With a multimeter and a reference electrode, you can measure the potential difference between the boat’s underwater conductive surfaces and the boat’s ground (negative battery terminals). You can check the conductivity of every branch circuit and also the effectiveness of your bonding system. Conduct these tests after the boat has been in its slip for several hours to allow the cathode system and sacrificial anodes to polarize the water molecules in direct contact with the underwater surfaces.
You will need a multimeter connected to a reference electrode immersed about six inches behind the conductive surface. The reference electrode should have a cable length of about 25 ft to be able to reach the water from all of your metal components and be close enough to the meter to write down the voltage readings from the various circuits. You will also need a heavy-gauge copper wire ground electrode—10- or 12-gauge—with an alligator clip at one end and a jack plug on the other end to fit the black socket on your multimeter. A record book and a helper makes the testing and troubleshooting process much easier.
What To Test
Make a simple drawing or write up a complete list of the metal underwater fittings. Include through-hulls, prop shafts, and anything that is part of the boat’s electrical bonding circuit. Include the stern drives, trim tabs and any other metal part that is normally in the water. Mark the location of every sacrificial anode on your drawing.
Branch Circuit Testing Procedure
Connect the reference electrode to the positive terminal of the multimeter. Drop the silver reference cell over the side and wait a few minutes for the seawater to soak into the porous housing. Connect the multimeter’s negative jack to the boat’s negative battery terminal using the copper wire ground electrode.
Switch all DC breakers off. The DC voltage potential you measure should be around 0.7 to 0.8 VDC Turn on each circuit breaker in turn, connecting the alligator clip to the circuit’s ground. If potential in any circuit reads more than 1VDC, you have internal stray DC current flow in that branch. This circuit should be investigated and corrected to prevent DC stray current corrosion to the underwater conductive metals.
Bonding System Testing Procedure
Connect the ground cable alligator clip to the metal fitting nearest to a sacrificial anode. Take a reading and begin to work your away from the anode, following the bonding circuit to the next fitting. Record each reading on your drawing. If your boat’s metal through-hull fittings are not bonded, you will need to touch each fitting; even with a properly grounded system, you should do this to detect any breaks in the bonding system.
Interpreting Your Readings
To help interpret the readings for various metal alloys, consult the generalized list of mV readings.
Generally Accepted Potential Range For Various Metals
Bronze: -500 mV to -700mV
Steel: -750 mV to -850 mV
Aluminum: -800 mV to -1,050 mV
If you do not obtain the same reading for a fitting in the same bonding circuit, you have an interrupted ground, and that fitting is in peril. If you obtain a number that is less than the acceptable range for the metal—for example a bronze seacock gives you a reading of -400 mV—then the item being tested is eroding. The smaller the number, the more the erosion that has taken place.
Protect Your Investment
This concludes our series on corrosion. If you as a result have obtained a working understanding of the boat electrical system and the process of corrosion, you will be able to detect this destructive process sooner, and correct problems before expensive damage can occur. This knowledge can also help you evaluate the condition of a boat before purchase. Corrosion is a condition that should be continuously monitored, since damage can be caused by shore power problems or by nearby boats. The implications involve not only maintenance, but also safety.
Marine Electrical Systems, AC/DC-Electrical Circuits, Grounding, Lighting, & Safety; United States Power Squadrons; 2008.
Does marine corrosion seem complicated to you? Are you not sure where to start in applying best practices to your boat? United States Power Squadrons has a solution for you. Marine Electrical Systems is an elective course dedicated to onboard electrical power–-both AC and DC. You will learn about electrical circuits, wiring, terminating, and routing wiring for safety and trouble-free operation. You will learn how to use multimeters and tools to check, diagnose and repair electrical problems. You also will learn about galvanic corrosion, stray currents, isolation, grounding, and safety of electrical systems.
The course material is organized in seven chapters and is intended to be presented in the classroom over nine two-hour sessions:
Day 1 – Introduction, Chapter 1: Properties of Electricity
Day 2 – Chapter 2: Boat Electrical Wiring Practices
Day 3 – Chapter 3: Direct Current Power
Day 4 – Chapter 4: Alternating Current Power
Day 5 – Chapter 5: Galvanic and Stray Current Corrosion
Day 6 – Chapter 6: Lightning Protection
Day 7 – Chapter 7: Troubleshooting Part 1; on-the-boat optional
Day 8 – Chapter 7: Troubleshooting Part 2
Day 9 – Review and Examination
This course is the best way to learn about electrical factors that can save you from sustaining considerable damage, especially if you berth your boat at a marina, or if you make any repairs or modifications to your OEM electrical system.
If you are interested in taking this class, indicate your interest using the contact page.