Large Salt water battery ideas and questions

[Mungo]

So here is the basic project:  My wife and I have been working on a moderately large houseboat which we intend to move aboard early next year.  A large portion of the  motivation is reducing and taking control of our energy usage.  Accordingly, the original idea was to utilize solar charged deep cycle batteries with a generator backup to drive a paddle (stern) wheel and to provide general power to the vessel.  But recently I've come across information on large salt water cells, and the prospect of using one or two pretty large battery wells to provide power, replacing the electrodes and flushing out the electrolyte with new seawater as needed has become interesting.  So here's the concept we're playing with, and I'd love any ideas, comments, or information the community could provide:

As an aside I should say I am aware of much of the youtube content on such matters, as well as of the Enviro-Gen 12 emergency battery system.

The area I have to play with would create two battery wells, each 16" X 12" X 30", divided into cells (probably 15 from the information I've been able to glean regarding salt water batteries).  The flooded plates would measure at least 12X10", copper and zinc.  Now I'm sure that there is some formula somewhere that would show how much current and voltage such a cell would create, but I've been unable to find said.  It would be possible to create a continuous isolated flow of electrolyte through the batteries.  The salt content would run between 20 and 30 PPT, but that could be amended to a higher concentration.

The motor we were considering creates about 5 hp from 12 volts, geared down 225/1 and series wound.  (I got this idea from some folks building small electric sternwheelers using wench motors as they are already 12 volt and include reliable planetary gearing.)  The batteries would also need to run a modicum of marine electronics and some LED lighting.

Ideas?  Comments?  Anyone able to calculate the likely output?

M

[Mungo]

To add a bit of additional information, Seawater has a specific gravity of about 1.028 (though it varies depending on the place measured).

[profitis]

you might want to try magnesium plates instead of zinc and graphite instead of copper.it will pack much more power and last longer.

[Mungo]

probably an excellent idea.  Im a little concerned, though, about availability.  This is rather on the lines of a "big dumb battery" concept.  Any idea for good sources for magnesium and graphite plates?

[profitis]

printing plates,old used ones from any print shop should do fine.otherwize aluminum might be a less better but easier option,parts of a ladder perhaps or thick foil roll.graphite can be cut to size plates by order from industrial suppliers of graphite products.do tests on small scale first by removing carbon rods from zinc batteries(non-alkaline ones,cheapo,s,its safe)and rinse and then heat the rods on your stove hotplate or over flame to evaporate all wax binder inside the rod and then it becomes non-waterproof and excellent for tests as a cathode(+).