Its all about the difference in nobility of the disemilar metals in the water. SS has a nobility of .24, Zinc is 1.04. Subtract the SS noblility from the Zincs and you end up with .8V positive. The Scotty manual that came with my BB sez that your boat naturally (without the BB's influence) should be somewhere between .7 and .9 V positive. The box will adjust it from there. Above .9 and you are "hot" and have a direct short to the hull, you then have to do a step by step process to find the culprit. Battery switches, ground points, etc. Below .7V and you do not have enough zincs or your zincs are not clean, always clean them with a SS brush prior to fishing in the salt. If that does not work you will need to add zincs. The old Glasspar that we had always was in the range with clean zincs, our newer sled w/pump does not always read above .7V when you turn the box on (the box checks the natural current for the first 10 seconds or so). I have added more zincs, but I think that the pump impellor sleeve (which is one large piece of aluminum and the nobility is .71) may have an effect on this. Another thing that is recommended that I do not see many folks do is to make sure that your trolling motor is also grounded with the rest of the system, for thoses folks with electric start that has already happened. For those with pull strat motors you need to connect the trolling motor to the main motor with a wire to ground it.
Interesting enough when I typed in the Scotty USA website it is under construction and the Pro-Troll is there instead, sounds like maybe they have broken off with Scotty of Canada and forged out on thier own, it does have the Cabelas label on the unit shown in the pic. Here is the link http://www.protroll.com/blkbox.html I have had a BB for a few years and I am convinced that they work. You really will not see a difference when the fishing is good and everyone is catching, I think they pay off when it is scratch fishing and it just gives you an added edge.
Good luck and tight lines, Jeff

Thanks Jeff. I was just reading the info on that site. Very interesting stuff. Who would da thunk it? Catching more fish through science.

Wow! What an awesome respones and some interesting posts about this subject. This absolutely convinces me that my wife...Whoops! Santa Clause is going to have to get me that for Christmas...Sheesh! I hate knowing what I'm getting....

Well, I went and bought one from Cabelas. I figure it will be a $100 experiment for Christmas.

I am just a sucker for new toys!

I usually run below the suggested numbers. For big kings, I like .530 and big silvers I run 600-645. This could be my boat. I run a 20' Trophy. I have the quick disconnects in the Scotty BBox line. Keep plenty of corrosion resistant oil on these and keep your battery terminals clean. Corrosion in these wires can do funny things. I keep a digital voltmeter on board. The .530 yielded a 40#+ king at westport this year. The lower numbers also seem to increase the size of blackmouth too. The begining of the year in the south sound I was not getting any fish. At the end of the day I checked the voltage. It had jumped to .9. Unplugged the BB wires and got fish on. I had the wiring go bad under the boat. Replaced the wiring and was back in business