Galvanic corrosion on saildrive

[sinnerman]

Stray underwater electrical current can also pass through a boat, entering one underwater fitting and exiting another on its path from source to ground. It will take the easiest path possible and if your boat provides a "shortcut" for the current, it will take it...

If there are no boats docked in nearby slips, then it may be the marina's dock power leaking, however good luck arguing that one...

BTW, AC current will do more damage than DC current ...

[DougR]

Originally Posted by guyrj33

How would you be able to keep the sail drive isolated? Isn't it connected to the engine which in turn is connected to the starter motor and battery?

In years past Volvo used isolated electrical systems on saildrive engines, where the engine electrical systems were two pole systems with no electrical connection to the block.

Today the engines have single pole electrical systems, but the sail drive is bolted to the engine with isolating washers, sleeves and gaskets and the torsional damper is constructed such that the drive side and driven side are electrically isolated.

[boatpoker]

Originally Posted by boat driver

Another vessel had a bonded system that the bonding was corroded from the zinc- and the saildrive was the next sacrifice.

That is simply not possible as the bonding conductor is more cathodic than the anode.

[andrewparker]

Originally Posted by guyrj33

How would you be able to keep the sail drive isolated? Isn't it connected to the engine which in turn is connected to the starter motor and battery?

I believe the saildrive is mostly (but not completely?) isolated from the engine with the clever use of insulators in the drive train and non conductive washers underneath connecting bolts.

Via this episode from Mads at Sail Life.

https://www.youtube.com/watch?v=zD9ySbgtku4&t=20s

[boatpoker]

Originally Posted by FabioC

There is a school of thought that does not recommend grounding and is aligned with your position. In fact, some production boats (e.g., Jeanneau's) are not grounded.

That is only true if fitted with Volvo's. Yanmar will void warranty for corrosion issues if not bonded.

[Scaramanga F25]

Nature of the beast: saildrives.

[boatpoker]

Originally Posted by sinnerman

BTW, AC current will do more damage than DC current ...

As a retired Certified Marine Corrosion tech . I say TOTAL BUNK !

[guyrj33]

Are there any sail drives that are made out of something other than aluminum?

[Thumbs Up]

I don't think that anyone has mentioned copper antifoul as the possible culprit.

[goboatingnow]

Originally Posted by guyrj33

How would you be able to keep the sail drive isolated? Isn't it connected to the engine which in turn is connected to the starter motor and battery?

Volvo ones are isolated from the engine

[Thumbs Up]

Volvo sail drives are isolated. You can check with an ohm meter probing the engine and the sail drive housing. Sometimes the shift cable or bracket can become grounded. If this is the case, it should be corrected.

[boatpoker]

Originally Posted by Thumbs Up

I don't think that anyone has mentioned copper antifoul as the possible culprit.

Many anti-fouling paints contain metals cathodic to aluminum (carbon, cuprous oxide, titanium dioxide). This is only an issue if the paint is in contact with the drive and the bonding system (if the leg is bonded).

[Thumbs Up]

volvopenta recommends a copper free antifoul. If you use copper, you at least need epoxy primer under it. It would be good to see some photos of this corrosion.

[Sailmonkey]

Originally Posted by sinnerman

BTW, AC current will do more damage than DC current ...

Ummm....no. Please show a citation supporting your belief.

[thunderhoof]

The manual for my Volvo/Perkins MD2030 makes it as clear as possible that the saildrive must be electrically isolated from the boat grounding system. A simple test is to use an ohmmeter between engine and saildrive. It should read a high resistance. I am not sure how high it will read when the boat is in the water because there is a salt water conductive connection between the two. When the boat is out of the water and washed clean, there should be close to an open circuit.
My guess is that in the water the nice clean prop is an excellent discharge path, especially on shore power, compared to the to the painted keel and other fittings.
Actually, I expect the prop blades would be rapidly damaged if the saildrive were the main leakage path into the water.

The question I cannot answer is how a saildrive is different from the engine/transmission/ shaft drive system. In both cases there is current in the water. The only difference I see is that a shaft zinc is easy to change. -any ideas?