Sterndrive corrosion

[MapisM]

Just bought a 20' boat. It has pleased the previous owner for five years, all spent on a lake.
Very good shape overall, but there are signs of corrosion here and there on the sterndrive.
I posted a picture, hope somebody can suggest what should be done to fix or at least avoid growing.
Many thanks in advance.

http://web.tiscali.it/mapis/AlphaOne.jpg

 

[byron]

I am no outboard/outdrive expert and unfortunately your shot doesn't show the prop. It is my belief that there should be a sacrificial anode ring located between the prop and the leg. If this is missing because it has been eaten away then this is most likely the cause of your corrosion, though the word corrosion should be replaced by electrolosys.
I look forward to seeing replies from more knowledgeable people than me.

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[bernard_foster]

My outdrive leg an AQ80 was among the first built in the early 60s, and has less corrosion, I believe this to be as Byron suggests due to the sacrificial anode, which on mine is a collar fitted between the bottom of the leg and the prop, this anode should be replaced once it has decayed by no more than 15% to my knowledge (quoted from handbook). in the meantime, with a bit of grind work and an alluminium welder, that could be repaired.

 

[BarryH]

You say kept on a lake, now i might be wrong, but I'm sure that iread somewhere that anodes used on fresh water are different to ones used in salt water.

Just a thought, when you sort the corrosion, be sure to etch prime before repainting, good luck

 

[scottie]

Re: zinc? annodes

Boats used in frsh water should have Magnesium annodes with drives
zinc annodes dont work in fresh and usually end up covered with a brown deposit whilst magnesium annodes just disappear in salt water

 

[MapisM]

Thanks for your hints !

You are most likely right, everybody. Thanks a lot.

Unfortunately, I will be able to check that only in the Easter weekend (I am not lucky enough to live near my favourite toy...), but:
1) The anodes are in very good shape - too much actually (one of them is mounted above the cavitation plate, and is visible in the picture). They are also somewhat brownish. I guess they are zinc, not magnesium, and probably never changed. Thanks Scottie!
2) There is no collar anode near the prop, but I am pretty sure this is normal on Mercruiser. I've always seen it only in Volvo duoprop. The other anode (besides the one visible in the picture) is the small fin behind the prop, under the cavitation plate. It can also be rotated to compensate the prop rotation (I would not bet on the efficiency of that, though...).
Anyhow, what I told before can be applied also to this anode.

I read somewhere about galvanic current.
Is that something which proper anodes can avoid, or is it due to other reasons?
If so, what else should I check other than the anodes?

 

[duncan]

who put the new anodes on the leg? If the previous owner then th ebetting is that the old ones had 'gone' hence the damage. Putting the right anodes on and replacing them when 50% gone should save you further trouble but an expert should look at the damage so far and recomend course of action.

happy boating

 

[Piers]

Sounds like the wrong anodes. Call M G DUFF (UK maker of anodes) and discuss with them. 01243 533336.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm