Unequal depletion of anodes

[ss2016]

I have a VP D6 with a DPI stern drive. After some 16 months in the water the anode above the cooling outlet on the drive leg is significantly depleted and needed replacing, while the anode at the bottom of the transom shield is almost as new. Is this normal?
Is the reason that since VP painted everything there is very little exposure of unprotected metal to the water except the propeller which the depleted anode has protected?
Interestingly the anodes on the SS trim tabs are also eroded significantly. Is this because the antifouling has not bonded well and they present a significant surface area?
Thanks, Steve.

 

[Portofino]

Reason is irrelevant .You just replace them when worn .The suggestion is 1/2 to 2/3 rds depleted .It’s convenient if that coincides with the annual lift , buts that not always the case .

Hard to comment further as so many variables, eg how your neighbours boats protected , how much metal is near like marina posts , how much other stuff is bonded to your existing under water protection, if there’s any current leak from a semi submerged 12 v appliance in your boat like a bilge pump switch going bad etc etc .

Good news is anode depletion where convenient or not is actually imho a good sign .Better they go rather than something expensive .In other words worry if they seem to last for ever , never need replacing.If that makes sense ?

 

[jrudge]

Check with a voltmeter that it is electrically connected to the chassis of the stern drive. The resistance should be as near as damn it zero. If it is then assuming the anode is the correct one for the water you are in ( zinc / magnesium) then ignore it. If there is a resistance then remove it, clean it and reattach .

 

[QBhoy]

It may be that you have a mix and match of anodes. Some outdrives come new with anodes that are multi purpose aluminium. That are a compromise to suit both fresh and salt water use. These are usually replaced later to specifically suit the actual use of the boat. So zinc for salt and magnesium for fresh. Obviously more effective at that. Perhaps you have a mix and seeing different wear rates. Other possible explanation might be if you have stainless props. These can have an effect on the nearest anode.

 

[BruceK]

16 months and significant wear sounds about right. Anodes are doing their job, the ones not wearing Id be more concerned about.

 

[ss2016]

Thanks for your replies. I think perhaps I proposed the right answer in my initial query. The anode above the cooling outlet is protecting the significant amount of exposed SS on the prop and therefore eroding. The anode on the transom shield, because the paint is good and there is effectively no exposed unpainted metal has nothing much to do, therefore no significant loss. The voltmeter test was a good tip.