Short answer is there is no issue with solar charging AGM and if you go over 40w get a controller.
If you do go with solar read this.
Installing A Small Marine Solar System Photo Gallery by Compass Marine How To at pbase.com
I am totally rewiring and upgrading my boat which happens to be the same size as yours and probably designing for a similar purposes so I just made all the decisions that you are about to.
The best advice I got (too late) was get a way to measure your consumption
first. Then keep track of your consumption
for a while.
You've been living aboard
15 years and are probably very used to unlimited power with your shore charger
This will get complicated quickly because you will find that your consumption will be higher than what you have available.
The best sequence I have decided on is this
your consumption and decide a target daily use for your boat I will guess 50-80 amps like me
2. Decide how many days you want to go without recharge compared to battery
space - for me this became 2 days
3. Size the battery bank at 50% consumption - for me 3 batts @ 100 a/h = 300/h
4. I have 200 a/h house and ~25% of start batt to use or - 125 amps useable
5. Decide your charging strategy.
1. 70amp alternator
run ~30 minutes a day = 20-25amps
2. 100w solar @ 80% efficient X 8 hours/day (tropics) = 53amps
My energy budget
has to come in around 70 amps
I don't know how you feel about aesthetics but getting more than 100w solar on your boat or my boat starts to get ugly quickly.
- Living on hook for a while you will need to know about your batteries. I have done the research
and the Smartgauge is the way to go today.
The only drawback is that it doesn't measure or track amps consumption and I have not yet found a separate (cheap) gauge to do that.
The Victron BMV602 does this but for "monitoring" the charge status it is inaccurate (due to practical operator limitations not the gauge). You need something like this for step 1.
I have almost convinced myself to get both but I am still looking for a cheap
digital amp counter.
If you've gotten this far and are still hanging on the last thing is how to charge two banks with one system - The simplest may is to wire everything to house and put an ACR (charge relay) between house and start)
So a ton of complexity.
Now - You already have a shore charger - I think another way to go for you is forget everything above - get a suitcase hondagen and a gas can. I recommend buiding a small vented box on the foredeck rail somewhere and store fuel
out of the cabin