This is one subject I'll dive right in on. I had this conversation with old Blainie Scott (Owner of Scotty Products) and his supplier from Russell Electronics Ltd.(The folks that make the black box) a year before Blainie died. Both agreed that if you're running a fibreglass boat with one motor and two riggers, the black box will do absolutely nothing for you except act as a placebo. Your boat is not producing enough of a current to matter one iota. If however, you run a metal boat, have more than one motor, or fish 3 or more riggers, then the black box will be helpful. The more metal in the water, the more current you're producing. Unless of course you're a complete git when it comes to wiring.
They both laughed when I asked about a 17 foot boston whaler, with a 50 Yammie on it, but said it was the average fishermen that didn't need one, that made them their money.
I believe that the average fishermen will buy anything if they think it will improve their fishing chances, and if after using the product, they catch a fish, they will automatically assume that it was the product, and the product only, that caught them that fish. Remember Dumbo's feather?