Anodes- Mg vs Zn

[Guest]

I understand that for fresh water I should have magnesium anodes, not zinc (which should be used on salt water). Given that I have zincs at present, but will over winter in fresh water and don't want to crane out till the spring, is it possible to bolt a load of magnesium anodes together, bolt them to a long electrical lead, and dangle them over the transom?

I remember reading ages ago about saving the part used anodes, and doing this, but (1) I can't remember what you need to bolt the other end of the wire to, and (2) I don't know if you can mix zinc and magnesium this way.

Anyone know?

 

[markc]

Yes you can, I did it on my last boat. You will need to attach the other end to a 'bonding' point, such as the bolts that attach the p-brackets (assuming it's shaft drive) on the inside of the hull. This also assumes that your existing anodes are bonded to all underwater gear. As an alternative in the future, try aluminium anodes, they work in both fresh and salt water.

M

 

[byron]

Don't worry about it, just leave your present ones as is. I keep mine salt water ones on all the time despite wintering on fresh water. Never had a problem yet.

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[Guest]

Thanks for that - I'd never heard of ally ones. I've got sterndrives, so I presume just bolt to some part of the block?

 

[kimhollamby]

Don't assume that there is a good bond between engines and legs on outdrive rigs. Would have thought you are better simply lifting the legs and giving the ring anodes a once-over occasionally over the winter; if anything is going amiss under the waterline aluminium legs will be the first to suffer.

There's another anode tip that's worth remembering, namely that the anode has to be in line of sight of the bits it is supposedly protecting.

 

[Guest]

Can't see bugger all under the swim platform, so up or down makes no difference, and I'm not going diving in the marina in December. Ring anodes? Do I have those?

> There's another anode tip that's worth remembering,
> namely that the anode has to be in line of sight of the
> bits it is supposedly protecting.

Does this mean there's no protection in the dark?

 

[jfm]

Re: Aluminium eek!

Whooaa! Not my specialist subject so I'll stand corrected if wrong, but your outdrive leg is made of ally, so if you use ally anodes aren't you risking the outdrive sacrificially dissolving so as to protect the anode? Doesn't the anode have to be further up the table of elements from the metal you're trying to protect?

Sorry if above is total garbage.

 

[paulineb]

I'd love to help you here Graham but I had my anodes out at the same time as my tonsils were removed when I was 7yrs old.

Pauline B

 

[Guest]

T'would explain a lot...

 

[kimhollamby]

Almost certainly yes, immediately ahead of the prop(s). And/or an anode on/under the cavitation plate. These are the important bits on any outdrive boat.

Solution to visibility: nick 20-30 bodies from local clubhouse/pub, get them all to teeter on the foredeck and you should find that the leg becomes visible! The round afterwards might be a bit expensive, however...

 

[kimhollamby]

Re: Aluminium eek!

Correct. Aluminium used a fair bit by Dutch steel manufacturers but not a good bet on outdrives.

 

[Guest]

Re: becoming suspicious

I am becoming more and more suspicious about the soundness of MBY's technical advice.

1) Is my coachroof going to take 2000kg+
2) More to the point, how do you get rid of 20-30 drunks from your boat?

 

[kimhollamby]

Re: becoming suspicious

Former MBM'er, so MBY safe. I was, in part, joking. But we have used the technique (okay, maybe not always 20 on a boat of 29 Targa size) when having to deal with fouled outdrive problems. Usually cheap labour for us...we simply got cruise in company participants to help and let the owner serve the drinks.

Interesting aside...as the years have passed so the angles at which new outdrive models lift have diminished. The original 270 Aquamatic made nearly 90 degrees when raised; the 290 only manages about 45 degrees, or not much more. There's an analogy there, somewhere.

Anyway, the serious bit of the advice is to keep an eye on your outdrive anodes. Better use of your energy than hanging anodes over the side. Magnesium still works in fresh but its problem is that the surface furs over and needs to be eroded from time to time to stop it from becoming completely isolated from the surrounding water. As Byron says, you can often get away with magnesium on fresh....I did too, but I did take a late summer swim to scrub them off pre-winter.

Oh yes, another thing. Where are you going to be keeping the boat? Big difference in anode erosion depending on water quality and surrounding structures. Worst scenario - marina with steel-reinforced banks and overhead powerlines. Ditto next door neighbour with earth-leakage problem on shorepower.

 

[markc]

Re: Aluminium eek!

True - I did mention that it was on shafts, no I wouldn't put them on sterndrives either!!!

M

 

[jfm]

Re: Aluminium eek!

markc yes you did say shafts, I was just making the point because graham's boat is a Targa with sterndrives.

 

[Scubadoo]

My anodes seem to last the year and replace them when the boat is out for antifouling etc. Once I got a generally idea of the life of anodes I was happy, you really can't check them unless you go for a swim.

RM.

 

[martin]

My last boat had Zinc anodes on Volvo drives as per dealers advice. Kept on upper thames for part season and lower thames for remainder. End of first season horrible pitted drives and volvo saying not covered by warranty due to wrong type of anodes fitted for conditions. Two attempts to stop the rot over two years and lots of legal posturing later managed to get them to fit new drives FOC. Sold boat and vowed always read the manual. Now have two sets and change them as the boat is craned into water as per conditions.

 

[byron]

Re: becoming suspicious

Ooops! that was remiss of me Kim. Yes! when my boat comes out for anit-fouling I do give the anodes a quick wire brushing to remove any fur.
Interesting what you say about local conditions. Years ago I had a mooring on the Hamble and my anodes were going almost monthly. Fairey Marine investigated and found that in the mud below my mooring there was a load of old iron. This was hauled out with a digger and the problem was solved.

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[Guest]

Re: quite attached to my legs

Rossiters in Christchurch. It's well up river, and fresh water, but still tidal, although we're in the Pool, so no river flow through. Natural mud/stone banks, and wooden piles, so no steel to worry about. Boats either side do not stay on shorepower (no-one does for quite a way round, actually), but we are because of the dehumidifier.

Any advice welcome, as I'm quite attached to my legs, and would be keen to find them still attached to the boat in spring...

 

[BarryD]

Re: becoming suspicious

So how do you know if your anodes are erroding quickly - at £100 or so a lift it must be expensive to pop the boat out each month to check. Can I guess that you speak to others in the area and see what their wear rate is? Don't really want to wear the boat our before I've used it!

Barry D.