Galvanic corrosion on saildrive

[boatpoker]

Originally Posted by 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?

No "ideas" required. Simple science. The aluminum saildrive body is anodic to all other metals involved.

[GordMay]

As Boatpoker & Sailmonkey have indicated, the intensity of damage, by DC stray current corrosion is much greater, than that by AC corrosion.

[Afrinus]

Boat poker, can you reference Volvo not warranty-ing if the engine or saildrive is bonded.

I'm in the process of buying a new boat and the surveyor has a finding that the engine/saildrive is not bonded, and that it should.

I've pushed back, but he insists that this is normal.

Pieter

Originally Posted by boatpoker

Volvo will not warranty if the engine/saildrive is bonded.
Yanmar will not warranty if it is not bonded.

[DougR]

I think it’s important to read the fine print of both Volvo Penta and Yanmar warranty statements concerning corrosion.

Warranties are written to provide protection to the product owner against defects in materials and manufacturing, and corrosion is very infrequently considered to be such a defect. Thus, saildrive corrosion is generally not covered under product warranties, regardless of whether the saildrive is isolated or not isolated.

[thunderhoof]

My Volvo MD2030 Manual states on page 39 "3. The drive/reverse gear and flywheel cover are
electrically isolated (A) from the engine and must
never be used as a ground.
IMPORTANT! The drive/reverse gear and
flywheel cover must never be used as a
ground or be electrically connected to other
equipment such as the radio, navigational
equipment, the rudder, bathing steps etc."

[Afrinus]

Originally Posted by thunderhoof

My Volvo MD2030 Manual states on page 39 "3. The drive/reverse gear and flywheel cover are
electrically isolated (A) from the engine and must
never be used as a ground.
IMPORTANT! The drive/reverse gear and
flywheel cover must never be used as a
ground or be electrically connected to other
equipment such as the radio, navigational
equipment, the rudder, bathing steps etc."

Would this mean that the engine itself is or should be grounded?
Is your engine grounded?

[Thumbs Up]

The engine is grounded. The adaptor plate (bell housing) has a plastic gasket and the bolts use plastic tee washers. The drive plate (damper plate) is coupled with vulcanized rubber. There should be no continuity between the engine and saildrive. Volvo only.

[thunderhoof]

Afrinus:
The MD2030 manual clearly shows the engine block connected to the keel with a ground wire. The manual states that other equipment including electronics and metallic structure (mast, railings, etc.) should also be connected to the same point. I'm sure the keel is shown as a typical ground point. If it is not available some other large metallic panel in the water could be used.

Yes, my engine is grounded.

[DougR]

Yes, the engine block of the MD 2030 is grounded with the negative battery cable, as well as the engine block of the more recent D1, D2 series.

However, the saildrive of the 2030 and the D1 & D2 series engines is isolated from the engine block and therefore not grounded to the vessels 12 volt system.

[CarlF]

Most common cause of stray current corrosion is low voltage DC from your boat - often a cut in the insulation of a bilge pump float switch wire or a butt splice that's fallen in bilge water.

Go through your bilge carefully looking for wet wires. I'd replace the float switch in any case.

[bglad]

A definitive check to see if a VP saildrive is isolated requires the boat to be out of the water. 0 ohms connection to electrical system. In-water will require a silver/silver chloride half cell. The potential of the drive should be that of the zinc and aluminum somewhere around -1050. If it not then trouble is afoot.

[fstbttms]

Originally Posted by CarlF

Most common cause of stray current corrosion is low voltage DC from your boat - often a cut in the insulation of a bilge pump float switch wire...

That is precisely what caused this. The prop fell off within weeks of this pic and the owner being notified of the issue.

[vicbusiness]

https://photos.google.com/share/AF1Q...ZWSEJBZ1JNMWNB

[tamicatana]

@bglad’s above statement wrt checking drives: This is NOT, I repeat NOT, true.

You can check VP drives in water:
Put an ohmmeter from engine ground to any part of the transmission inside the boat. If you have continuity, you have a problem. My experience is that the shift linkage tends to be a culprit, provided the other isolation protocols are in place.

My saildrives are from 1992, by the way…