CAPAC and Hull Potential question

[jtalberts]

Ok, I had a low capac reading all last year. It was .75. I cleaned up the anodes and replaced the keel anodes. The Capac is reading .95. I have the reference electrode from boat zincs and did a few tests. I tested at the front of the boat and it was .925. Near the Capac it was .95. At the rear of the boat I was at .85. Would this be due to my stainless rudders without anodes or lack of shaft isolation? Both of which I am going to correct this weekend. I disconnected my AC power and DC breakers and had 0 change on the readings. I also disconnected anything to the batteries and this didn't change it either. Which is good that my electrical is in good shape but now I have to figure out how to get the rear readings up.

[jralbert]

First time I heard of anyone taking measurements at different locations - interesting results. Sounds like you have brought the boat right into range. Rudders, as I have read in Marinette literature and specs reported by John Althouse (see his site for details), require anodes. Trim tabs, too. And if your shafts are not isolated, that needs priority correction.

[Harryb]

Hi you need zincs on both the rudders and trim tabs as well as a predetermined amount on your hull. My 32 calls for two 3 foot strips on the sides of the keel and 12 inch pieces on the trim tabs and 6 inch strips on the rudders. I don't know how much you need on a 41 footer but I bet someone on the forum knows, good luck.

[jralbert]

Just checked at Althouse's still functioning web site and here are the anode specs for M's:

Anode specifications for Marinette aluminum boats

Material:
Although anodes are commonly referred to as "zincs" the proper anode for use on the Marinette hull is actually a specially developed aluminum alloy with the designation of KA90 or SEALLOY 150. These aluminum anodes, because of their material make up and the processing that they go through, give the highest protection possible. KA 90 and SEALLOY 150 were the anodes that the boats were supplied with from the factory. Magnesium can be used, however Marinette Yachts does not recommend its use. If magnesium is used, it can only be used in freshwater and it will deplete at a much faster rate than the aluminum anode possibly leaving the hull unprotected if not monitored closely. Commonly available zinc is too close on the galvanic scale to provide adequate protection and should not be used. High purity zinc can be used but it is difficult to find and does not provide as much protection as the aluminum anode.

Attachment:
Anodes should be bolted to bare metal with Stainless Steel bolts. Do not use wire brushes made of steel or copper alloys to strip the aluminum.

Anode requirements:

26', 28' and 29.5' Models
30" on each side of keel, 12" on each trim tab, 6" on each rudder, Total single engine 7' 6"
Total twin engine 8' 0"

32' Models
36" on each side of keel, 12" on each trim tab, 6" on each rudder
Total 9' 0"

37' Models
54" on each side of keel, 12" on each trim tab, 6" on each rudder
Total 12' 0"

39', 41' and 44' Models
60" on each side of keel, 18" on each trim tab, 6" on each rudder
Total 14' 0"

[jtalberts]

Well replacing what I had On the factory mounts

12 feet on each side of keel.
6 feet on each side under the rear of the boat
total of 4 feet on each trim tab
1 foot on each rudder.

I can't believe the amount they have on it.

[barkleydave]

Your readings are great... do not worry about it GO BOATING!

safe boating,
dave

[Rockit]

Dave is right.

It's a funny thing about us owners. We can't help pushing that button all the time to see what the readings are at the dock, at the entrance to the creek, at anchor, in different water temperatures, etc. With the good advice on this thread and elsewhere, preventing electrolysis is a minor maintenance item. Don't let those guys with the plastic bathtubs tell you differently--the skipper in the slip behind me loves to tease me about my "dissolving". And I say "No blisters on my baby!"

[jtalberts]

Well I figured out the starboard isolation while on a conference call today. After putting in a couple bolts I knew were perfectly isolated and removing the others I was still getting continuity. The flange has about 2 cm of gap between it and the cv joint. I ran a piece of paper through the gap and cleared out a bunch of grease and dirt. Shaft is now isolated. I am going to clean it out a lot better tomorrow.

[jtalberts]

Dave is right.

It's a funny thing about us owners. We can't help pushing that button all the time to see what the readings are at the dock, at the entrance to the creek, at anchor, in different water temperatures, etc. With the good advice on this thread and elsewhere, preventing electrolysis is a minor maintenance item. Don't let those guys with the plastic bathtubs tell you differently--the skipper in the slip behind me loves to tease me about my "dissolving". And I say "No blisters on my baby!"

Well I am very touchy to this as my previous aluminum boat was horribly corroded. This has me on edge.

[jralbert]

Well replacing what I had On the factory mounts. 12 feet on each side of keel. 6 feet on each side under the rear of the boat total of 4 feet on each trim tab, 1 foot on each rudder. I can't believe the amount they have on it.

You can actually "overprotect" the boat. Too much anode and it sets up another recipe for corrosion. It's not a case like, say, body armor, where more is better. So, follow the recommended setup and you will be fine. A reading that is slightly under (or slightly over) won't cause damage.