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

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?

i forgot,welding suppliers sell large carbon arc-rods by the dozen..

I did many experiments with salt water batteries.

Copper, aluminium, steel and carbon just do not give enough power to make it worth while. The reaction also stops pretty soon due to deposits on the surface of the metals.

Magnesium works better, but is expensive and kind of dangerous in larger quantities (fire hazard, toxic reactions). Also, a magnesium salt water battery (with carbon as the other pole) does in no way compare to a standard 12 V battery, it just can not come near it regarding the power output.

There is a reason why salt water batteries are not used, they do not work well.

The best option on a boat:

Fife to ten 100 Ah 12 V batteries charged by solar cells and a small wind generator. That really works.

http://www.emarineinc.com/pages/Wind-Generator-vs-Solar-Panels-Which-is-Better-For-Your-Boat.html

Greetings, Conrad

What you want is a bank of Nickel-iron batteries... will live longer than you!

http://en.wikipedia.org/wiki/Nickel%E2%80%93iron_battery

I did many experiments with salt water batteries.

Copper, aluminium, steel and carbon just do not give enough power to make it worth while. The reaction also stops pretty soon due to deposits on the surface of the metals.

Magnesium works better, but is expensive and kind of dangerous in larger quantities (fire hazard, toxic reactions). Also, a magnesium salt water battery (with carbon as the other pole) does in no way compare to a standard 12 V battery, it just can not come near it regarding the power output.

Thanks for the comment, Conrad.  A couple of questions:  What is"pretty soon"?  With no generator to mess with and no other batteries to service, I've no problem doing maint. on this system every 5-10 hours or so if all i'm doing is wiping off a few plates and changing out the electrolyte.  I'm not demanding lithium cell convenience here, nor it's performance.  If this thing is the size of a kitchen table and takes a daily regimen of low level maint. to keep it running, that's fine.

SeaMonkey I actually just completed such an experiment, and was able to pull three volts + from six cells, lighting an LED for several hours.  The hydrogen buildup you described was noted, and did indeed affect output (all I had to do was shake the cathode a little and the cell output would bounce back up).  I ran the salt solution at 1.030 sg , a bit above ocean water in concentration (btw for anyone doing this, one great cheap hydrometer solution is to hit a pet store that deals in tropical fish.  Hydrometers for measuring salt concentration in aquariums can be had for $8-12.)

I ran the experiment with both copper and carbon cathodes, and with both zinc and aluminum anodes.  The best reaction I got was with the copper/zinc arrangement, but I suspect that the quality of my electrodes had a great deal to do with that.  I'm having trouble finding a decent carbon electrode at an affordable price (here suggestions, especially for the final project, would b appreciated).  Copper and Zinc are much more readily available, especially in clad form.  The aluminum came from a can, sanded free of its coatings, but rather thin.  In general, I was getting .6-.73 volts from each cell, though the metallurgical purity of ALL of my electrodes is, to say the least, suspect.  I did not, at this stage, attempt to measure current, but the cells did light a 3V white led to full luminance for several hours.

As this is technically an aluminium-air or zinc-air battery, its likely that production finally stopped when the O2 in the solution was exhausted.  Vigorous stirring or the addition of a little hydrogen peroxide kicked the battery back to life (oddly, I remember an old Gilligan's Island in which the Professor did just this to charge their radio batteries, probably with galvanized nails and pennies
; the stirring thing I mean, not the peroxide).

Which brings me to some potential solutions, and I'd love to hear what folks thought of them.  A larger cathode, certainly, would make for less impact of the hydrogen bubbles, but two other options occurred to me.  One was to attach some kind of vibrator to the cathode rack which would steadily or periodically shake the hydrogen bubbles free.  The second, which might kill two birds with one stone, might be to place an airstone in each cell (the bubblers you see in aquariums).  The bubble stream could act to dislodge the hydrogen bubbles on the cathode, and would also serve to oxygenate the electrolyte.  Such air pumps consume tiny amounts of energy(about 2-3 watts at 110V), and other, non electrical means could be used to pump air into this system.

The next, pragmatic phase of this experiment will be to actually build a battery of cells at near full size (probably, given the voltage produced, around 20 cells)to produce 12v dc, and place a load on the system (Im contemplating an inverter and a small electric fan (35w ), along with any shaker or air pump used).  Your suggestions for electrode materials and where to find them would be most helpful, bearing in mind the parameters of the original experiment:  Common materials, easily accessed and replaced, and inexpensive. This whole contraption will have to wait until my next check as it is

I thank you all in advance for your comments and suggestions.  This board has been an immense education for me, and I wanted you all to know you were appreciated