Bénéteau 36.7: Experiencing with current flows

The issue on the boat:

There is current flowing between the mast and the keel. A little bit of current might be expected: there are sacrificial anodes on the boat, they create a galvanic cell thus a current. However our saildrive is not supposed to be connected electrically to the ground, see this article. So this current is not coming from a galvanic cell on board, it's probably a current leak from one of the wires going up inside the mast.

All the lights on the mast as well as the instruments and the radio are functioning correctly. We checked all the cables and made sure none was electrically connected to the mast. Most of them were not except the shield of the VHF antenna. This coaxial cable shield is connected to the connector, which is connected to a metallic bracket fastened with rivets at the top of the mast: Here is the electrical path, the cable of the VHF antenna.

We measure the current flowing through the keel from the lightning protection cable, this is the only electrical link to our keel. So we placed a amp meter in series with that cable:

The picture above shows a very small current flowing: 3 µA or 0.000003A which is almost nothing, we don't have to worry about a lot of corrosion here. The issue is when we plugged the shore power to the boat, that value jumped to 300-2000µA. Here some concerns can be raised about corrosion as the cord is usually plugged for very long hours.

As the only electrical path identified before is the VHF antenna shielding or ground, we disconnected the antenna: 

When the antenna is disconnected we get very low current: 0.5µA or 0.0000005A: close to nothing.

So to summarize:

• Boat in the water, VHF antenna connected, shore power disconnected: very little current.
• Boat in the water, VHF antenna connected, shore power connected: non negligible current.
• Boat in the water, VHF antenna disconnected, shore power connected: close to nothing.

 So apparently two majors actors in the equation: The shore power and the VHF antenna.
We decided to carry on, keep the VHF antenna connected, let the shore power cord plugged on the boat, and trip open all the circuit breakers, one is in the lazarette, one by the nav station.

 The circuit breaker of the lazarette disconnects two of the tree wires in the cord from the electrical system on board. It disconnects the hot and neutral wires. The ground (used as a safety) stays connected. We measure again the currents and got the same results.

• Boat in the water, VHF antenna connected, shore power disconnected: very little current.
• Boat in the water, VHF antenna connected, shore power connected: non negligible current.
• Boat in the water, VHF antenna disconnected, shore power connected: close to nothing.

Conclusion, we have a grounding issue with the shore. What can be tested: disconnects every power equipment on board one at a time and measure. If there is no difference in the measurements the component we disconnected is not likely to be the cause of our problems. So we started by disconnecting the battery charger:

On the picture above you can see the hot and neutral wires disconnected and taped (the tape prevent accidental shorts and sparks if you forget about it or power up the system again) You can also see on the picture above that the ground is still connected, it should be disconnected (I took the picture at a bad time).

After measurements no changes, so the charger is not likely to be the cause of our issues. We then try to disconnect the ground from the water heater. Besides the fact that we never use it, the water heater is connected to the shore power in case of the crew needs to take warm showers while staying on dock :)

The picture above shows the ground cable (green) disconnected from the water heater, et voila ! The measurements are not fluctuating anymore weather or not the shore cable is connected. I don't say we fixed all the issues, but we isolated one of the major contributors to our current flow.

As we never use the water heater and it was obviously a source of problems we chose to disconnect it completely from the power panel in the nav station. The green wires all disconnected are the grounds of the on-board equipments: Heater, charger, outlets.

Note: Be sure that nothing is connected to the outlets while making that test, this might be a source of problems too on your boat if one of the appliances is leaking current.

The the power panel the heater in completely disconnected and the poles are taped, this reduced a lot the current flowing through the keel while connected to shore power.

Conclusion:

• We had currents flowing through the ground of the shore power, then thought the heater and somehow back to the electrical ground of the VHF and finally the keel. This electrical path is now broken open and no current can flow.
• Our approach is maybe a little bit radical, we might test if a diode based galvanic isolator on the shore ground would solve this issue or not.
• How is the current flowing from the heater to the ground of our VHF? There are no links between the 12V electric system and the heater, and the battery charger was disconnected... Don suggested that the water heater might have been damaged a year ago when the engine overheated. Something might have been damaged inside the water heater or corrosion might have developed and offer a path for the current to flow. The current might flow then though the engine coolant to the engine, then as the engine is connected to the 12V ground this causes an issue with the keel which was connected to the negative pole of the batteries though the VHF antenna.
• We also plan to isolate the bracket holding the VHF antenna from the mast. There is no need for that, and it cannot hurts to do it. The good of that is that we'll break another electrical path for currents to flow and prevent further damage.

Be careful while opening all these electrical boxes and playing with shore power, this might hurt or kill you...

