CHECK YOUR SWIM PLATFORM BACKING PLATE MOUNTINGS

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.


Here's a closeup:


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:







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.



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



and the solution...

(photo is sideways)


and the solution.

My background is twin 30 amp 120 volt systems. Never dealt with 50 AMP 220 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

Interesting. I will need to check mine out

A couple of things could be in play. As these backing plates were not bonded to the boat if your boat was leaking current then it could eat itself. Unlikely however... More likely is that boat was slipped next to another boat that had an electrical problem and that boat was eating your boat. That happened to me; take a look at some of my posts in the 500 520DB forum section. As your boat doesn't have an isolation transformer it then has one or two galvanic isolators which are essentially a type of diodes that can fail over time.
Regardless, the aluminum plates were not bonded and as they are one of the less Nobel metals they will be the first to go from a corrosion aspect. The Garolite is probably a good answer to replace but I would probably use a 316 Stainless plate; the Garolite does flex but I think it will be fine provided it is at least 1/2 inch thick.
Those bolts and washers that SR uses are 316 stainless steel; the best for corrosion but I've seen in exact same applications stress corrosion conditions in the bolting so make darn sure the entire bolt except for the head is completely sealed and salt water cannot get to the shank or threads. Bed that backing plate in 5200 and the bolt/washer also in 5200...
As with any boat one of the first things to do is completely go through the bonding system and get it back in repair then look at the galvanic isolation then look at the AC power to make sure the neutral is terminated and isolated when on shore power then bonded to the ground/bonding system when on generator power..
A very common mistake when people add inverters is incorrectly arranging the neutral AC wiring and the boat becomes both an electrical hazard and corrosion issues become apparent.

I suggest testing your existing isolators if they exist, that is a long time in service. Replace or add as needed. Second the above suggestion to inspect your bonding system and make repairs.
Also, you can clamp a high quality ammeter on your shore cord and verify it reads exactly zero (all current in = all current out) and your boat is not the leakage point. I actually tested all the cords around me in my marina.

I'm just learning the 450 DA as compared to my 10 Meter.

Those bolts/backing plates have zero corrosion inside the hull and are above the water line. They are tied into NOTHING.

She's still out of the water this week while undergoing rudder repacking, new transducers and numerous other items.

Appears all underwater metals are tied into the bonding system, (except where the connection is broken or obviously dirty)

As compared to my 10 Meter there are two bronze plates running fore/aft on the bottom side of the hull.

I don't understand what/how these contribute to the bonding system as there is no zinc anode attached.

Is there a factory galvanic isolator on these vessels?

BEST !

RWS

Why not tie each of these bolts into the bonding system? You. Old leap from bolt to bolt in the inside and have the final one terminate at your bonding block. Just a thought.

I don't have anything different to add to all of the advice already given, but another possibility for the cause is that it appears the type of bottom paint being used has copper in it. Painted right up to the aluminum plates is yet another disimilar metal (see TT's post about nobility) in contact with the Al. Of course this area will get splashed by salt water, but even just the air is considered an electrolytic solution - salt air being worse than fresh water "air".

Maybe A Dancer said:

I'm just learning the 450 DA as compared to my 10 Meter.

Those bolts/backing plates have zero corrosion inside the hull and are above the water line. They are tied into NOTHING.

She's still out of the water this week while undergoing rudder repacking, new transducers and numerous other items.

Appears all underwater metals are tied into the bonding system, (except where the connection is broken or obviously dirty)

As compared to my 10 Meter there are two bronze plates running fore/aft on the bottom side of the hull.

I don't understand what/how these contribute to the bonding system as there is no zinc anode attached.

Is there a factory galvanic isolator on these vessels?

BEST !

RWS

Click to expand...

Those bronze plates are the ground reference and lightning protection. Do not paint those; clean them to bright metal before splashing. I wouldn't worry too much about bonding the swim platform bolts; just make sure they are sealed. As a side note - replace the bonding cables / connectors to the rudders; they always fail due to having to rotate with the rudders.