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garyh
#1 Posted : Monday, April 27, 2009 6:45:45 AM(UTC)
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Replacing a rectangle anode on the rudder.

Is it OK to use the round magnesium style in place of the rectangle?

TKS!

Garyh
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pfhlaw
#2 Posted : Monday, April 27, 2009 7:43:25 AM(UTC)
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Gary
If your rudder is steel then the round pancake style zinc anodes are what you want.
Peter
1981 32' sedan bridge
twin Chrysler 360 cu. in. 250 hp engines
Raw water cooled
Nimbus II
Home port: New Buffalo, MI
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garyh
#3 Posted : Monday, April 27, 2009 4:23:28 PM(UTC)
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It is the original on an '86 32 ft.
The original anode was the rectangle and was magnesium I assumed and the round pancake one is magnesium and just was not sure if ok to go from the rectangle to pancake style.

You have me thinking now I should use zinc instead of magnesium?
Gary
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tr_guy79
#4 Posted : Tuesday, April 28, 2009 12:09:09 AM(UTC)
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Take a magnet to it. You'll know pretty quickly what material it is.
"McKenna Renee"
1972 '32 FB Express. Twin 318 / Edelbrock 1409s
Chesapeake City Md (Upper Bay / C&D Canal)
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dougrose
#5 Posted : Tuesday, April 28, 2009 12:29:15 AM(UTC)
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I think, based on what I have read elsewhere in this forum, that you want zinc in salt water, and magnesium in fresh to protect the aluminum parts.

But your rudder is steel. I think they use zinc to protect steel on the great lakes. Check the electrical isolation between your rudder and the hull with an ohmmeter. Clean up where the zinc (I use a round one near the top of the rudder) will go with a wire brush and mount it. Tape the zinc. Prep the bare rudder with OSPHO, and put at least one coat of epoxy primer on. Now you are ready for bottom paint. Don't forget to take the tape off before launching. (!)

This has worked well for me in brackish water, but your conditions are different, so be sure to heed the advice of the fresh-water experts.....


1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida
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Barkleydave
#6 Posted : Tuesday, April 28, 2009 1:18:55 AM(UTC)
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John Althouse has the CORRECT anodes for your Big M's KA90/SEALLOY 150

Zinc collars on SS shafts help protect shafts/props (Fresh or salt)

Mag. ONLY use in fresh water. It will depleat very quickly in salt and will readings will be TWO high and can attack alluminum welds and blishter paint off the bottom.

I just pulled my M here on the Cumberland River and I put new correct anodes on three years ago. The boat has been in the water all that time. Here are my findings and observatons.

1. When I installed them I made sure they hull, rudders and tabs were bright clean.
2. I put a bead of RTV silicone around the edges to help seal out water from anode and hull contact. 95 percent of the silicone was in still in good shape after Three years. Contact point to hull, rudders etc. where very clean no scale continuity was still very good.
3. Late last summer my readings dropped to 7.5. I added a mag guppy and it brought readings back to 1.0 to 1.05.
The reason the readings dropped was the anodes were incrusted with calcium and a few zebra mussels. If I had gotton under the boat with SS brush I am sure the readings would have gone up.
3. After removal and comparison to a new piece they at least 90 percent. They were pitted but no need to replace.

Why did they hold up so well?

1. Shafts, rudders thru hulls are properly isolated!

Using a supplemental mag guppy in fresh water as long as the voltage is in the safe range will greatly reduce the load on your anodes while at mooring.

safe boating,
dave
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Jazman
#7 Posted : Tuesday, April 28, 2009 1:58:26 AM(UTC)
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You can also get the proper anodes from these guys. On the left of the screen pick Hull Anodes then Aluminum.

http://www.boatzincs.com/index.html

They even say the bars are for "Marinette"
Bill

1975 37' FB Sedan
Sleepless Knights
Bohemia River
Chesapeake City, MD
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pfhlaw
#8 Posted : Tuesday, April 28, 2009 2:10:56 AM(UTC)
pfhlaw

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Here's a link to some good info on this subject:

http://www.aquafax.co.uk...rosion%20Card%202005.pdf
Peter
1981 32' sedan bridge
twin Chrysler 360 cu. in. 250 hp engines
Raw water cooled
Nimbus II
Home port: New Buffalo, MI
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garyh
#9 Posted : Tuesday, April 28, 2009 4:05:46 PM(UTC)
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My rudders are steel and in fresh water.

The old anodes were the rectangle mag. and have lasted for several years so maybe I should stick with mag. as I had spare set except for 1 rudder and will go with the mag. pancake style for that one as marina has and I want to splash friday.

Thanks All!!
Gary
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