How important is it to change zincs in fresh water?

[labatt]

In another thread I just learned that, when in fresh water, you need to replace your zinc sacrificial anodes with magnesium. Since I kind of hijacked that thread I figured I would start a new one. Our boat entered fresh water Mid-May and will be hauled in October (so figure 5 1/2 months in fresh water) and we'll probably be bringing her back to salt water next year.

Is it worth the expense of hauling her to replace the zincs? Will they "coat themselves" with zinc oxide so quickly that they are useless? I'm concerned that I'll have to haul her twice - first to measure the prop shaft zinc and the hull plate zinc - and second to actually replace them once I find ones that fit. Even hauling her once is a few hundred $$$'s.

I'm assuming that what will happen is that they will become coated with zinc oxide, becoming useless for their purpose, which will allow other metals to give up their electrons faster than the zinc? Is galvanic corrosion still a risk in fresh water?

If I need to do it, I need to do it...

Thanks!

[Fstbttms]

Originally Posted by labatt

Is it worth the expense of hauling her to replace the zincs? Will they "coat themselves" with zinc oxide so quickly that they are useless?

Haul the boat to replace anodes?!!! There has got to be a diver around that can do it much more quickly and cheaply.

[sailingdog]

Labatt-

Galvanic corrosion is still a problem in freshwater. However, zincs made of zinc are not generally reactive enough to provide your boat sufficient protection in fresh water. Fresh water boats should have either aluminum (brackish water usually) or magnesium (pure fresh water) zincs. If you've got zinc-alloy zincs, they can often not be affected while the aluminum parts of your boat get attacked.

You should also keep a notebook on the boat, preferably in the maintenance log, that lists the important consumables and what their size/part numbers are, so you don't have to haul the boat to figure it out. Oil filters, fuel filters, impellers, zincs, clevis pins, shear pins, and other things like that should be on the list IMHO. Also, might want to list various fuses and what amperage is used for what circuits.

[sailingdog]

Originally Posted by RickBowman

Who would install aluminum parts below the waterline on a sailboat?

Ummm.. lots of new boats are equipped with saildrives, and the saildrive housing is generally aluminum...