Volvo Sail Drive sudden rapid galvanic attack damage and management question

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boaterbob

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Feb 17, 2021
17
Bavaria 41 Olympia
2004 Bavaria 41 with Volvo D255 and sail drive. Problem free for 10 years of saltwater ownership. Put on Grid May 19th to inspect bottom. Minimal sail drive growth and zinc appeared well connected to sail drive. Had trouble replacing bow thruster zinc so left it bare. Bow thruster has no 12v except when relay delivers 12 volts. Tried bow thruster June 2 and nothing but humming motor, no thrust. Back on grid June 3. Bow thruster visualized, support dissolved and prop lying in tunnel. Sail drive white with Al oxide corrosion. Pitting of metal notable but may be smoothed by sanding or grinding.

So what would you do? What might explain aggressive attack in two weeks in slip??. Boat tested by two engineers with silver chloride reference. All voltages ok. No explanation. Galvanic isolator installed in 2016. No ground wire current. Is Sail drive worth grinding? Insurance Geico has no mention of galvanic damage sudden or not in policy review.
What ideas here on sudden attack ( now all fine) and choice of how to handle sail drive damage?
BoaterBob
 

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Davidasailor26

Davidasailor26

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May 17, 2004
6,110
Beneteau Oceanis 37 Havre de Grace
Significant corrosion that fast is usually associated with DC faults of some sort, reducing the chance that the problem is with the galvanic isolator or nearby boats. Is it possible there was some DC circuit on for some of that time period that wasn’t running when the silver chloride tests were done?

With the thruster being damaged so much that the prop fell off I’d look carefully at that as the possible source. I know you said it has no 12V unless the relay is delivering it, but my guess is there are two circuits - one for a controller and one for the motor itself, and the relay only turns one off. The other could still be energized and leaking 12V through the leg into the water, back to ground via a thru hull or the keel.
 
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Don S/V ILLusion

Don S/V ILLusion

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Sep 25, 2008
7,668
Alden 50 Sarasota, Florida
As stated above, evidence suggests there is/was an active circuit, possibly a totally different piece of equipment which shares a common ground loop through which the erosion occurred.
 
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D

dlochner

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Jan 11, 2014
13,951
Sabre 362 113 Fair Haven, NY
As others have said, rapid corrosion is usually a DC fault somewhere. The bow thruster is a "person of interest" as the damage there was greatest and the thruster was not working.

The damage to the sail drive unit is probably related. Without photos it is difficult to assess the damage and even with photos it is best to have a qualified technician inspect the unit.

Read your insurance policy. It may contain an exclusion damage due to maintenance issues, which they might assert in order to deny any claim.
 
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boaterbob

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Feb 17, 2021
17
Bavaria 41 Olympia
Helpful comments. Measured voltage at bow thruster as 1 volt. Not measured activated by helm control, however. Assume it will be 12 volts. Posted pictures. Cleaning up existing saildrive would save me $15K (new). Contractor against that. Lost saildrive material should not cause structural problem with saildrive. Grind and smooth and epoxy seal? Replacing lower unit of bowthruster necessary. Cause remains unproven.
 
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dlochner

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Jan 11, 2014
13,951
Sabre 362 113 Fair Haven, NY
There are 2 factors to consider on the sail drive. How much material is left, that is, it is not the surface area affected, it is how deep is the pitting. And if the stray current issue is solved.

Unless the sail drive is contained within a watertight compartment, the boat will flood and sink in quick order if the corrosion penetrates the drive. An ultrasound test could determine how deep the corrosion is, but money spent on that might be better spent on replacing the sail drive. To keep the CFO happy, it might make sense to forgo repairing the bow thruster and replace the drive.
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
23,308
Catalina 34 224 Maple Bay, BC, Canada
Bob, I'm sorry to hear about your issues. A colleague trucked his boat from Georgia to Florida. Unbeknownst to him the ground wire to a battery he had installed under his aft cabin berth fell off during transit, ended up lying on one of the bolts to his strut. Much dissolution ensued.

Intermittent problems, especially electrical, are often difficult, if not impossible, to specifically identify, other than "Oh, it's gotta be a DC power issue." Don and Dave are right. I see you've added photos to your OP. Given the appearance of the metal, I would investigate much further with a different attitude; instead of one of trying to save as much as possible change to replace as much as possible, if not everything, IF you are planning to keep the boat long term. If not, different choices.

ITWMB, as part of the electrical investigation, I'd also replace the relay(s).

Good luck.
 
