"Zinc" anode basic questions

[Njlarry]

Hello all.
Last year was our first spring commissioning. I changed to aluminun anodes on the hull and swimplatform gear. The props shafts rudders and swimplatform lift arms were well coated in zinc paint.
In the fall the aluminum anodes were way wasted by over 40 percent or more. I did not check for stray currents or the galvanic isolators. However none of my neighbors reported a problem. So assuming no stray current. We are in the brackish water of the upper Chesapeake.
1. Could the zinc paint have caused that much aluminum anode sacrifice?
2. I know wooden hulls can be over zinked but is there any harm in adding anode collars to the shafts? All my previous boats had this and I am not concerned about them slipping.
3. What is the function of the ground plate under the port side of the hull? Ours had a slight halo around it on survey prior to purchase.
Thank all for your advice.

 

[dwna1a]

Know you are on the upper bay but I think I would stay with zinc. I’m in the mid James River and tried aluminum once. They did not last at all. I’ve changed back to heavy zinc.

It sounds to me you have a stray current issue but the aluminum did do its job. This year I’ve used Vivid which is a zinc copper paint.

I’ve never understood the bonding plate so I will be interested to see what others post

 

[ttmott]

The fact your boat is consuming the anodes is a good thing. If they weren't being consumed then for sure other more important and expensive metals would be going away.
Aluminum anodes are not ideal for brackish or salt water; You'd be better off with zinc anodes.
Even if there isn't stray current there is still galvanic electrical activity between the boat and water. So, the need to have those sacrificial anodes.
Damaging stray current can come from your boat, another boat, or the electrical on the docks. Presence of stray current is easily checked with a silver-silver chloride test cell. Just follow the procedure. Your boat's bonding and electrical system can also be verified with that test cell. I purchased one from boatzincs.com and use it whenever things change around my boat.
That bonding plate, also called a dynaplate, is to provide a dedicated ground for the boat's electronic systems. The earlier VHF radios really needed that ground to water to keep electrical noise down. It also serves as a bit of protection in the event of a lightning strike. My 2000 400DA had one but my 52DB does not.

 

[Lazy Daze]

TT's first sentence is important.

Contrary to TT's statement about composition, I've had excellent results with Al anodes with multiple boats - primarily stern drives which are harder on anodes, as well. I also use the Performance Metals brand - which is a step up over most others.

BUT... the best answer I can give you is to check with other people in your area with a similar setup. If they have been in those water for at least a few years and have been having good results... just copy them.

 

[quality time]

I just realized that your avatar was taken at BBYH, not sure if you're still there or not. I've had boats there for 30 years and had always used zinc but decided to switch to aluminum a few seasons ago. They were gone within a few months so I went back to zinc and change them annually.

 

[Njlarry]

We winter on the hard at Bohemia and really love the place but season down in Rock Hall with higher salinity.
Know any good bottom paint guys there? Ken at Bohemia says they are too busy this spring to strip and re coat.