
dumb question, but how or where did you put the anode on? I'm guessing it was a collar type around the propshaft, and you had the boat on the slip?. if that was the case, and all that was done, I think the above responses would be on the money, probably not related.
And Shaggies idea, I bought one of these lithium jump starters, thinking it would be a great idea for the boat. Have used it around the garage a few times, and it's virtually idiotproof, you can reverse polarity it, even short the terminals together whilst its on by accident and no sparkies. Downsides:charges by usb or a 12V socket (but if your battery's already flat???)
I also envisaged I could use it as a little battery bank, to transport ejuice if you like, a jerry can for voltage. Charge it at home, and connect it to the boat batteries when they are low, especially during winter when the solar isnt doing much. The potential voltage difference between the charged jumpstarter and the 'flatter' boat batteries would effectively 'charge' or top them up right, given a bit of time?
No, not in practice. If you are using it connected to crank your diesel over, it would work providing a power source boost, but not in the trickle example described above, the unit doesn't activate or effectively switch on.
Could just be me, quite likely, but something to know maybe.