I have actively followed the forum for several years. I just do not post very often. My question is related to the most popular subject other than painting and that is corrosion control and Capac readings. I have never been able to get readings over .70 on the meter in the 3 years I have owned our 1987 28' Express.

Our boat is in fresh water on the Little TN river. Last year the bottom was taken down to bare metal and redone with the correct bottom paint. At the same time the anodes were replaced with the recommended amount of Sealloy bar stock. I have also bolted a guppie I got from John at Marinette parts to the trim tab. I recently checked, cleaned and reattached the engine and system grounds. I also grounded the rudder linkage to the hull. Two weeks ago the strut bearings were replaced with new non-metallic bearings. The old bearings were also non-metallic. During the haul out it was evident that a small amount of corrosion was back following the new bottom paint last fall. I should add that our 1987 has the stainless rudder posts and rudders.

I have been all over the boat with a digital volt meter. The driver shafts are properly isolated at the coupler. I cannot detect any stray DC current leaking to hull from the engines or DC electrical system. I also checked the bilge water, nothing. The factory wiring harness is mostly intact. I have tried disconnecting the batteries, no change. It makes no difference if I am hooked to shore power or in the middle of the lake the readings are nearly the same. So an AC leak does not appear to be the issue.

I am using a recently purchased reference anode and the digital volt meter for my readings. However, the Capac readings I am getting are close to the same.

Do any of the "experts" have any suggestions!

are you getting corrosion anywhere?

Bonding your steering linkage will put your SS rudders in the loop.. and that will deflect your readings.

dave

I appreciate the quick response from all of the dedicated members of the forum. To answer some of the questions raised;

I do have some small spots of "popcorn" corrosion on the transom and on the hull aft of the rudders. As I noted in my post the bottom was completely redone in Sept 08. The boat is in the water year around.

I have confirmed using the reference anode and volt meter that the Capac analog reading is fairly close to the digital read out.

I tried bonding the rudder linkage to the hull after doing some research in Nigel Calder's "Mechanical & Electrical Manual". He devotes a chapter to electrolysis and corrosion control. However, I may be on the wrong track when an all aluminum hull is involved.

Jeff, I recall reading your reference to a block of magnesium as a supplemental anode before. Any ideas where I might find magnesium in that form?

Thanks Guys,

Deffinitely remove the bonding wire to your steering linkage. The bearings on your rudder shafts (posts) should also be non-metalic.
After you un-ground the linkage, check to make sure your rudders and shafts (posts) are isolated from the aluminum hull. Right now, I think the aluminum hull is acting as a sacrificial anode to the more noble metal of the stainless rudder posts.

The mounting screws for our trim tabs have plastic sleeves.

Plastic sleeves on the trim tab screws are news to me, but I have not looked too closely there. That being the case, then all is as it should be - stainless and bronze parts isolated from the aluminum hull, and the hull protected by keel plates.