What is this for?

[hargsnz]

On my 366 I have 4 of these chrome fittings set into the Aluminium radar arch & hard top. They are all reacting with the Alu & it does not look like they actually do anything? I need to remove them & repair the alu / paint underneath. Obviously there was an initial purpose - but now?

[Phrancus]

great, always like these questions

Looks like a place to attach a hook. Such as on the end of a rope or elastic band, for a cover or something like that. Perhaps a bigger overview showing the locations relative to each other helps to guess their purpose.

[hargsnz]

Well yes, they are at each end of the shelf that goes across the inside of the arch & each side of the arch itself. But as dis-similar metals they are eating each other I think.

[Midnightsun]

Those are lifting lugs. You attach to this via rope/slings to assemble/disassemble the boat. You cannot see them in the picture but the ropes going under the arch are attached to exactly the fitting you have in your picture.

[Viper]

Ya they're lifting rings. Good pic Hans.

The fixture itself is probably isolated from the aluminum because of the caulking, it's the ss screws that are causing the problem as they are in direct contact with the aluminum. When you install them again, use Tef-Gel or other anti-corrosion product on the threads. These will prevent the galvanic corrosion associated with dissimilar metals in contact with each other.

[hargsnz]

OK thanks all for the replies. In the meantime given they serve no current putpose, I think I'll just take them off until I can get to doing the repair & respray. Really made a mess of that shelf before I really noticed it under the towels & foam noodles that live up there.

[Midnightsun]

Source of the post OK thanks all for the replies. In the meantime given they serve no current putpose, I think I'll just take them off until I can get to doing the repair & respray. Really made a mess of that shelf before I really noticed it under the towels & foam noodles that live up there.

If you are in salt water this is probably the reason. For electrolysis to take place there needs to be an electrolyte present. Common water can do it but salt water is as evil as it gets. Damp or wet objects on those parts would cause this reaction to take place. Really does not help that we are talking stainless steel to Aluminum contact, both of those are at the extreme of the metals scale for reactivity to electrolysis.

Mercruiser had huge problems with the duoprop drives in the past, hanging dual stainless props off of an aluminum drive, mixing water and electricity was a disaster. Took them a few years before they figured out how to control this but there were many unlucky customers.

[Viper]

Source of the post.....Mercruiser had huge problems with the duoprop drives in the past,....Took them a few years before they figured out how to control this......

I think you mean "have" huge problems. Nothing's been figured out to control it really, you can take measures to try and minimize the effects but you can't completely get around the phenomenon. This goes for Volvo drives too by the way, they are just as bad, and big outboards of all makes. The introduction of electronic active protection systems such as Mercathode which is standard on Bravo drives helps, but it's still a huge problem and is an ongoing maintenance concern, even in fresh water applications.

[Midnightsun]

Thanks for the update, thought they had figured this out.

I had a Bravo 2 drive on a previous boat and figured it would be beneficial/cool to swap from an aluminum prop to Stainless, the experience taught me something important. After just 1/2 a season there was enough corrosion going on that I sold the prop and went back to aluminum. Sanded and painted the drive and no more corrosion ever showed up.

Fuel flow figures by the way showed no improvement in economy or speed with the stainless prop and being so darn heavy there was a clunk when engaged into gear which I am sure would have caused premature wear somewhere in the drive. The aluminum prop in comparison was a butter smooth shift into gear. Stainless is not all what it is hyped up to be and certainly has its disadvantages.

[Viper]

Pros and cons to both types. Way more flex on the aluminum prop than the ss, as such ss props are a must for a lot of applications. You can't even get aluminum props for a Bravo III, newer Volvo DP, and large outboards. Even with aluminum props on a Bravo I or II, the amount of ss mass of the prop shaft in the water (electrolyte) is enough to cause a problem for the aluminum casing. Keep the casing's coating intact and replace the anodes every year.