Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

Let’s assume the reading you get is within

the prescribed range. You’re not done yet:

1 With your meter still connected and the

reference electrode still submerged,

remove the electrical connections from

the boat to the dock one at a time and

watch for a change in the meter readings.

2 If the reading gets more negative, then

there was leakage from the boat to the

dock through the shore cord you just dis-

connected. The subject boat may be help-

ing to protect other boats at the dock with

its anodes.

3 If the reading gets less negative when dis-

connected, there may be a DC stray-

current problem on board the boat.

Further checking will confirm or deny

this possibility.

4 If there is no change in the reading with

the boat plugged in versus not plugged in,

two possibilities exist:

• The most likely reason is the boat has
  an isolation transformer or galvanic iso-
  lator that is doing its job.


• There is a no-ground situation at the
  dock—a potentially deadly situation. It
  is essential that you check the integrity
  of the dock ground with the SureTest
  tool (Chapter 4).
  Next, let’s test the integrity of the bonding
  system on board the boat. We’ll use the
  DVOM to check the hull potentials:

1 First, perform this test with the engine(s)

off.

2 Find all the metal objects in the bilge that

are attached to the bonding system.

3 With your reference electrode still in

the water near the boat, use your

positive meter lead and check the

112 electrical systems troubleshooting

Checking potential at a seacock (touch the lead to any

metal part). The meter reading is within 1 mV of the over-

all hull potential shown in the bottom photo on page 111,

indicating that all is well.

potential at each bonded metal fitting

one by one. The readings should be the

same, or within about 50 mV of the

basic hull potential; otherwise, the

possibilities are:

• If a reading is more positive than the
  basic hull potential, a poor connection
  in that leg of the circuit is indicated, and
  it should be tracked down and repaired.


• If a reading is dramatically more posi-
  tive or negative than the basic reading,
  it may indicate DC stray current,
  which should be remedied.

4 Now repeat the test with the engines run-

ning. Dramatic changes in the readings

may indicate DC stray-current problems

induced by something electrical that is run-

ning on the engine, typically an alternator.