Cobaltchat.com

Been browsing thru Cobalt Chat for the past year to get familiar with the brand. Went for a ride last year in one and it was a really solid and nice boat vs some of the Sea-Rays and Four Winns.

Fast forward a year and I have been looking at a few used Cobalt's. One had a volvo penta duo-prop with some corrosion on the lower unit housing in front of the propeller and leading edge of the skeg. The upper portion didnt show any signs of this, just the bottom. Is this a common issue with volvo units? I have read some of the older merc-cruisers had corrosion problems.

Have a picture?

Here is a picture. Boat is on a lift.

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Hey Canterbury. So glad you found us!

That looks like galvanic corrosion due to dissimilar metals in the water. It appears from your pic that the skeg and propeller shaft housing may be the only portion of the outdrive in the water on that lift. Outdrives have anodes to "sacrifice" themselves instead of the aluminum outdrive. Once the anodes are sacrificed you just unbolt and replace them. It's cheaper than replacing an aluminum outdrive. Saltwater can really magnify this issue. In your pic the majority of the anodes on the outdrive are not in the water to keep this from happening.

The owner added an anode that is not normally there to try to combat this but again it does no good unless it's submerged. I’m curious, is this a saltwater boat?

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Normally there’s also a sacrificial anode at the base of the propeller shaft area and believe it or not a cone shaped one that can be placed at the back of the propeller that looks like a torpedo. There are quite a few other locations for anodes on the outdrive as well including a large round one on the underside of the area circled above.

How is she powered? And welcome aboard mate.

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Thank you for the reply AsLan7. This is a fresh water boat but on a lift so you are correct that the anode isnt in the water. I have only viewed pictures and will need to see this one in person to get a clearer idea if this is just surface or deeper.

The picture is kinda small so hard to tell exactly but from what I can see, that looks pretty bad.

Normally you don't have corrosion like that in fresh water unless there is an electrical problem at the dock. My boats, both Mercruiser and Volvo Penta sit in the water all summer and they have ZERO corrosion. Of course not having the anode sitting in the water doesn't help things.

I'm wondering why they didn't trim up to keep the whole unit out of the water?

I will get a better look at the issue, but what finer details should I be looking for? Obviously if there is metal degradation where there are holes and large pieces eaten away a much larger problem. If this is mostly surface, this could be smoothed, filled and repainted. Does corrosion like this typically eat into the housing behind the props so now you are dealing with seal concerns?

In the bigger picture, are the lower unit casings separate pieces like on an outboard so this could be removed and replaced? Or is the whole casing from the transom back one piece?

If really needed you can replace the lower unit only. I'm sure you could sand it down and fix it. I'm guessing that's the height he keeps it on the lift and the waves keep hitting the drive and that's what happens. The anode on top of the drive is normal for that year. Volvo mounted them like that. Also most volvos didn't have a anode on the prop what like merc.