First off Simon, no energy transfer is 100% efficient. Because your charging sources are putting out a certain amount of power it does not mean that all of that power is going to reach your batteries. There are losses, in the form of heat, that while minor do reduce the effectiveness of the recharge. Let's ignore those for this hypothetical discussion.
Also, your charging sources are rarely operating for long periods of time at 100% rated capacity. So the total amperage that will be varying widely in terms of what is reaching the batteries at any given time. You now know basically everything it took 70 plus pages of the Solar thread to communicate.(G)
Each of your recharging systems has, or should have, a controller, or regulator, that serves to regulate the amount of energy being sent to the batteries. Assuming we are running all three at once consider, hypothetically, that all of their energy is entering a funnel, at the bottom of which is your battery bank. If your battery bank is discharged, which due to the nature of batteries we consider to be at 50% of charge, the funnel is letting all of the charge through to the bank. As the charge in the batteries is restored, and usually at or around 80% of charge, the funnel starts to constrict at it's bottom. This is a function of the batteries themselves. It is called acceptance. As the batteries get closer and closer to full charge they will only accept energy at a reduced rate. The only way to get them to 100% of charge is to continue to supply current to them, at a reduced rate, for a period of time. As a very rough rule of thumb, it takes about the same amount of time to raise them from 50% to 80% as it does to go from 80% to 100%. This is why most boats are generally operating in that 80% range. It takes the same amount of diesel fuel
to run an engine, and it's alternator, to get that last twenty percent as it did to get the first 30%, maybe more.
Now when your batteries are fully charged, the charging devices continue to operate but their controllers, or regulators, will only send to the battery what it will accept, the rest is shunted and lost to heat or the charging device is automatically disconnected. The latter is basically what an alternator does-it only switches on when the voltage is low. If these charge limiting devices were not present we would "cook" our batteries. For instance, when charging a wet cell and we are trying to force too much energy into it, at too great a rate, we will boil the electrolyte in the cells, in the process releasing hydrogen gas, and possibly damage the battery. This is why we need to taper, or regulate, the charge.
Adding the extra batteries gives you a longer operating time in the batteries operating range. Below 50% charge the battery still has energy, but it is not really usable as the voltage drops too low for most uses. So, in practise, we usually end up operating our batteries somewhere between 50-80% charge and anything that lengthens that period will be of value to us. Extra batteries do just that. You would wire them in with your two existing batteries so that the incoming recharge will charge them all at once. If one is out of balance the charge will flow to the lowest charged battery, the one with the greatest acceptance. Due to the variables of our charging sources, both in availability and output, the longer we can extend the time we are operating in that range the better. We will also get more bang for our charging buck on the recharge as our batteries will be at their highest acceptance for our charging time expended. Anotherwords, we can get those four batteries charged back to 80% much more effectively than we can get two batteries charged to 100% and have the same total energy, roughly, available for our use.
I'd recommend that you pull up the archived threads on the topic of batteries. There is a ton of info out there on this. So much, in fact, that if you don't understand what one poster is saying you can skip to the next for another explanation. When you've read all of that, which should be around the year 2525, if man is still alive, you can commence with the reading of Solar World thread. If you get through that you'd probably wish to sell your boat as you'll need the money for long term institutional care!(G)