Please also use a meter when trying to find issues concerning galvanic corrosion on a boat. All the legends that you hear on dock, at the yacht club or in the yard are most of the time based on other sailboat wiring designs and might not apply in your case. Just use a meter and isolate the issues, this is your best bet!

BoatUS provides also a nice article on issues from shore power. Basically what we did above was trying to disconnect the shore ground from the conductive elements of the boat in the water.

Thanks Don for providing the boat and the camera!

Bénéteau 36.7 Saildrive electrical tests

The Volvo Penta saildrive was designed to be electrically isolated from the ground of the electrical

Figure 1: Volvo Penta engine and saildrive assembly

system onboard. From the care and maintenance, Volvo states: “Volvo Penta sail drives are designed to provide the sail boat owner with many years of trouble free boating enjoyment.There are two main components to the galvanic protection system on your Volvo Penta sail drive. First off, the entire
sail drive is electrically isolated from the engine and engine grounding system. This isolation is accomplished by the use of plastic bushings, gaskets and washers in the sail drive mounting hardware. Never compromise this protection by placing any type of ship’s ground wire on the sail drive housing or mounting bolts. Don’t use wire wound hose in the water pickup system, and don’t use the sail drive as an SSB ground plane.“

Why? Because if the saildrive is grounded or connected to the electrical system, there is a risk that current is going to flow through it and damage parts of it (internal or most likely the prop). Now there are many zinc or magnesium sacrificial anodes to protect against galvanic corrosion, but this will not help if another component of the boat, i.e. the battery charger is leaking current to the ground (imposed current). The best way to avoid galvanic corrosion is to cut out paths for the electric current to flow! As it can be seen on the Figure 1 above, the green parts are the engine; the gray ones are the saildrive. By using a simple digital meter, measure the resistance between these two parts, the meter should display -1 or infinity. If not you have a problem, current is allowed to flow from the grounding though your saildrive…

What to check:

• No electrical cable linked between the saildrive and anything else. This is the easier point to check, just open all the panels around your engine and visually check that no electrical cable is attached to your saildrive. If there is a cable then simply remove it.

• No metallic hose for the water intake, the water intake hose should be an industrial rubber hose which does not conduct electricity. The ones with a metallic sleeve around might conduct electricity.

Figure 2: Gearbox cable end to the saildrive:

Cable that can be electrically connected to the saildrive. 

But should stay isolated from the engine ground.

• Throttle cable should not be grounded. The original cable I've seen was secured to the gear selector bracket by a cutter pin. This method of fastening the cable to the bracket does not provide electrical isolation. As it can be seen on Figure 2. The red sleeved cable is attached to a bracket which is how gears are selected. By using a standard ohms meter, you should measure a resistance of 0 Ohms between the cable and the saildrive: The gearbox cable is electrically connected to the saildrive, thus might provide a path for electricity. This is why the other cable coming from the engine control handle: the throttle cable is designed to be electrically isolated from the engine. If the throttle cable is not isolated, you provide a path for the electricity from your engine, then though the throttle cable, through the handle, and a return to the saildrive thought the gearbox cable. We’ll see in the next point how to check for that.

Note: On Figure 2, a green/yellow grounding cable can be seen; this is to ground the gas tank gauge sensor to the ground of the system. The sensor needs a ground to operate so this is not a bad option for doing it. But again this cable should not touch the saildrive. 

Figure 3: Throttle cable end and the engine:

The black plastic outer casing holder, make sure no 

conduction is possible between the cable and the two screws, 

this is an isolating holder. 

At the end of the cable, isolating washers/sleeves make 

sure no conduction is possible between the cable 

end and the engine ground. 

• Test that the throttle cable is isolated from the engine ground: In order to do that, use an Ohms meter again and check that there is no conductivity between the ground of the engine (or batteries ground) and the cable itself. As it can be seen on the Figure 3, the throttle cable is isolated from the engine in two ways. First the cable holder screwed to the engine is made of plastic. This prevents conductivity from the engine to the shell of the cable. The tip of the cable is isolated from the throttle bracket by two plastic washers/sleeves. Something that was wrong on the boat I’m sailing on is that the two screws where holding the cable by a metallic plate. Therefore offering a path for electricity thought the plate and the screws to the cable.

Measuring Points:

Note: We do not care about the polarity here; black/red cables can be inverted. We want to make sure that no current can flow which means that there should be an infinite resistance between the difference components.

If everything is well isolated you should not measure any resistance between the saildrive and the engine. If you measure some, check all the protective sleeves of the cables. Try to detach one by one the cable until you break the path for electricity to isolate the issue. Make sure no conductive elements (hose, screws, cables) are accidently touching the saildrive. If nothing can be found maybe the saildrive isolation gasket is damaged and should be check by a Volvo specialist.

Thanks Don for the providing the boat and the camera!