First, as they've pointed out...the zinc is on backwards... BTW, it does need the 1/4" or so gap between it and the strut or the cutless bearing won't get enough water and can score the propshaft badly.
Second, if you're in fresh water, the zinc should actually be Magnesium or Aluminum, rather than a Zinc. Magnesium is required if you're protecting aluminum parts... like a saildrive or outboard. Aluminum should work in fresh water or brackish water and will out last a Magnesium "zinc", but should not be used if the boat is stored in salt water.
One problem Zinc "zincs" have in fresh water is that they are not as "electrically active" as the magnesium or aluminum "zincs" and if you've got aluminum bits to protect, they will often get attacked before the zinc does—in fresh or brackish water. Also, IIRC, the Zinc "zincs" can get a coating of zinc oxide on them that will essentially insulate them and prevent them from working properly.
There is another good reason to have that bullet zinc on the shaft. If anything happens and the shaft disconnects inside the boat, it can prevent the shaft from pulling out of the stuffing box completely and help prevent your boat from sinking.
When you replace it...remember to seat the zinc firmly with a tap of a hammer. Also, remember to scotchbrite the shaft to get it nice and shiny, so that the zinc makes good contact with it.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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