The system I am familiar with feeds 4 (24v rated) 36 volt mpp panels into 48 volt mppt charge controller. It is not this simple on a boat. Any partial shading of a panel brings charging to nothing when using crystaline panels. To guard against this it might be best to use micro inverters on every panel. This can be in the form of buck/boost dc to dc converters or as is practiced in some arrays that put out an ac current.
I would suggest using high voltage panels like the for mentioned 36mpp panels. Remember that losses can be high with dc to dc converters. Choose them based on efficency not on price all converters are not equal. Some have losses as high as 15% or more and that will be a lot in terms of boat real estate.
Kevin
These nominal 250W panels actually vary in voltage from about 24-30V and outputs of 225W to 270W for the same roughly 55"x39" panel size. Since the charge voltage for a 48V FLA bank tops out at about 57V in the final phase of charging I would be sure to pick a panel that is closer to 30V.
The Flexmax80 charge controller I am using recommends a minimum 60 Volt input for a 48V battery bank so if your panel has a 24V open circuit voltage you'll actually need to string 3 of them together in series to get the required voltage. While this will work fine, I think it's better to use two 30V panels. Panels in series are more susceptible to greater losses from partial shading and working with an integral of 3 panels in series to make up your array can be more problematic.
These voltage numbers are specific to FLA and are a little different if you're charging AGMs or LiIon but I don't know what those values are.
Carter
I agree with using the "24v" 250w panels mentioned as they are a lot less expensive per watt than 12v panels. Connecting 2 in series will get you the 48v you're looking for to charge your propulsion batteries.
To run the 12v accessories I'd have a 12v "house" battery and charge it from the 48v system using a 48v-12v charge controller.
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