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Black Box Techniques

A voltmeter with a scale of zero to one volt will measure the natural voltage of your wire. Here the natural voltage reading is .782 volts. Note the positive lead is touching the downrigger wire. The negative lead is connected to the engine.

1.  Inspect the inside of the hull.  If the boat is fiberglass or wood, there should be a copper bonding    wire running along the bottom of the hull connecting all the underwater metal fittings together.  For example, it should run from the engine or outdrive to the metal fuel tanks, metal water tanks, through hulls, trim tabs and motor shaft and stuffing box.  Be sure the bonding wire is not broken and that the connection to each fitting is clean and tight.  The connections are easy to check with a volt/ohm meter.  With the boat in the water, touch the positive lead from the volt meter to each fitting and the negative lead to the bonding wire.  If the meter shows a reading of .010 volts or higher, clean the connection and recheck.  If it is below .010 check the same connection with an ohm meter to ensure continuity of less than one ohm.  If the boat is out of the water on a trailer, you can use the ohm part of this test by itself.


If the outboard is an electric start, it is automatically grounded and nothing further needs to be done.  If it is not an electric start, it can be bonded by running a wire from the metal on the motor to a ground point on the boat hull.  If you are not sure the motor is grounded, you can use a volt/ohm meter to check.  To test for bonding, connect the negative meter lead to the negative terminal of the battery and test for continuity to the boat hull or bonding strap as well as the outboard motor.  If there is no continuity, install a bonding wire (10#) from the negative terminal to the hull and to the outboard.  One of the most common problems is the failure to bond a kicker motor to the hull.  Many are not electric start and must be bonded to avoid problems.





















2.  With the boat in the water, lower a downrigger cable into the water a few feet. It is best to do this away from marinas or docks where a number of boats are moored. Stray electrical currents from battery chargers or electrical systems can distort your readings. It is also best to have a vinyl-covered downrigger weight and an insulated end snap connecting your weight to the cable.


3.  Turn off everything electrical on the boat. Turn off the master connect battery switches if you have them. Then connect the negative lead from your volt meter to the negative battery terminal, the engine or to one of the bonded metal fittings on the hull.  Connect the positive lead to your downrigger cable near the spool or along the arm. In saltwater or most bodies of freshwater, you should get a natural voltage reading of between .7 volts and .8 volts. If the reading is significantly outside this range, you have a problem (see later problem section).  The voltage may vary slightly with different water conditions.


4.  One by one, turn on the boat's different electrical systems and watch the voltmeter. Start first with the battery switches. Next, turn on the bilge pump. Start the engine and then each of

the other electrical devices. 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 or a problem in the negative battery circuit. These are quite common in

accessories like bilge pump connections where a slight amount of positive electricity

can leak into the water in the bilge.  If you have electric downriggers, be sure to turn

them on as part of your checkout.  To test them,you will have to lower them deeper

and then hold your positive lead against the moving downrigger cable as it rewinds.

Chapter III. How to Test Your Boat's Electrical Charge


The effects of underwater electrolysis and the corrosion damage it can cause to metallic boat parts have concerned boaters for many years. Only in more recent years has it been learned that these same factors can have a significant role in how a boat fishes. This chapter describes a step-by-step procedure that you can follow to see if your boat is adequately protected from galvanic corrosion (electrolysis) and is set up to maximize the positive effects of a positive voltage on fish.
























Even if you may never intend to purchase a Black Box, you should carefully follow these steps. You can save yourself a lot of grief from corroded boat parts and may considerably enhance your fishing results.


Whenever a boat is in water, the different underwater metal parts interact with each other to form a weak battery. Electrical currents flow from one metal part to another depending on the type and placement of the metals involved as well as the mineral content of the water. Typical metals used on boats include aluminum, copper, steel, brass, stainless steel and zinc as sacrificial anodes. If a boat is set up properly all the corrosion is channeled so it dissipates harmlessly in the zinc sacrificial anodes. As it does so, it creates a positive field around the vessel which can be helpful in attracting fish. There are three seperate electical sections:


1. The zinc anodes are located on the outboard motor, outdrive or propeller shaft.  Because of the low nobility of zinc, these will be charged positive.


2. The hull itself and all the metal  parts in contact with the water will have a negative charge.  These parts include the hull if it is metal, motor shafts, outdrives, electric trolling motors and thru hull fittings.  All these metal parts must be interconnected with a bonding wire.


Other metal objects in contact with the water like stainless downrigger cable or wire fishing line that are not connected to the boat will be charged positive.  By using a voltmeter with a scale from zero to one volt, a series of measurements can be taken which indicate whether or not the boat is being protected from harmful electrical corrosion and if the downrigger cable has the proper positive field to attract fish.  Following is the step-by-step procedure.
























All metal parts in contact with the water should be connected together by a heavy bonding wire

Leakage of electricity from the plus side of a boat's power system creates "hot spots" and will ruin a boat's fishing ability. Here an isolated screw on a battery switch shows a leakage reading of .59 volts. This charge will pass along the fiberglass, through the bilge and directly into the water around the boat. This switch needs to be removed and all surfaces cleaned free of the dirt and crust that are allowing the electricity to pass.

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