Magnesium or aluminium? 4 or 2 cutlass bearings?

[Fjord John]

Hi, my twin shaft GRP cruiser has just been lifted. She is based mid Thames, with just one trip into the Thames estuary. The magnesium anodes have dissolved by about half in 3 years (longer than I would normally leave her), and the stern gear is in great order. Happy days!

Over the next 2 years I will do one or two east coast trips, to Ramsgate, maybe to the Norfolk broads, but overnighting in marinas. Otherwise 100% freshwater based.

Should I switch to aluminium or stick with magnesium? Advice appreciated.

Also, she needs new cutlass bearings. She has one in each p bracket and 1 in each shaft tube. But I don’t yet know which ones are worn. It’s likely they are the original ones, from 2007. Should I await advice from my engineer on which ones to replace or just ask for all 4 to be replaced?

Thanks John

 

[Greg2]

Aluminium anodes work in both fresh and salt water and are what we used for years based in fresh water with trips out to sea. My understanding is that magnesium corrodes quite quickly in salt water. Aluminium would be my choice.

On the cutlass bearings those in the P brackets are more likely to need replacing as those in the hull can last for a very long time. Question is though, what makes you think they are worn and need replacing?

 

[Gustywinds]

You only need to use magnesium anodes if you are protecting aluminium alloy with a very high magnesium content - i'e high strength aircraft grades like the 7000s. For regular aluminium alloys, pure aluminium anodes are plenty good enough

 

[Fjord John]

Aluminium anodes work in both fresh and salt water and are what we used for years based in fresh water with trips out to sea. My understanding is that magnesium corrodes quite quickly in salt water. Aluminium would be my choice.

On the cutlass bearings those in the P brackets are more likely to need replacing as those in the hull can last for a very long time. Question is though, what makes you think they are worn and need replacing?

Hi Greg, a mix of things: high pitched ping ping noise when changing gear, some mild vibration, particularly around 1400 revs and what feels like cavitation when going astern. John

 

[Bandit]

When you gift the boat lift up the shaft and prop if you get more than 1.5mm of play they are worn.

Inspect them you see any damage to cutless bearings, strips of rubber hanging out?

If you remove them mark the top of the cutless bearing then when pulled mark p or s is there more wear in one area than the rest, this can indicate an alignment problem. Bear in mind flexible engine mounts do sag over the years and May nee adjustment or replacing.

 

[Greg2]

Hi Greg, a mix of things: high pitched ping ping noise when changing gear, some mild vibration, particularly around 1400 revs and what feels like cavitation when going astern. John

I am not sure that the noise when putting into gear or cavitation are likely symptoms of worn cutlass bearings but I stand to be corrected. As Bandit says, a lift out and inspection will tell you if that is the problem.

 

[Tranona]

You only need to use magnesium anodes if you are protecting aluminium alloy with a very high magnesium content - i'e high strength aircraft grades like the 7000s. For regular aluminium alloys, pure aluminium anodes are plenty good enough

That is not correct. Magnesium is used instead of zinc in fresh water because zinc becomes ineffective in fresh. on the other hand magnesium erodes quickly in salt water. This is a dilemma for those who boat in a mix and aluminium anodes are equally effective in salt and fresh so are the preferred choice.

 

[Gustywinds]

That is not correct. Magnesium is used instead of zinc in fresh water because zinc becomes ineffective in fresh. on the other hand magnesium erodes quickly in salt water. This is a dilemma for those who boat in a mix and aluminium anodes are equally effective in salt and fresh so are the preferred choice.

Zinc will not protect aluminium in any water.

 

[Tranona]

Zinc will not protect aluminium in any water.

Yes it will. Otherwise all the stern drives and saildrives will have corroded away by now. They all use zinc (or aluminium with the same potential as zinc) as anodes to prevent galvanic action between the stainless components and the aluminium housing. It is not the material that is important but the difference in potential compared with the item being protected.

 

[Gustywinds]

Yes it will. Otherwise all the stern drives and saildrives will have corroded away by now. They all use zinc (or aluminium with the same potential as zinc) as anodes to prevent galvanic action between the stainless components and the aluminium housing. It is not the material that is important but the difference in potential compared with the item being protected.

Zinc is only very slightly more noble than most aluminium alloys and will offer little protection, it is normally used to protect steel.

 

[Hacker]

Yes but it is more noble!

 

[Tranona]

Zinc is only very slightly more noble than most aluminium alloys and will offer little protection, it is normally used to protect steel.

Then I wonder why all the drive units I mentioned use zinc successfully to prevent corrosion of the aluminium housings. Zinc does nothing for steel as that just rusts which is not galvanic action. Not sure you have good grasp of the subject. This might help en.wikipedia.org/wiki/Galvanic_series

The aluminium anodes that are increasingly use to replace zinc have virtually the same potential as zinc and therefore being lower than the alloys used for the housings will work instead of zinc. volvo seem to have worked this out because they now only use aluminium based anodes.

 

[Fjord John]

When you gift the boat lift up the shaft and prop if you get more than 1.5mm of play they are worn.

Inspect them you see any damage to cutless bearings, strips of rubber hanging out?

If you remove them mark the top of the cutless bearing then when pulled mark p or s is there more wear in one area than the rest, this can indicate an alignment problem. Bear in mind flexible engine mounts do sag over the years and May nee adjustment or replacing.

Thanks Bandit. I saw the boat out of the water on Friday. No stripes of rubber hanging out. Thought to my untrained eye, possibly some fins more visible than others. With some force a bit of slack felt in the stern tubes. A previous engineer did say one engine a bit misaligned but nothing to worry about. Though a new and well recommended engineer currently working on her. I have noticed a slight kick when switching engines off but wasn’t sure if normal or not. I’ve not heard back from the engineer yet, so he’s no doubt considering these things. But I don’t want to risk paying for a second lift out I’ll ask him to inspect the mounts and alignment - do you agree that’s the best next step? Thanks John

 

[Greg2]

Engine alignment can be checked whilst the boat is in the water so it would indeed be prudent to talk to your new engineer to ask them to check it if only to eliminate a possible cause before you go for the expense of another lift out.

 

[Bandit]

You can’t check engine feet unless they have totally failed or sheared. It comes back to alignment and whether the engine and shaft are aligned to the sternseal cutless beaning and whether the P bracket is aligned to the latter.

 

[Fjord John]

You can’t check engine feet unless they have totally failed or sheared. It comes back to alignment and whether the engine and shaft are aligned to the sternseal cutless beaning and whether the P bracket is aligned to the latter.

Thanks Bandit. I’ve had all 4 bearings replaced and the alignment will be checked when she goes back in the water.