Chiriqui - I suggest you read my post about the small but sufficient panel I installed on my H27. My setup is pretty similar to yours. I have a 8hp Nissan / Tohatsu engine and a chart plotter + depth unit, and no shore power hookup.
I was having trouble keeping my batteries charged up with just charging off the engine (2-3 times a season when they were so low they would hardly start the engine, I would take the batteries and charge them with a regular car battery charger). Replacing all my lights with LEDs helped, but I still had to charge the batteries from a portable charger once or twice a season.
After replacing my batteries and installing a new panel and charge controller this spring, my batteries have never dropped below 75% SOC (state of charge), even on a 4 day cruise. I know with complete confidence now that when I get to my boat the batteries are always fully charged.
http://sailingit.com/blog/boat-projects/solar-panel-mounting-and-installation
Additionally, if your leaving the mast light on, and it's not a LED light, that can easily drain a single battery by itself over night if it wasn't fully charged to begin with. A typical mast light like you probably have draws roughly 0.8 amps, if left on for 12 hours that's roughly 10ah alone.
If your battery is a typical group 27 rated around 70ah, and you don't have a charger that has a float stage, you are probably never achieving over 80% SOC which means your battery only has about 56ah of actual gross capacity, and 28ah of usable capacity (to maintain 50% SOC or higher) .
Throw in the chart plotter and sounder (12 hours x 1a = 12ah), plus the anchor light (12 hours x 0.8a = 10ah) and you are already at 22ah of use, or 60% SOC.
Then try and run 2-3 (non LED) cabin lights through the night (3 hours x 1.4a each = 4.2ah each x 2-3 lights = 8.4ah to 12.6ah total) and you are now between 30ah and 35ah, which will pull the battery down to between 46% and 38% SOC depending on cabin light usage. None of this takes into account the peukert effect which will likely leave the battery even lower than calculated here.
If you instead started off with a full battery that was topped off with a good solar charger and the exact same usage, you would end up with a remaining charge of 69% SOC before cabin lights, or 57% to 50% SOC after cabin light usage.
Changing nothing but ensuring your batteries start off full will give you an extra 12% of battery capacity after your first day on the water. If that is done through solar, you will actually do even better as the panel (depending on size of course) will be able to replenish much of that usage of the plotter / sounder during the day. Adding LED lighting will also make a huge difference.
