Best voltage to have is around 0.6 to 0.65 volts. quickest fix is to add more zincs.
SPECIES VOLTAGE
Chinook (King) Salmon .600 volts
Coho (Silver) Salmon .650 volts
Sockeye Salmon .750 volts
Kokanee Salmon .650 volts
Halibut .450 volts *
Laketrout (Mackinaw) .650 volts
Rainbow & Brown Trout .650 volts
Cutthroat Trout .650 volts
Black Bass .750 volts *
Sharks .400 volts *
Striped Bass .650 volts *
Sturgeon .500 volts *
Catfish .500 volts *
* Further research may change this recommendation.
This information is from Pro-Troll who make a black box.
The Pro-Troll Quick Boat Check Procedure.
1. Use a voltmeter that has a DC scale that will read zero to one volt. If you already have the Cabela's Black Box you can use it as your voltmeter in the natural voltage mode.
2. With the boat in the water, lower a downrigger wire into the water five or six 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 wire.
3. Turn off everything electrical on the boat. Turn off the master battery switches if you have them. Then connect the negative lead from your volt meter to the negative battery terminal, the engine or to some other grounded metal on the boat. Touch the positive lead to your downrigger wire near the spool or along the arm. 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).
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. These are quite common in battery switches and accessories like bilge pump connections where a slight amount of positive electricity can leak into the water in the bilge.
If your boat fails test 3 or 4, you are probably repelling fish rather than attracting them. You need to find the problem on the boat and clean it up.
Initial things to check if your readings are not normal
If your readings are low (below .500) most of the time the problem is either your zinc not making good contact with the water or your downrigger cables are not making good contact with the water. Boats that are kept on trailers out of the water, often get oxidation on the zinc and it will become covered with a white powder. This insulates the zinc from the water and causes a low voltage reading. The solution is to clean the zinc with a stainless steel brush.
Downrigger wires also get corrosion and don't make good contact with the water. The older the downrigger wire, the more likely it is covered with scum or corrosion. It will give you a low reading even with a good zinc. For testing purposes you can scrape or clean a section of the wire with steel wool. If the wire has broken strands it's probably time to replace it. Otherwise the best answer is to install a Black Box which will bring the voltage back where you want it. Different downriggers on the same boat will frequently show different natural voltage readings because the wire on one will be older or more corroded than the other. The Black Box will correct the voltage on up to six downriggers.
Boat bonding could be a problem. Inspect the inside of the hull. If the boat is fiber glass 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, thru 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 its below .010 volts, check the same connection with an ohm meter to insure 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 your outboard engine is an electric start, it is automatically grounded and nothing further needs to be done. If it is not an electric start it must be grounded (bonded) to the boat ground system. This can be done 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.
I do keep the natural boat voltage at 0.65 -0.72 with zincs added to my tabs.