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B

boaterbob

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Feb 17, 2021
17
Bavaria 41 Olympia
dlochner said:
There are 2 factors to consider on the sail drive. How much material is left, that is, it is not the surface area affected, it is how deep is the pitting. And if the stray current issue is solved.

Unless the sail drive is contained within a watertight compartment, the boat will flood and sink in quick order if the corrosion penetrates the drive. An ultrasound test could determine how deep the corrosion is, but money spent on that might be better spent on replacing the sail drive. To keep the CFO happy, it might make sense to forgo repairing the bow thruster and replace the drive.
Click to expand
Helpful, thoughtful comment about saildrive. I think I will keep the boat and replace.
 
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colemj

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Jul 13, 2004
1,036
Dolphin Catamaran Dolphin 460 Mystic, CT
dlochner said:
Unless the sail drive is contained within a watertight compartment, the boat will flood and sink in quick order if the corrosion penetrates the drive.
Click to expand
This isn't true. The entire saildrive leg could be snapped off and no water would enter the boat.

Mark
 
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colemj

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Jul 13, 2004
1,036
Dolphin Catamaran Dolphin 460 Mystic, CT
I just saw the pics. It is difficult to make any estimates of how much damage there is until you remove all of the aluminum oxide scale. Aluminum "rusts" like steel - where a lot of "rust" scale can be produced by a small amount oxidized metal.

But as others have said, saildrive corrosion issues are pretty much 100% caused by DC electrical issues on the boat itself.

Mark
 
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dlochner

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Jan 11, 2014
13,951
Sabre 362 113 Fair Haven, NY
colemj said:
This isn't true. The entire saildrive leg could be snapped off and no water would enter the boat.

Mark
Click to expand
If the cooling water passages corrode above the boot, then water could enter. Or the corrosion around the shaft where the boot meets the shaft could cause a leak. Low probability events for certain. However, with stray current corrosion possible.
 
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colemj

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Jul 13, 2004
1,036
Dolphin Catamaran Dolphin 460 Mystic, CT
dlochner said:
If the cooling water passages corrode above the boot, then water could enter. Or the corrosion around the shaft where the boot meets the shaft could cause a leak. Low probability events for certain. However, with stray current corrosion possible.
Click to expand
Still can't see how that could happen. Above the boot is the sealed upper half of the drive, which is pretty much electrically isolated from the lower half with the boot, a gasket, and locktite on bolts. There are no cooling passages through this part of the gear case. The cooling passage (which isn't really a cooling passage, but a water intake passage) ends at the valve for providing water to the engine cooling.

Maybe it might be possible to imagine the area containing this valve corroding off, but it is a very thick piece of aluminum, with only a tenuous (if at all) electrical connection to the lower half, and I'd guess that the probability would be so low as to completely discount it entirely.

The boot is above all of this, so no corrosion possible above it, with the exception of the immediate area around the valve, as above.

Mark
 
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colemj

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Jul 13, 2004
1,036
Dolphin Catamaran Dolphin 460 Mystic, CT
The Volvo D2-55 engine should be, by design, electrically isolated from the saildrive attached to it. In order for a DC stray current to attack the drive, there needs to be an electrical path to ground through it. Volvo intentionally isolates the drive to prevent this. You should check to make sure the engine is still electrically isolated from the drive.

Some culprits might be the plastic isolators and washers on the shift bracket are missing, you have metal reinforced cooling hose with metal worn through to the seacock, or you had engine/drive work done in the past where the isolation gaskets and bolt spacers were not installed.

It is curious that your anodes galvanically protecting the leg show little wear. You need to clean that scale off the leg to make any determination.

Mark
 
D

Dave

Forum Admin, Gen II
Staff member
Feb 1, 2023
126
colemj said:
Still can't see how that could happen. Above the boot is the sealed upper half of the drive, which is pretty much electrically isolated from the lower half with the boot, a gasket, and locktite on bolts. There are no cooling passages through this part of the gear case. The cooling passage (which isn't really a cooling passage, but a water intake passage) ends at the valve for providing water to the engine cooling.

Maybe it might be possible to imagine the area containing this valve corroding off, but it is a very thick piece of aluminum, with only a tenuous (if at all) electrical connection to the lower half, and I'd guess that the probability would be so low as to completely discount it entirely.

The boot is above all of this, so no corrosion possible above it, with the exception of the immediate area around the valve, as above.

Mark
Click to expand
Thanks for the explanation. While the design seems sound there is room for human error. A phenomena I am all too painfully aware of when it comes to dealing with "marine techs." Nonetheless, a thorough examination of the sail drive is warranted to assess its integrity.
 
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