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!