Black Box when used in Relationship to Downrigger Trolling

Polarity ; West coast commercial salmon trollers for years have known that some boats will out-fish others, this can usually be traced to stray electrical current generated from the boat and then into the trolling or downrigger wire. If you remember your chemistry from high school, when you place 2 dissimilar objects (stainless wire and zinc anode or a aluminum boat in this instance) in a conductive solution (salt water) you create an electrical charge.

I had one old time troller tell me of his friend’s boat where one side of the boat would catch double that of the other side, and those fish that did hit were quite deep as compared to the other side where depth did not seem to make a difference. One winter during layup, they finally went thru the boat, checked all the thru the hull connections and grounded them to the negative side of the battery. What they found was that the sea water intake for the motor cooling system was on the side of the boat that was the fish non-biting side. Apparently somewhere in the wiring or plumbing, there was a electrical leakage to it. After the grounding process, his catch rate on that side went up considerably.

Currently most sport fishing boats used on salt water are made of fiberglass and these boats appear to be more forgiving than say an aluminum boat. I have seen it written somewhere to NEVER ground your hull if it is aluminum. I am not an expert in this field, so can not really say, but in some circumstances, this may be rather hard to accomplish.

Check it Out ; If you have any doubts as to whether you need a black box, you can simply check your downrigger wire with a volt/amp meter that can read 1 volt or less on the DC scale. With all the electrical equipment on the boat turned off, connect the negative lead from the volt meter to the negative battery terminal, or something that is already grounded. Touch the positive lead to your extended downrigger wire while in the water. Ideally it should have a voltage of + .6 to +.7 from your battery ground to the downrigger wire. If the voltage is low, you can add zincs to your motor/trim tabs, etc. If it is high, then it is suggested you check things out and ground everything metal that touches the water.

If a boat is set up properly, all the corrosion is channeled so it dissipates harmlessly in the zinc sacrificial anodes. Therefore first, I would check the zinc anodes on the boat and on your outboard or out-drive. They should NEVER be painted. If they are pitted or corroded slightly, a wire wheel or sandblaster can help. If they however are more than 50% dissolved, they should be replaced. They should be cleaned off each season anyway.

To check if any of your electrical units need looking at, one by one, turn them on and watch the above indicated voltmeter. Start first with the battery switches. Then start the engine and then each of the other electrical devices including the bilge pump. If your natural voltage reading changes by more than .05 volts from its starting point with any of these steps, you have an electrical leakage problem.

One thing most overlooked would be your trim tabs. They will usually be bolted thru the fiberglas transom, if so just run a bare #10 wire as a ground from you negative battery terminal to the bolts and put a second nut holding this ground wire. Do this to anything metal that goes thru to the outside of the hull.

If your downrigger cable is the stainless steel wire and is more than 2 years old or has been used consistently in saltwater, it may have become corroded somewhat and should be replaced. Always wash down the cables with fresh water and then with Salt Away each time you pull the boat out. I have seen some so rotten that even a year’s use made them so weak & starting to fray that any even slightly fast stop in letting the ball down broke the cable. Replace it to save yourself some grief.

Do not use a metal snap on the end of your downrigger cable and tie it directly to an uncoated downrigger weight. If the ball is tied directly to the cable with a metal connection, you have just increased your voltage many times. The best method to break this electrical connection would be use a Scotty nylon snap or connect the weight with a short piece of tuna cord to insulate the weight from the wire. The Scotty snap is also made to pull loose, then drop the ball in case of a hung ball. Silver Horde makes a pre-made molded nylon strap about a foot long with a special snap that is a quick disconnect plus it is an ideal insulating unit. When using this Silver Horde strap or a homemade tuna cord about 12″ long, this also gives you something to hang onto when lifting the balls over the side.

A Cure, the Black Box ; For the small boat fisherman, there is another alternative. Scotty and Pro Troll both make a “Black Box” that can be adjusted to give the desired electrical charge to your wire. If the voltage is outside the above listed range either way, it will benefit you to consider doing something to correct the problem. The price on the Scotty is usually over $400 while the Pro Troll is $99.

In usage there are wires running from the black box to each downrigger. A electrical wire is attached to the boom that has bare pig tail loops that is placed so that the downrigger wire can be readily placed inside this loop that you need to place in line with the location of the wire from the spool to the pulley. The wire just rubs on the inside of this pig tail loop making connection. When you turn the black box on, you can read either the existing natural voltage, or turn the knob to induced voltage and then adjust what output you desire.

Any voltage leak or implied voltage to the downrigger wire will dissipate as the distance around the wire increases or the depth from the boat increases. This is why many knowledgeable fishermen watch their how far back they run the lure from the downrigger wire. When using the black box, the farther back the lure is, less implied voltage will attract the fish. Here 15′ or 20′ is a lot better than 70′.

Different types of fish react to different electrical charges. These recommended voltages are supplied when you purchase the black box.

Scotty Black Box Pro Troll Black Box


Use of a Black Box can also be effective in mooching, jigging and still fishing.  Here, the downrigger or wire is connected to the Black Box as if you were trolling, but the ball is lowered to the fishing zone


To contact Scotty’s website for some information use this link


Pro Troll sells a booklet for $8.95    They also have a series of very complete “HOW TO” articles on their website that is recommended reading.

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