This is actually a good question, and maybe some of the real electrical
experts could give an opinion.
charger needs to be sized right both in relation to your bank, and to the generator. With regard to the battery bank -- the optimal charger for regular FLA batts is probably about 20%C, or for you, 240 x .2 = 48 amps. That's an amount of power which regular FLA batts will accept without too much bubble and fizz, during the best part of the bulk phase. Smaller is ok and will actually be more gentle with your batts, but with a petrol powered generator, you don't want to extend the run times more than necessary.
That's, by the way, a pretty small bank for a 40' boat
-- can you expand it?
A 48, say 50 amp charger puts out maybe 700 watts of power and will maybe consume 840 watts (but they vary a lot in efficiency -- look at the specs). That's within the capability of the Honda EU10i, but near the edge. These generators are variable speed inverter
generators, and when they are putting out near maximum power, they will be screaming away (quietly, but screaming) at redline. I think that for a 50 amp charger, you might really want to think about the next bigger one, especially if you plan to use AC power for anything while you're charging
Or you could go down to a 40 amp charger which will be comfortably within what the little Honda can do.
By the way, if you can afford to increase your solar capacity as well as doing this, you will be awfully happy you did. It will greatly reduce the amount of generator run time and will allow you to shut down the generator earlier. I wish I had listened to advice
a couple of years ago to do this, instead installing the bloody useless wind generator
My father's boat
, about the same size as yours, has I think 500 watts of solar and is based in Florida
. He has no generator of any kind and is basically self-sufficient in power. He does have a lot more battery capacity than you do -- a couple of 8D's.