CHECK YOUR SWIM PLATFORM BACKING PLATE MOUNTINGS

[Maybe A Dancer]

New to me 450DA w/ Factory standard swim platform.

Aluminum backing plates are used both inside and outside the hull to spread the load out on the swim platform supporting bolts.

Surveyor pointed out significant galvanic corrosion between the stainless bolts/washers and the aluminum backing plates.
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Here's a closeup:
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Surveyor recommended removing the bolts ONE AT A TIME and replacing the outside aluminum backing blocks with 1/4" GPO3 or G10 fiberglass material.

I bought this sheet on Amazon and cut the new blocks out:

Amazon.com: GPO3 Fiberglass Sheet (1/4") .250" x 24" x 36" : Industrial & Scientific

This may come in handy for other projects as well.

The inside the hull aluminum backing plates are FINE.

Had to chip away the corrosion to get to the bolt heads, which were surprisingly in PERFECT CONDITION, but take a look at these backing plates:

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This 1996 DA was a DOCK QUEEN in a large California marina.

Removal of her rudders indicated a bit of galvanic corrosion in her past.

I don't know if this is a chronic problem that comes with age & saltwater boats where y'all need to check yours, or just a specific issue with this one.

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Anyway, I thought it would benefit CSR to share the problem...

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and the solution...

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and the solution.

My background is twin 30 amp 120 volt systems. Never dealt with 50 AMP 240 volt systems.

So, moving forward...

• should these above the water line bolts be bonded?
• Should I consider a galvanic isolator?

Galvanic Isolators Explained | BoatUS

Is this more or less an isolated situation?

I understand the corrosion, but was surprised by the severity, especially with these being ABOVE THE WATER LINE.

Thoughts?

BEST !

RWS

 

[TonyA]

Nice work. You should sell some pieces of your material to recoup some $$ AND save $c for others.

 

[copb8tx]

• Should I consider a galvanic isolator?

Galvanic Isolators Explained | BoatUS

Interesting read. Thanks for the post

 

[Korkie]

Correct me if I am incorrect. But - - - - I was always told not to use SS and Aluminum in the same application as they will do just this.

 

[Maybe A Dancer]

Correct me if I am incorrect. But - - - - I was always told not to use SS and Aluminum in the same application as they will do just this.

>>>>>>>>>>>>>>>>>>

I believe the aluminum backing plates to be factory installed, as I have photos of other 450's under consideration during my search.

BEST !

RWS

 

[PlayDate]

The boat is 28 years old which means a number of things could have contributed to the corrosion.

What is the support material for the swim platform that the bolts and backing plates attach to? The surface looks painted.

The problem with older bottom paints is that they contained high amounts of copper. Copper plus stainless creates battery where ions flow from one metal to another.

In your case, the aluminum became the sacrificial anode.

This is why when painting metal structures near or under the water line…..most yards would use Tri-Lux 33 which is engineered to prevent corrosion on dissimilar metals.

Your solution is even better insofar as the aluminum goes. I’m a bit surprised that original structure was not bonded but even if it was…..I don’t think it would have prevented that corrosion.

Great pictures and write up.

 

[Njlarry]

I installed a galvanic isolator on my sailboat, a very easy job. However you may already have one. On some models they are hidden behind the master electrical panel.
I can see no reason to bond above the waterline. Some noted experts even recommend not bonding below the waterline for isolated hardware like seacocks. Others disagree. Seen it done both ways.

 

[NotHerDecision]

I replaced ours on the 460 with new stainless bolts and giant fender washers. We didn’t have the aluminum blocks but the bolt heads were really rusty. Unfortunately the 316l washers I used 3 years ago are rusting and I’m sure it’s another Chineseium 316l. Anyway I’ll probably replace the washers again at my next haul out.

Thanks for sharing your experience. I always learn something.

 

[dtfeld]

That boat should already have Galvanic Isolators installed, so you should not have to add them.

The corrosion on the rudders may indicate that the bonding system was/is not up to snuff. Make one of your jobs to go inspect/correct all the bonding wires. They get corroded, broken, detached etc. I want to say Sea Ray used green #6 wire in my 2001 410.