fine there during those months but not the rest of the year, but I wouldn't be cruising then anyway.
One has to determine their needs/wants and then work around that. I use to backpack and take canoe trips that were 8-10 days and had no fridge and had a great time. We started with an ice box, actually 2 in the boat, One with food and ice and the other with just ice or frozen food until it defrosted. For us we didn't like this situation due to where we went and how we used the boat (much different than most). We found it restricting where we went and how long we were able to stay there. Upper Priest Lake in Idaho has no marinas so we weren't able to stay there as long as we would of liked to. Even Priest Lake didn't have a lot and planning on where we were on the lake to get to one was a pain. Lake Powell is a similar situation. Forget marinas in parts of Florida like the 10,000 Islands and the Everglades and even in the Keys they aren't everywhere. Due to weather you can't always stay on a set schedule that will put you where ice is and it is amazing that one really big problem in parts of Florida is actually finding a place you can go ashore for supplies. We also just aren't marina people although we recognize a lot of people really enjoy them. If you are only staying in them to recharge batteries they aren't cheap and a few nights will start to buy a lot of solar.
One solution for someone that doesn't need solar most of the time is making some mounts for the panels that make adding or removing them easy. Say have a 100 watt panel on the boat all the time and just add another 100 watt for trips. 200 watts will take care of a fridge like ours about anyplace one is going to sail. If you start dipping into the low 50% range start shutting things down. Another option is buying one of the small generators for $100 or less and stow it away with a compatible charger and take it out those few times you might need to help the solar out.
The other option that costs more and is more work is to make is a permanent fridge with Stu's 'super insulation'. We did that on the Endeavour and it is almost twice the size of the portable and yet uses about 2/3's the amp/hours. Most of the big boats that have fridges are not super insulated and do have the amp/hr. usage that Stu commented on.
So determine your goals and work towards a solution, there is one,
Sumner